My radiator explored into pieces on the way to climb Mt Washington because of the temperature sensor at the top right of the rear radiator failed. After changing the temperature sensor, the car has no issue to climb to the top of Mt Greylock. I noticed today the engine cooling fan is not turning when the AC is on.

My question is:

1. Should engine cooling fan turn on automatically when AC is on?
2. What can be the possible cause for the cooling fan stay off when AC is on?
3. Is there a switch to control engine cooling fan?
4. How to diagnose the bad switch

My volvo 940 Specs:
Engine Model B230F Engine Power (kW) 86.5012 Engine
Configuration In-Line Engine Brake (hp) From 116
NCSA Make Volvo NCSA Model 940 Trailer
Engine Info w/o Bendix EGR

Thanks,

 

I believe that switch controls the electric fan, there is also a relay. The fan should also turn on when the AC is on OR when a high temperature is reached.
If you changed the switch, look at the relay. It's mounted in front of the battery and it says Cooling Fan Relay on it.

 

Thanks for the good tips, The FCP asks $78 for the Volvo A/C Cooling Fan Relay - Genuine Volvo 9442933, I will check the relay perhaps service the relay before making purchase on a new one.

Your suggestion leads to the other question:
Is the cooling fan relay acted as dual switch receive single from both temperature sensor and AC power? I asked this question is because when the engine temperature is high the engine cooling fan did turns on but when the AC is on when engine is cold (just start the engine) the fan does not turn on. I assumed the relay only work on high temperature side but it does not work on the AC side which leads to my assumption that the relay is a dual switch.

I hope I am clear and not confusing you. Thanks

 

Makes sense, no argument here. You can try and inspect the relay for cracks on the solder joints and refloat them if need be.

 

1. When ac pressure is high enough
2. Bad switch, bad connection, wiring problem, bad relay, fuse?
3. For the AC the switch is mounted on the condenser, for the engine it's in the head or the top of the radiator
4. Test light, jumper wires - the low and high pressure switches should turn the compressor off when unplugged, fan switch should turn the fan on when jumped

The electric fan as mentioned is turned on by the relay part number you found - It looks like the standard two speed relay used on many Volvos. The AC switch to tell the fan (through the relay) to come on is mounted on the bottom of the condensor. If yours looks like pictured with three switches (33,34,35)- one is low pressure compressor shut off, one is high pressure compressor shut off and the other is to turn on the fan when the pressure is high enough. Sorry can't tell you which one is the fan switch, I don't have a wiring diagram for that year. Looking at the relay - 30 is B+, 1 and 2 are the two fan speeds. The two round connectors are for the signal wires to tell the relay to turn on - I'm certain you have found many of those round terminals with melted/destroyed rubber insulation on your car so far - there is a website "prancingmoose.com" where I have seen replacement pigails to fix those melted round connectors.

 

Thank you lev and hoonk you rock!! I checked the cooling fan relay (as shown below) by connecting 12V (+) to terminal 30 and (-) to terminal In 1 then I have continuity from terminal 30 to terminal out 1. When I switched (-) to In 2 I got continuity from 30 to out 2 which means the cooling fan relay is working. save $78 for the new relay.

The next test I plan to do is to find which pressostat is to turn on the fan. Since my AC is working properly and the compressor is cycling on and off, I am going to unplug one pressostat at a time and leave two others plug in. My theory is that in order for compressor to work both high and low pressure sensors should be closed for electricity to run to the compressor.

1. If I unplug the high pressure sensor, the compressor will not run when AC is on
2. If I unplug the low pressure sensor, the compressor will not run when AC is on
3. If I unplug the fan switch, the compressor will run as usual when AC is on
4. If I jump the fan switch, hopefully the cooling fan will turn on ( when pressure is high the switch will closed (negative trigger) for cooling fan to turn)

 

hoonk, Great advise and chart. I am able to follow your advise to work on my 30 years old Volvo

Today I found the gray pressostat #6848584 is the pressure switch to turn engine cooling fan on when the condenser pressure is high. I jumped the socket (image below) and the fan turned. The blue/black wire from the gray pressostat connected to cooling fan relay "In 1" which is low speed fan (see image below). I am glad I figure it out through you guidance. I hope other people who can also benefit from the post. Thank you!!

gray pressostat #6848584 is the pressure switch to turn engine cooling fan:


Blue/Black wire to relay "in 1" low speed:


The Black/White wire from radiator temperature sensor connect to "In 2" relay round prong to turn engine cooling fan in high speed on when temperature is high: