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.
$38.89
1997 Volvo 850 Wagon Project 