First of all, this is a 1993 940 GL. My AC will not turn on, so I charge the AC and cannot get it to turn on at all. I took the wire that goes to the AC pump clutch and gave it 12 volts from the battery to engage the clutch to see if the pump is bad and it works, and puts out cold air. Unless there is some other issue with the system, it should turn on and work. There must be a bad sensor, relay, or wire somewhere that is not letting the AC clutch engage. Any thought? Thanks for your help.

 

i believe the clutch power goes through a couple sensors ... I have the schematic here for a 92 940, assuming you have manual climate its probably really similar or identical...

from the back of the climate controller in the dashboard, pin10 is +12 for AC on and goes to a green/red wire that goes to a low pressure switch on the dryer-extractor beercan, then another green-red (or maybe its green) goes to a high pressure sensor on the condensor behind the radiator, then it goes to yet another green or greenred that connects to the black wire that goes to the compressor clutch solenoid.

so... I'd switch the AC on (with the car running) and check for +12V on the low pressure switch thats on the 'beer can' extractor/dryer thing. with AC switched off, that +12V should be gone. then I'd check that the +12V is on the other side of that switch and on the high pressure switch on the condensor manifold and it should be on both sides of that too, and then it should be getting to the compressor. my drawing is a little confusing, it shows multiple wire paths some with connectors, some without.

 

Thanks, I will have to take a look and that when I can. Now, my house AC is now having issues with the main blower fan, so it takes more priority than the car with this heat wave coming it.

 

FWIW, I had a similar problem with my 740. Cold air when I jumpered the compressor, no voltage at the low pressure switch on the accumulator. Spent a lot of time looking for a relay that is apparently part of the AC control panel. Also suspected a bad on/off switch. Got a used control panel from a wreck at the junkyard, problem solved.

 

I will keep that in mind. I have two 1993 940s (one wagon and one sedan) so I can swap them for the test. The AC in the wagon works, but the sedan is the one having this issue.

 

I'm not 100% positive, but I am pretty sure those high pressure, low pressure AC switches will require evacuating and recharging the AC system to swap.

 

I have this same problem with my 92 740.I could not find a relay anywhere.I replaced the control unit with a used one that I was told was working.What would I need to check next in order?I jumped 12volts to the compressor and it blew cold air.When I checked the coolant pressure it was where it needed to be. Thanks for the help.

 

Nope. Schrader valves.

 

You have no voltage at the compressor so you need to see if you have voltage at the low pressue switch on the accumulator. With the AC switch and fan in the on position you should have voltage in at least one of the two wires on the low pressure. If yes, then the switch isn't closing as it should. It has a schrader valve, so you can just unscrew it and try another. If no voltage on either wire, the problem is probably with the control panel (most likely, see above posts) or the wire between the switch and the panel which will probably be a PITA to trace.

 

first start the car, and switch the AC on (button out), with the fan on '1'... then get a volt meter and read between a engine or chassis ground and either side of the low pressure switch thats on the accumulator/dryer...

if you have voltage on one of the low pressure switch wires but not the other, then the switch is open, which either means the pressure is too low or the switch is defective. If you jump across this switch the AC should come on if thats the only problem.. if you don't have voltage on either side, then you'll need to look at the control unit. if there's voltage on both sides of the low pressure switch, then you need to find the HIGH pressure switch, whihc is one of the three switch things on a manifold on condensor at the radiator... again, if there's voltage on one side but not the other, than that switch is open and jumping across that switch should start the compressor. if there's no voltage at the high pressure switch and there was voltage at the low pressure switch, then the wire in between is bad. if there's voltage at both sides of the high pressure switch then the wire between it and the compressor is bad.

 

Thanks, now I know where to start.

 

I found a relay behind the battery that has cooling relay on it.I have not seen this location described before so I thought I would let people know.There are three plugs that connect to this relay.I just unpluged all the wires and plugged them back in and the compressor started working. Thanks for the help.

 

Could you take a picture of this location? My AC won't turn on either, and I would like to find the location of this relay as well.

I'll test for power to the compressor this weekend and update you all.

 

Chris850T: what year/model car? AFAIK, 240/740/940 have no relay in the compressor clutch circuit. they may have relays for the radiator fan, however.

 

I have a 96 850 GLT

 

k, this is the 240/740/940 section of this forum. the a/c on the 850's is quite a lot more complex, with a variable rate compressor and other new features, you probably should post in the 850/s70/v70 section.

 

New guy here and I just want to say thanks.

I found a 150 page wiring schematic online but it was terrible to follow. The glossary of abbreviations (like A4-32) was hundreds of pages from the images. AAAUGH!

A google search found me here. I'll point out, that on my 91 740, pin 10's output from the climate control head is a GREEN wire. It turns green/red later under the hood. Just thought I'd clarify that for future readers. It is also a slightly fatter wire than most in there. If my implication is not complete, I had OPs issue of working AC with the clutch jumpered. (Was nice of them to put a junction on a clip between two red/green wires on the engine block behind the alternator for testing purposes.)

My power antenna snapped off before I picked up this heap, so I repurposed its switch to AC clutch control. I like having HVAC on the driver's side of the dash, this is a start to that goal. BTW, I'm confident the switch can handle the 6 amps or so the clutch should take.

 

K, thanks. I did a search and this was the first relevant thread I found. Didn't think to look which section it was in. Thanks again...

 

So, I finally got back to check out the AC. I have 12v at the beer can when AC switch is on, 0v with switch off. Beer can sensor showing it is closed with ohm tester. I then found the condenser sensor (far left of the three) that has 12 v when AC switch is on, 0v with switch off. That condenser sensor is also showing closed with ohm tester. So, that means those sensors are all good and should be passing the 12v to the compressor if I understand correctly. If there a sensor on the evaporator I need to check? There is also a green wire with red stripe exposed behind the beer can which has a connector similar to that right next to the AC pump, and I also have 12v there with AC switch on, 0v when off.

I did find the relay behind the battery and reseated it with no change. Not sure what it was used for. I did notice with my gauge on the AC low side and the power to the pump bypassed, that when the pressure got to a certain point, the radiator fan would kick on, then pressure would drop to a point and the fan would kick off. I saw it do that several times to the same limits, so I know those sensors are working.

I could swap out the relay behind the battery from my other 940 with working AC and see if that works. Does anyone know what that relay does?

Thanks again for the help, sorry for the delayed response, too many other things going on.

 

if you've got +juice at both contacts of the high pressure side sensor on the condensor, then you SHOULD have power on the alternator unless the wire between those two is broken.

I show no relay in this circuit, at least on a 92 740/940 with manual climate control. if you have the optional electronic climate control (ECC, has a temperature dial marked in degrees instead of a cold/hot ****), then there is a relay, but its before the high and low pressure switches so again if there's power on the low pressure switch.... .....

errr, wait, do you have a 16V/DOHC B234F ?

THAT has a cutout relay on the AC which is designed to disable the compressor on full power. this relay is over behind the right headlight. it gets signals from the low pressure AC switch, from the fuel injection ECU, and from the full throttle switch. if you jumper pins 1 and 2 of this relay, it will be as tho its not there. its not a simple relay, it has some logic in it.