Archive for the ‘Cooling’ Category

#91 Replace Leaky Heater Core / Window Fogging

March 2, 2014

As mentioned in task #87, while checking for coolant leaks I discovered that the heater core was leaking.  I fixed the bigger leak at the upper radiator hose and left this heater leak for later.  It seemed just a minor nuisance where I could keep topping off the coolant level, plus my impression was that replacing the heater core was a big job.

Since then I’ve come to realize that replacement isn’t terribly difficult or time-consuming.  More importantly, I learned the hard way that a leaking heater core can be a real safety issue because it will fog up the windows.

This is a very common problem on 850s– probably on every car from the factory because of a poor design in the heater core itself where the plastic frame is not properly bonded to the heat exchanger.  After ten years or so (and all Volvo 850s are now at least 16 years old) the heater core will leak a little coolant.  Not too much, but enough to be a situation that needs attention.

Even if you are willing to just keep topping off the coolant, it’s a real problem because if you ever use your heater a leaky core creates a fine aerosol of coolant (water and antifreeze) which deposits a moist, oily film on the windows.

Leaking heater core puts a moisty, oily film on inside of windows.

Leaking heater core puts a moist, oily film on inside of windows.

This then attracts dust and such in the cabin which just obscures the windows even more.  Eventually it’s hard to see out the windows, particularly at night and when the sun is low.  Poor visibility is a real safety issue and is what convinced me I needed to fix the leak.  It took several passes with cleaner and rags to get the windows really clean and there is a lot of glass on a wagon.  This film is also being deposited on the passengers and everything else in the cabin.  :(

You will probably first detect a heater core coolant leak with your nose.  When any heat is turned on some air flows around the heater core and the vapor from the anti-freeze chemical moves into the cabin.  Most of us recognize the unique sweet odor of ethylene glycol in the coolant.  If you’re not sure, just remove the coolant reservoir cap and give it a sniff.  If you smell the same odor inside the car with heat on, you’ve got a leak.

Confirmation of a leak is coupled with an observable drop in coolant level in the reservoir.  The heater core is located forward of the transmission/gear shift lever under the dash just above the floor.  Access requires removal of the dash board lower covers and then pulling back carpet and removal of plastic covers under that.

When I opened up the panels on this car, I found a wet spot on the inside of one of the panels and shiny wet spots around the base of the heater box.

Obvious wet spots around and under heater housing.

Obvious wet spots around and under heater housing.

IPD has a great video showing the replacement procedure:     It is for a 1998-2000 V70 so isn’t exact but only a few details are different.

MVS has a really good step-by-step procedure written up:

General procedure to replace the heater core:

1. Remove lower dash covers on both driver and passenger sides.

Remove dash lower covers.

Remove dash lower covers.

2. Peel carpet against center console back on both sides.

Carpet pulled back.

Carpet pulled back.

3. Remove black plastic panel behind carpet to access heater box.

Black plastic cover removed.

Black plastic cover removed.

Coolant leakage is very obvious.

Coolant leakage is very obvious.

4. Unscrew core frame from heater box, two screws on each side.

Shop manual shows two screw locations.

Shop manual shows screw locations.

Access to these screws is somewhat awkward.  It helps to push the seats back, put a cushion on the floor, lay on your back working up and have a good work light.

5. Pinch off both heater hoses on engine side of fire wall to prevent draining engine coolant into the car.

Pinch off both hoses between engine and heater box.

Both hoses between engine and heater box pinched off.

Hose access improved by removing heat hose to air cleaner.

Hose access improved by removing heat duct to air cleaner.

6. Pull evaporator drain fitting out of hole to provide space for removing the heater box.

AC evaporator drain tube pulled up and out of way.

AC evaporator drain tube pulled up and out of way.

7. Unscrew tubes from heater box.

Single screw secures tubes to heater core.

Single screw secures tubes to heater core.

8. Pull heater core and tube block apart.  Be prepared with rags or towels for quite a bit of coolant leakage (residual in the hoses and tubes plus inside core).  It may take some prying and effort to separate.

Separate core from heater tubes.

Separate core from heater tubes.

9. Rotate heater box back and out the RH side (US passenger).

Rotate core out of heater box and out passenger side.

Rotate core out of heater box and out passenger side.

Note access to the heater box may be slightly different on earlier model 850s but the general idea is the same.  On models with manual transmission there is also a gear shift cable passing under the heater core to work around.

Heater core is now out of the car and can be worked on in a convenient location.

Heater core is now out of the car and can be worked on in a convenient location.

10. Remove screws holding the heater core in the frame and pull the old core out.

Four screws on end hold core in frame.

Four screws on end hold core in frame.

Core removed from frame.

Core removed from frame.

Wet spots at top (and bottom) of core typical of plastic separation from aluminum tubes.

Wet spots at top (and bottom) of core typical result of plastic separation from aluminum tubes.

Now ready to install new heater core.  While many people insist on using factory heater core replacements, the design is still flawed and will happen again.  They are also a bit expensive.  I chose to go with an after-market part that claimed to have an improved design.  It also comes with two new O-rings for sealing the tubes and is at a very good price on eBay.  This is likely a knock-off of a better part but they claim that their design fixes the expansion problem by crimping the tubes onto the plastic tanks.  If it lasts five years or more I’ll be happy; not sure how much life the car has left in it.

New core at right is an exact match to old one on left.

New core at right is an exact match to old one on left.

New core crimps tubes to plastic to avoid separation problem.

New core crimps tubes to plastic to avoid separation problem.

This new core came with foam strips installed.  If yours doesn’t, you would have install new foam strips to match old core.

11. Clean up the frame to remove oil and dirt.  Disintegrating foam strip leaves a lot of debris behind.  It also would not be sealing well, meaning some air flow through the heater box will bypass the heater core, reducing heat efficiency.  New foam will seal better and give better heating.

Old core foam strip was disintegrating and really gummed up the frame.

Old core foam strip was disintegrating and really gummed up the frame.

Cleaned up core frame before installing new part.

Cleaned up core frame before installing new part.

12. Put new heater core in frame and screw in place.

New and improved heater core installed in frame.

New and improved heater core installed in frame.

13. Remove O-rings from ends of tubes from firewall and replace with new ones.  If your tubes are corroded they should be cleaned up before installing O-rings.  If your pipes are steel (older models) consider replacing with newer aluminum tubes.  Always use new O-rings because the old ones will be dry, hard and not pliable.

Old, dry and hardened O-rings were removed.

Old, dry and hardened O-rings were removed.

Tube ends were slightly corroded.  Cleaned up the surface with strip of emery paper.

Tube ends were slightly corroded. Cleaned up the surface with strip of emery paper.

New O-rings installed with a bit of petroleum jelly.

New O-rings installed with a bit of petroleum jelly to improve seal and preserve elasticity.

14. Maneuver heater box back in place and push onto tubes to mate properly.  Screw core onto tube block to secure.

Core positioned in heater box.

Core positioned in heater box.

Pushed tubes into core to mate.

Pushed tubes into core to mate.

Screw core back onto tube block.

Screw core back onto tube block.

15. Screw core frame back into heater box on both sides, making sure the rubber seal is in place all around the seam.

16. Remove clamps pinching off heater hoses.

Don't forget to unclamp heater hoses!

Don’t forget to unclamp heater hoses!

17. Start the car and check for fresh leaks.  Shouldn’t be any with new O-rings.

No leaks with engine running!  This is an important check before buttoning things up.

No leaks with engine running! This is an important check before buttoning things up.

18. Top off the coolant level as needed, depending on how much was lost in this process.

Added fresh coolant to replace what was lost (about half a tank).

Added fresh coolant to replace what was lost (about half a tank).

19. Reconnect evaporator drain.

20. Install plastic panels on both sides of heater box.  Note the white plastic tab on heater core frame mates with slot in black plastic panel.

Replace plastic panels.

Replace plastic panels.

21. Push carpet back into place on both sides.  It’s probably a good idea to let the car air out for several hours (windows open) to evaporate much of the spilled coolant from inside the cabin.

22. Install lower dash panels.

I’m going to wait a week or two before cleaning the inside windows again.  There is likely some residual coolant in the ducts and air vents which has yet to be pushed into the cabin.  Once it’s dried out we can clean the windows for good.

Now I get to do this again on my 850 sedan which also has a heater core leak, just not as bad (no coolant loss and no foggy windows but antifreeze odor with heater on).

$39 for low-cost “improved” design heater core

Technical Note

The heater core is essentially a small radiator, much like the big one in front of the engine.  Hot coolant from the running engine flows through the core and air flows through the fins where the heat from the coolant is transferred to the air.

On Volvo 850s coolant flows constantly through the heater core; there is no shut off valve or regulator.  Heat into the cabin is determined by temperature settings on the climate control.  This regulates air damper position to direct more air through the heater core as you increase temperature.  So while hot coolant is always circulating through the heater core, it does not exchange heat to the cabin until the operator selects some heat.

#88 Coolant Temperature Sensor/Engine Stalling

February 8, 2014

Long post so I’ll give an executive summary first:  Occasional random events of engine quitting while driving caused me to narrow down possibilities.  Focused on coolant temperature sensor and learned that a loose connection here can cause surging and stalling.  Discovered that the sensor was aged and out of spec so needed to be replaced.  More importantly, the connector had major issues.  Replaced the temp sensor and replaced the connector contacts for reliability plus added a missing connector bracket.  Hoping this cures the engine stalling problem.

Now for the details:  For months now we have experienced the nuisance of having the engine just die while driving slowly.  It occurs infrequently and these intermittent problems are the worst kind to solve.  So far it seems to happen at half throttle; not at idle and not at highway speed.  No error codes on the ECU but when the engine quits the instrument cluster lights up with all those warning lights.  No real self-diagnostics to use here.  Car will usually start right up after these incidents.

Idle stalling problems are often caused by a dirty throttle body or a gummed up idle air controller but this is not a problem at idle.  In 850s stalls are frequently due to fuel delivery problems (fuel pump, fuel filter, pump relay.)  However, the pump was recently rebuilt, the filter is less than one year old and the relay was renewed so these are unlikely causes.  Ignition (spark) is also suspect although this is more likely to throw an error code which we are not seeing.

I was leaning towards an intermittent sensor to explain this problem.  Two sensors are used by the ECU to adjust fuel injection into the cylinders based on various conditions.  First is the mass air flow (MAF) sensor.  This one was new when we got the car and it seems to behave normally.  The other is the engine coolant temperature (ECT) sensor.  This one seems to be original and is turning colors with corrosion plus these are known to fail with age, coolant quality and overheating incidents.

The ECT sensor is sort of hiding under the upper radiator hose at the thermostat housing.

ECT sensor located at thermostat housing under upper radiator hose.

ECT sensor located at thermostat housing under upper radiator hose.

Looks a bit crusty now and has been there a long time.  Some people recommend they be replaced with the thermostat although I have never done so.  Flaky sensors are known to cause various problems from hard starts to poor fuel economy to mystery stalls such as we are experiencing now.  At any rate, since these are known to be problematic and this one is apparently old (likely factory original), I started investigating it.

There are resistance and voltage specifications for a proper functioning ECT sensor.  It is basically nothing more than a two-terminal resistor which changes value corresponding to temperature.  See tech notes below for details.  I unplugged it to check the cold resistance and was surprised to discover that the connector on the wiring harness side was broken and falling apart.  The two contacts were loose and not being retained in the connector housing.  The contacts were also fairly corroded.  I know that poor electrical contacts can cause all sorts of problems, particularly intermittent failures where they work most of the time but occasionally lose contact.  So this automatically became my main suspect for the stalling problem.

Socket contacts loose and corroded are a huge problem.

Socket contacts loose and corroded are a huge problem.

Sensor pin contacts properly retained in housing but also corroded.

Sensor pin contacts properly retained in housing but also corroded.

I suspect that the sensor signal cuts out intermittently with vibration while driving, which is interpreted by the ECU as a cold engine, which responds by dumping more fuel into the intake to a warm engine.  That gush of fuel effectively chokes the engine and it dies.  This was confirmed experimentally by running the engine, then while pulling the connector apart, the engine would suddenly surge a few times.  When plugging back together again, the engine died, just like the problem we are experiencing (although this experiment was at idle).  So the theory that a bad ECT sensor can cause the engine to surge and/or stall is proven true.  No error codes occur so it may be that intermittent contact is short enough to cause a surge but not long enough to register an error.

Cold resistance was measured with a multimeter and showed about 5200Ω.  Compared against a new sensor which measured 2400Ω and the sensor in my 850 sedan which measured 3000Ω, this cold value is quite high.  Factory spec is 2800Ω at 68°F (it was slightly warmer than this so the new part was right on spec). Ran the engine up to stable temp on the instrument gauge and checked the resistance at this level.  Measured 300Ω compared to my sedan at 200Ω and subsequent reading of 212Ω with the new sensor installed.  Factory spec is 150Ω at boiling, which we are just under so the value should be in the 150-200 range.  This old sensor is about twice the resistance it should be at both hot and cold.

I also have a nice code reader which will display the real-time data stream of monitored parameters.  This old sensor runs hot and stable at 180°F compared to my sedan at 200°F; not too far off but seems a little low, which is consistent with its higher resistance.

Data stream coolant temp parameter stabilizes at °F.

Data stream coolant temp parameter stabilizes at 180°F.

All these diagnoses lead to two conclusions:  First, the ECT sensor is old and high in resistance and operating values.  Second, the bad connector is quite likely causing intermittent connections to the sensor due to corrosion and unsecure terminals.  This intermittent connection can cause surges and/or stalling engines so both the sensor and the connector need to be replaced.

To remove the old sensor requires that the radiator be partially drained of coolant, then the thermostat housing removed for access to the sensor itself.

Coolant partially drained and thermostat removed for access to ECT sensor.

Coolant partially drained and thermostat housing removed for access to ECT sensor.

Unscrewing the ECT is a pain because there is no room for a standard 19mm wrench.  Factory procedure is to release the pin contacts from the connector to allow the cable to pass through a box-end (ring) wrench.  Polarity of the wires does not matter so there is no need to orient them when finished if this is done.

Sensor connector removed to allow box end wrench to slide over wires.  You could also just snip the wires if you're throwing the sensor away.

Sensor connector removed to allow box end wrench to slide over wires. You could also just snip the wires if you’re throwing the sensor away.

A bit tricky to remove connector housing.  Basically pry out on sides to release latch then flip end up.  Hinged near middle of connector.

A bit tricky to remove connector housing. Basically pry out on sides to release latch then flip end up. Hinged near middle of connector.

19mm box end wrench slips over wires to loosen/tighten sensor.

19mm box end wrench slips over wires to loosen/tighten sensor.

New sensor compared to old one.

Installed new sensor with fresh crush washer then secured the thermostat housing back in place and snapped the sensor connector back on the wires.  Polarity doesn’t matter (both wires are black).

New sensor installed with thermostat housing back and coolant refilled.

New sensor installed with thermostat housing back and coolant refilled.

Then added fresh coolant back in.

To repair the loose connector with corroded sockets I gave up trying to identify a new part number to order so went to a Volvo junkyard to snip one off of a scrapped 850.  But after looking at several, they all had some degree of corrosion and loose contacts so this is obviously a common problem that all 850 owners should be aware of.  I took the best connector I could find; the contacts were crusty but they were secure in the housing.

Old contacts are corroded and plastic bushing is hard and cracked so contacts are not secure.

Old contacts are corroded and plastic bushing is hard and cracked so contacts are not secure.

Found a website where a guy sells Volvo wire harnesses and connectors and relays and other interesting things.  Nothing specific to 850s but he sells connector bullet terminals with new bushings that are pre-crimped on 24″ wires.  Ordered a couple of these to try out and they fit fine in the old connector housing so I basically repaired the connector with new contacts by splicing them into the engine wiring harness.

Here are the female bullet contacts I ordered from Dave's Volvo Page.  Choice of colors.

Here are the female bullet contacts I ordered from Dave’s Volvo Page. Choice of colors.

Pre-crimped terminals fit perfectly in connector housing.

Pre-crimped terminals fit perfectly in connector housing.



Verified good connection with sensor before installing either parts.

Verified good connection with sensor before installing either parts.

Solder spliced each wire into engine harness, then covered splice with heat shrink tubing before slipping wires back into split loom tubing.

Spliced new connector wires with solder joints covered by heat shrink tubing.

Spliced new connector wires with solder joints covered by heat shrink tubing.

Covered new wires with split loom tubing secured with cable ties.

Covered new wires with split loom tubing secured with cable ties.

This gives us a clean and secure connector to the sensor. This repair method can be used for many other connectors in the car.

Also installed a new retainer clip salvaged at the scrapyard to replace the one that was missing from this car.  Now this connector will be secured instead of swimming around on its own.

Installed missing connector retainer.

Installed missing connector retainer.

Plugged connectors together and secured on "new" retaining clip.

Plugged connectors together and secured on “new” retaining clip.

Sprayed contact cleaner on new sockets before plugging connectors together to fight off corrosion.

Ran the the engine to normal operating temperatures and verified that the sensor is behaving. Checked ECT with new sensor at normal running temp and it now reads 196°.

New sensor reads hot normal temperature of 198°F.

New sensor reads hot stable temperature of 196°F.  Was too low with old sensor.

Time will tell on this fix but I’m optimistic.  Assume it solved the problem unless I report otherwise.

$37.88 for a cheap after-market ECT sensor.  $10 for a sensor connector and retaining clip from a scrapped Volvo.

Technical Notes

The ECT sensor is a simple negative temperature coefficient thermistor.  That means its resistance decreases with temperature.

The Volvo 850 uses the known resistance curve to determine temperature proportional to this resistance based on the voltage across this sensor.  A pull-up resistor reference to +5V forms a voltage divider with the thermistor being the variable device.

ECT sensors are very important in the Volvo 850 (and most modern cars) because it is used for fuel trim, idle air, ignition timing and cooling fan operation.

ECT sensor resistance table per factory specs:

7300 ohms at 32 deg F
2800 ohms at 68 deg F
1200 ohms at 104 deg F
300 ohms at 176 deg F
150 ohms at 212 deg F

From the factory functional description:

The Engine Coolant Temperature (ECT ) Sensor supplies the Engine Control Module (ECM ) with a signal describing the temperature of the engine coolant. This gives the engine coolant temperature (ECT) sensor a measurement of engine temperature and influences the control of:

  • Injection period
  • Idling speed
  • Engine Cooling Fan (FC )
  • Ignition timing
  • On-Board Diagnostic (OBD ) functions.

The sensor incorporates a temperature-sensitive resistance with a Negative Temperature Coefficient (NTC ). The sensor is supplied with a stabilized voltage of 5 V from the engine control module (ECM).

The voltage across the sensor is a function of the engine temperature and, therefore, of sensor resistance. Voltage can vary between 0 V and 5 V .

The engine control module (ECM) uses substitute values if the signal from the engine coolant temperature (ECT) sensor is missing or faulty, however, substitute values can cause starting problems in very cold weather.

The engine coolant temperature (ECT) sensor is mounted in the thermostat housing.

#87 Coolant Leak at Upper Radiator Hose

January 18, 2014

Car was losing coolant at a rate where the coolant level warning light would come on once a month or two.  Not a huge leak but concerned me enough to look into it before the situation got worse and maybe stranded the driver with a blown hose or overheated engine.

The leak was not enough to leave telltale drips on the ground so it wasn’t obvious what was leaking.  I also didn’t want to run the engine to get hot fluid circulating if I was looking for leaks.  The usual way to find a problem like this is to pressurize the cooling system with a special tester.  I researched this and found a decent one at Harbor Freight Tools for maybe $55 with a coupon.  Then I thought about rigging up my own tester to save money and thought I could get a new coolant reservoir cap and drill a hole in the old one and add an air pressure port to it and inflate with a bicycle tire pump or something.  That might work but then I noticed that the brake fluid reservoir on the 850 appeared to have a cap of the same size and thread as the coolant reservoir.  Those Swedish engineers are so clever!  It is the same fitting and so I could use my brake fluid pump to pressurize the cooling system.  Here it is connected to the coolant reservoir instead of the brake fluid reservoir where it is intended:

Motive Power brake bleeder pressure pump hooks up perfectly to the coolant reservoir.

Motive Products power brake bleeder hooks up perfectly to the coolant reservoir.

I checked the pressure relief rating of the coolant cap and it shows 150kPa which is about 22psi.

Coolant reservoir pressure cap relieves at 150KPA (20psi).

Coolant reservoir pressure cap relieves at 150kPa (22psi).

So I pumped the cooling system up to 15psi and looked for leaks.  Since there were leaks the pressure slowly dropped at a rate of about 5psi in 10 minutes (half psi per minute).

Pressurized cooling system to 15psi to make leaks more evident.

Pressurized cooling system to 15psi to make leaks more evident.

Checked around all hoses and fittings, the reservoir and radiator looking for signs of coolant.  Any cracks or weak points will leak fluid when pressurized.  I found a fresh wet spot below the upper radiator hose connection:

Small leak at upper radiator hose connection.

Small leak at upper radiator hose connection.

Hose seemed to be otherwise in decent condition so started by tightening the hose clamp.  I tightened it about two turns and then cleaned up the fresh coolant to see if that made a difference.

Sure enough, no more fresh coolant leaking and the pressure loss rate dropped significantly to maybe 1psi in 20 minutes.  That indicates there is still a tiny leak but this bigger one was taken care of.  I expect the hoses will need to be replaced in a year or two but for now this should cut down quite a bit on the coolant loss.

Finding no other leaks under the hood I went looking for trouble under the passenger side of the dashboard where the heater core is located.  Sure enough, after removing the lower cover I found a wet rusty spot and then more wet spots when I pulled the carpet back and removed the console side cover.  This is consistent with the odor of antifreeze (ethylene glycol) when the heater is turned on (fortunately not often here in central Texas).

So that will be a separate topic because it’s a big project to replace the heater core.  I’ll decide if and when on this later.

$0  No cost repair! (at least for now, pending hose replacement and heater core).

#86 Air Guide Replacement

September 30, 2013

There is a large plastic cover under the engine called an air guide.  Its purpose is to protect the engine and also to help direct air flow for optimal engine cooling.  This air guide is also known as the belly pan, splash pan, skid plate or lower engine cover.

Since many Volvos are missing this part, some people don’t think they need one, but Volvo put it there for a reason.  Not only does it help guide air through the radiator, it also keeps water, dirt, gravel, rocks, wood, road grime, and debris off critical engine components like the alternator and belts.

Sometimes the air guide is removed for service and not replaced.  Also because the 850 is so low to the ground in front, the air guide frequently scrapes against parking curbs and such.  Eventually this will shear the plastic hangers and the air guide falls off.  That’s what happened here.  If you’re fortunate you will notice this and rescue the part.  If the part is left behind you’ll need to get a new air guide, louver and clips.

Black tabs on left and right sides are all that secure the air guide to the car.  When the guide shears off you have just the mounting tabs bolted to the chassis.

Black tabs on left and right sides are all that secure the air guide to the car. When the guide shears off you have just the mounting tabs bolted to the chassis.

Broken tab on other side.

Broken tab on other side.

On this car we were able to recover the broken air guide so it was a simple matter of transferring the louver to the new air guide.

Old louver transferred to new air guide.

Old louver transferred to new air guide.

Old louver transferred to new air guide re-using mounting clips.

Old louver transferred to new air guide re-using side mounting clips.

New air guide with old louver ready to install on car.

New air guide with old louver ready to install on car.

While the air guide is secured to the car with bolts at the sides, there are features that help hold it in place loosely.  Without these installing the air guide is difficult.

A line of clips at the front holds the guide on top of the front bumper ledge:


A slot at the bottom center of the radiator accepts the rear tab of the air guide:

Rear tab supported by slot in radiator.

Rear tab supported by slot in radiator.

Note that this radiator slot is frequently crushed by jacking or high curbs so may need some coaxing back into shape with pliers.

Supported in front on the bumper and back on the radiator, the air guide will stay in place where it can easily be secured with bolts at right and left:



When done it looks like this:

New air guide installed.

New air guide installed.

The round hole nearest this view is for radiator draining.

The Volvo diagrams show an additional smaller air guide piece that fits in or around the open rectangle.  I have this part but haven’t figured out exactly how or where it goes because nothing makes sense with it.  This main part should be all you need to protect your engine and improve air flow.

$17.98 Factory Replacement Air Guide (note shipping for this large item may be expensive, although with Volvo Parts Webstore it was not)

#68 Cooling Fan Relay

October 22, 2012

One hot day I noticed that the air conditioning wasn’t very cool while stopped at an intersection and I heard the AC compressor clutch cycling every 4 to 5 seconds.  When the car started moving again the air got cooler so I started thinking what might cause this condition.

The AC compressor will cycle with the temp control thermostat but that wasn’t switching off because it was still hot in the car.  The other things that control the compressor clutch are the high and low refrigerant pressure switches.  I thought about the refrigerant cycle and how the condenser (thinner radiator in front of the engine cooling radiator) exchanges the heat from refrigerant expansion and allows the hot gas to condense back into liquid.  If there is no air flow through the condenser (such as when the car is not moving), this heat exchange will be minimal so the refrigerant stays hot and gaseous at higher pressures.  That will cause the high pressure switch to open and shut off the clutch.

To prevent this condition, the cooling fan behind the radiator switches on with the air conditioning to keep some air flow through the condenser when idling/stopped.  That should keep the refrigerant cycle flowing properly without activating the pressure switches.

The cooling fan has two speeds, one for AC and one for engine cooling.  Engine cooling rarely requires supplemental fan cooling, so the fan doesn’t often come on when the AC is switched off.

The cooling fan is switched on via the cooling fan relay module.  This cooling fan relay is located just under the top shroud above the fan itself and is activated by control signals from the engine control module (ECM).  If the ECM wants to switch on engine cooling, it sends that control signal for high speed; if the AC is switched on, the ECM sends the control signal for the low speed relay.  Module circuitry prevents both from being on at the same time; the cooling speed relay has to be off to enable the AC speed relay.

Wiring diagram shows both relays in the cooling fan relay module.

I observed that the cooling fan was not running when the AC was switched on.  Tried wiggling wires and tapping on the relay module and discovered that the fan would start running if I banged on the relay.  Thinking there was a problem with the relay contacts, I removed the module and opened it up to clean the relay contacts.  Sealed it back up with silicone caulk because I damaged the edge of the cover.  Also sprayed contact cleaner on the connector pins and sockets before plugging them back in.

The fan still would not work until the relay was tapped hard.  I tried wiggling individual wires to see if one of them had a problem with no results.  Determined that something was wrong inside the relay, either a weak coil or poor contacts.

Replaced the cooling fan relay and now it switches on when the AC is working.


#65 Timing Belt

October 20, 2012

This is a big and important topic so bear with me on this long entry…

Replacing the timing belt and related parts is an important preventive maintenance (PM) task every 70K miles.  Volvo 850s have interference engines where valves and pistons share the same space.  If the belt breaks and timing is lost, valves and pistons may collide and be damaged.  On this old car such damage is practically catastrophic; it would be too expensive to justify a repair unless you have lots of time and experience and special tools to rebuild the engine internals, so this is a very important task.

This car is overdue for its second belt change at 150K.  Assuming original rollers, I decided to replace the belt, rollers and tensioner but skip the water pump.  There are no leaks and the water pump spins freely, smoothly and noiselessly and is likely to start leaking long before it binds up.  There are strong opinions on both sides of the question of replace or not on water pumps with the timing belt; after checking pump condition I decided against replacing it.

Bought a complete timing belt kit from FCP with factory parts (supposedly).  Glad I replaced the rollers along with the belt since they both had issues.

I won’t give a step-by-step procedure here, just some highlights.  There are good detailed procedures on other forums.  I like Ozark Lee’s writeup which is located at

Moving the coolant reservoir out of the way really improves access.

Access to the idler roller is easier if the serpentine belt tensioner is removed.

30mm socket on long ratchet used to rotate (CW only!) crankshaft and camshafts to line up timing marks. Removing spark plugs helps reduce force required but can be done with them in. Interior flap clamped out of way.

Highlighted camshaft sprocket timing marks so they are more easily seen to line up to reference notches in rear cover.

Crankshaft timing marks that many people have difficulty locating. Upper one is fixed on the engine block. Lower one on the pulley rotates with the crankshaft. When these are lined up the cam sprockets should align with their marks.  There may be a notch in the crankshaft sprocket valley at the same location instead of the raised spot on the pulley.

Old belt ready to remove.

This blue locking fixture from IPD is simple insurance against either camshaft moving while working in the area and routing new belt.  Need to get at least three pins engaged to really lock.

This is the pinch point under the crank pulley where it’s difficult to pull the belt through. It can be wrestled out with a lot of time and fussing but since the belt is getting replaced…

…it is easier just to cut the belt and pull the ends out.

Old and replacement idler rollers. The old wheel spun roughly with difficulty and seems to be on its way to freezing up. Replacement was essential.

Removing the tensioner roller is difficult because it’s hard to get a T45 bit in between the bolt and the wheel well. A standard Torx bit drive with a thin, long 3/8″ ratchet will fit and wedging a tool in the gap keeps it secure in the bolt socket.

This tool combination works to get the tensioner roller unfastened. The long ratchet gives lots of leverage and the thin profile fits in the narrow clearance.

This unibody drive on left allows a few mm more clearance than the traditional Torx 3/8 socket drive.

Old and new tensioner rollers. Old wheel spun noisily which indicates that the bearings are failing. Needed replacement.

Old and replacement hydraulic cylinders. Nothing obviously wrong with the old part but it’s advisable to replace it to ensure critical belt tension.

Water pump shows no signs of leaks and spins freely, smoothly and without noise, so I left it alone. If it was leaking, moved roughly or made noise, I would have replaced it.

New timing belt roughly positioned, ready for tensioner and final belt placement.

For me final belt positioning is the hardest part of this task.  I have done it four or five times now and I still don’t have a foolproof method.  Things seem different every time so it’s trial and error for me.  I have read many different guides and none quite seem to work precisely.

Mark some sort of label with mileage so that future owners and service shops know when the timing belt was changed.

All the parts changed on this job along with their replacements.

My brief walk-through of the process:

  1. Set parking brake and block rear wheels.
  2. Break loose the RH front wheel lug nuts.
  3. Raise front of car and secure on jack stands.
  4. Remove serpentine belt and its tensioner.
  5. Remove spark plug cover and fuel line clamp on timing belt rear cover.
  6. Remove timing belt front cover.
  7. Unplug sensor connector on coolant reservoir and move on top of engine.
  8. Remove front right wheel.
  9. Move splash guard flap in wheel well and clamp in open position to access crankshaft pulley.
  10. Remove guard under crankshaft pulley.
  11. Crank engine around in CW direction using 30mm socket until the camshaft sprocket marks line up with their notches in the rear cover.
  12. Verify timing mark on crankshaft pulley lines up with the mark on the engine block.  (Don’t trust existing belt position; it could be off one or two notches.)
  13. Install camshaft sprocket locking tool or secure the sprockets with cable ties, if desired for extra security.
  14. Remove timing belt rear cover.
  15. Remove timing belt tensioner, starting with the upper bolt.
  16. Remove old timing belt.  Fight it off at bottom of crankshaft pulley (hard and time-consuming) or just cut it (quick and easy).
  17. Remove idler roller.
  18. Remove tensioner roller.  This is a tough one without a short T45 tool and long ratchet.
  19. Install new tensioner roller.  No chance to tighten to spec since it is impossible to get a torque wrench in there.  Specified 18 ft-lb is not terribly tight, so an easy torque by hand.
  20. Install new idler roller.
  21. Install new timing belt starting at that pinch point under the crankshaft pulley.  It’s a bit of fussing and twisting but it’s much easier to install the belt than to remove one.  Rough in place loosely in proper routing path.
  22. Install new tensioner cylinder but do not pull locking pin yet.
  23. Make final routing of timing belt around tensioner roller.  Ensure it is tight (no slack), starting at lower right from crank pulley, maintaining tension as you go.
  24. Reinstall rear timing cover and secure in place.
  25. Verify timing marks one last time on crankshaft pulley and camshaft sprockets.
  26. Remove any camshaft locking devices.
  27. Rotate engine twice at crankshaft until camshaft sprockets complete one revolution back to their timing marks.  Verify that the belt did not slip any teeth (marks at camshaft and crankshaft line up at same time).  This is where it may slip a tooth if something was not quite right.
  28. Once you are confident of the timing and installation, pull the locking pin to release the tensioner cylinder pin.  It will now push the tensioner roller firmly against the belt.
  29. Rotate the engine through one more camshaft sprocket revolution and check timing marks one last time at both locations.
  30. Install white plastic spacer between tensioner end and tensioner roller pad.  Re-use old spacer if new one is not provided with new hydraulic cylinder.
  31. Start engine and observe that it runs well and the timing belt moves smoothly.
  32. Stop engine and button things up (replace guard, covers, flap, etc.)
  33. Install wheel and lower car.  Tighten lug nuts to proper torque (81 ft-lb).  Remove wheel chocks.
  34. Reinstall serpentine belt tensioner and serpentine belt.  This is the perfect time to replace the belt if needed or based on age.
  35. Move coolant reservoir back into place with sensor connector plugged back in.
  36. At this point everything should be back in place and secure.
  37. Start engine and verify smooth movement of serpentine belt.
  38. Record mileage on a label to indicate when the timing belt was changed.
  39. Drive the car a short distance through varying RPMs to make sure there are no surprises before you really need the car.
  40. Drink a beer or two to celebrate a job well done and hundreds of dollars saved.

This procedure took about four hours of deliberate, careful work; this is not a job to be rushed because if you miss something important it could be a big problem or cause you to do hours of re-work.  Newbies will take longer; this was my fourth time so I had some experience and all the right tools.  I was also taking photos along the way.

$214.95 Timing Belt Kit

Technical Notes

The timing belt in this vehicle (and most other modern cars) is a flexible toothed belt which links the engine crankshaft (where the pistons are driven up and down in each cylinder) to the overhead camshafts (which open and close the intake and exhaust valves for each cylinder).  Its purpose is to synchronize the movement of the intake and exhaust valves with the piston cycle in each cylinder so that fuel and air are introduced at the right time for combustion and then waste gases are pushed out at the proper time.  The belt is outside the engine on the RH side under a cover.  If the cover is removed you would see the belt as shown in the diagram below:

The lower gear is on the crankshaft; the upper right sprocket (front of car) is the intake camshaft; the upper left sprocket (rear of car) is the exhaust camshaft.  An idler roller guides the belt on the right and a tensioner roller is above and just to the left of the crankshaft sprocket.  A hydraulic cylinder pushes against the tensioner roller to keep the belt tight.  The large object to the left of the tensioner is the water pump for engine cooling.

This timing belt should be changed every 70K miles or so because if it breaks, the pistons will collide with open valves and wreck the valves and/or the pistons.  The rollers and tensioner should also be replaced at least every other timing belt change because they tend to wear out and can seize up.  Even if the belt is fine, a frozen roller can shred a good belt pretty quickly.  A weak, leaky tensioner can allow the belt to slip teeth on one or more sprockets, which causes the engine to run badly or not at all.  Many people recommend changing the water pump when the rollers and tensioner are replaced because they could also seize up and wreck the belt.

#59 Coolant Flush and Fill

July 3, 2012

The coolant on this vehicle was a suspicious rusty brown color and of unknown age and composition so I drained it, flushed with a chemical cleaner and refilled with known high-quality coolant.  This has to be done with a cool engine, of course, to avoid getting burned or sprayed with hot fluid.

Brown color could be from mixing different brands of antifreeze or could be caused by corrosion inside the cooling system.  Volvo 850s have an aluminum engine block and aluminum radiator so there is not much iron in the system to rust.  At any rate, the color and some mineralization evident from the thermostat replacement plus the uncertain age and quality of the coolant demand a good flush and fill.

The 850 radiator has no pressure cap to directly access the radiator; everything is done through the plastic coolant expansion tank:

Start by raising the front of the car and supporting securely on jack stands.  Remove the splash guard under the radiator:

Place a catch pan under the radiator drain cock on the driver side (LH) and open the drain port:

Removing the coolant reservoir cap allows air to get in to drain smoothly.  This car had an after-market radiator installed at some point, evident by the drain plug being different in size and type from a real Volvo part.  It also looks newer than the rest of the car.  I measured around 5 quarts from the radiator.

After draining the radiator, replace the drain plug and crack open the engine block drain plug accessed from underneath behind the engine towards the passenger (RH) side:

Coolant from the engine drain splashes all over the place so you need a large pan to capture it all.  Better yet, stick a hose over the port to direct the coolant into a catch pan, which I didn’t do out of laziness.  Didn’t get much coolant out of the engine block but it was good to drain out what was there.

Did this job along with the thermostat replacement (#58).  At this point the thermostat was removed and the housing re-attached so flushing could occur with no thermostat in the way.

After tightening the engine drain plug, I added some Prestone super flush chemical to the reservoir, then filled with distilled water.

As mentioned in the previous thermostat topic, it is important to use only high-quality low-mineral content water in the cooling system.  Mineral buildup causes chemical reactions and gums up the innards.

Closed up the reservoir and ran the engine for 10-15 minutes to get it all circulating through the radiator, engine block and heater core (put cabin temp controls on heat).  That should effectively break up any gunk and rust and minerals in the cooling loop.

Waited for the car to cool down, then drained as before.  After that, I refilled with distilled water only and ran the engine again to get one more good flush with no chemicals.   Drained once more after cool-down.  This is where I installed the new thermostat.

Now the car is ready for fresh coolant.  Something so simple can be so controversial.  Lots of discussion about best or proper coolant for Volvo 850s.  Many people insist only real Volvo coolant should be used, or at least a particular alternative.  Others say just use whatever.  Strong opinions all around and much of it contradictory.  I weighed opinions from reputable forum comments and decided to go with extended life coolant from a major supplier, Prestone for this job.

Mixed half coolant and half distilled water and filled the reservoir for the proper 50/50 ratio.  Ran the engine until it got warm and then topped off the reservoir to keep it level.  Also added more after it cooled off completely.

Of course after draining I re-attached the splash guard.  Also disposed of used coolant properly, not into the sewer.

Cooling system should now be good for a few years at least, with a new thermostat and fresh, high-quality coolant after flushing.

$4 Flush, $13 Coolant, $4 Distilled Water

Technical Notes

Coolant in the car is normally a mixture of coolant solution and water.  Water is the primary ingredient and in some warm climates is all that some people run in their cars.  Coolant keeps the water from freezing in cold climates, which is why many people call it anti-freeze.  By an interesting property of chemistry it also raises the boiling point of the water (so does the pressurized cooling system), so coolant helps in both cold and hot weather.  To call it just anti-freeze is not accurate; it is also coolant and the product you buy says both on the bottle.

Coolant also lubricates the water pump so if the fluid goes bad it can cause problems for the pump.

Extended life coolant adds corrosion-resistant chemicals which allow it to last up to five years, although many people change it more often than that.

#58 Thermostat

July 2, 2012

Most every car has a thermostat and this model is no exception.  The thermostat is a type of valve in line with the engine cooling system which closes to block cooling flow until the engine is warm, then opens when the engine needs cooling.

Thermostats can fail where they are always open.  This condition causes the engine to take a long time to warm up to operating temperatures and provide passenger heating, a problem mainly in cold climates.  The worst scenario is when the thermostat fails closed.  This chokes recirculating coolant flow between the engine and radiator and causes a running engine to quickly overheat.  This is a bad news shut-you-down type of problem that can leave the car on the side of the road.  Don’t want this to happen again (it happened once to us on a family minivan) so I replaced the thermostat in this project car.

After draining the coolant from the radiator, the thermostat is easily accessed on top of the engine.  Two large Torx screws hold it in place but these are usually buggers to break loose if they have been on awhile.  You also need a long T40 drive bit to get around the fuel line.  Many people disconnect and move the fuel tube out of the way but I was able to work around it with the long bit.  Also sprayed penetrating oil (PB Blaster) on the screws the night before and then again before trying to break these screws free.

Old thermostat was not in too bad of shape but it did have some mineral buildup and the gasket was starting to crack.

By the date code (Oct. 2005) it was six or seven years old. Not original to the car but getting long in the tooth.  Also scraped buildup away and cleaned the housing surfaces before installing a new thermostat.

The minerals on the thermostat and around the housing suggest hard water was added to the coolant in the past.  Only distilled or DI water should be used to minimize mineral buildup which can clog internal passages or gum up the thermostat or water pump.

Used a genuine Volvo thermostat to replace the old one.  Should give years of reliable service.



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