Long time lurker, first time poster. I'm at a loss of what to do about the A/C and need a sanity check before I throw parts at it:
Car: 1994 850 non-turbo with manual a/c controls
Problem: A/C does not work. Blower motor works fine.
Symptoms: When fan is off, blue light for A/C is on at switch. However, when fan is turned on, a/c light goes out.

Troubleshooting:
-All sensors (High side, low side) all have voltage. Shorting the LP sensor by the firewall doesn't do anything.
-Verified relays behind glove box have power. Even tested with another relay I had laying around and jumper wires.
-Charged system and holds pressure, even after a week - see below.
-Power is going to the compressor.
-Gaps on the compressor clutch are within spec.

Now, here's the strange bit: compressor just doesn't kick on by itself when all hooked up properly. However, if I power the compressor directly from the battery, I can hear the electromagnet engage when the car is off and the clutch engages when it's running, then it works and cold air blows. I did this when I charged the system to ensure R134a was circulating.

I'm thinking it's one of two things:
-Control board in the dash
-ECU

If it's the control board, how can I verify?

Otherwise, I'm out of ideas.

 

OK - you say "Power is going to the compressor." and it does not come on until you directly wire power to the compressor - it works then , you were able to charge the system and cold freon is created. Correct?

Power to the compressor goes from the switch on the panel, to the AC delay relay 2/22 (supplies power to the compressor a few seconds after the car is started, not immediately) then thru the high pressure switch (turns off power if too high) then the low pressure switch (obvious) then to the compressor. If it works directly wired I would suspect a bad connection in the delay relay - possibly a cracked solder joint that limits current flow. Your test light will light up - but the cracked connection won't send enough current to power the clutch.

OR - "When fan is off, blue light for A/C is on at switch. However, when fan is turned on, a/c light goes out." This could mean there is a bad ground, causing the blue light to come on at the wrong time. (and grounding through the compressor circuit instead of 31/15.) The ground connection for the control unit and the fan is at 31/15. 31/15 is near the passengers right side panel. Check for a melted/poor connection at 31/15 next to the passengers right foot (behind the panel) But I would think if there were a bad ground the fan would not work properly also. That leaves a poor connection in the control unit. That's a pretty old car and I've worked on many for many years - but have not come across a bad control unit in a 850 with manual climate system. After this many years - anything is possible.

 

Correct. The one wire that leads to the compressor has 12v to it.

Bad ground is something I'm thinking, too. The relays are fine behind the dash. I have not checked the sensor grounds, yet. I also haven't taken the control unit out to test for ground. It is a 30-year-old car, and I did find one ground corroded off the block earlier this year and reconnected it.

Thanks for the wiring diagram. I'll go chasing some more gremlins tomorrow.

 

I looked at the relay housing and all the wiring looks fine. I can look deeper during daylight tomorrow. I think I’ll pull the control board anyway and double check all the connections.

I'm struggling to find the 31/15 ground. I pulled the carpeting back and found wiring but can’t see where there’s a ground stud.

 

Found the ground. No scorch marks. Looked at the wiring harness for the control board, no obvious signs there's anything amiss, either. I will mention if I try to jump the a/c relay from 30 to 87 (12v to 12v), it kicked on once, then wouldn't anymore after that. Is there a way to test the sensors to see if they are bad? Also, how much current is supposed to be coming out of the delay relay?

 

You can test low and high sensors with a test light, ohmmeter, jumper wire, paperclip. Jump past the low side sw with a paper clip, jump past the relay with a jumper wire.

The compressor, low side sw,high side sw, relay, are wired in series. Start at the control unit and follow power from one component to the next with a test light.
How much current? Compressor probably needs at least 6 amps - not sure if that's your question. ( sorry I don't imagine you having a way to measure that)

Are you sure the compressor comes on and stays on if you wire it directly to power at the compressor? (make sure the radiator fans are running - they might not if no power is going thru the high side switch, a signal goes to the ign and fuel system after that, probably to turn on the fan.)

 

I don't have a test light, I'm using a multi-meter. Yes, this is what I'm looking for - if not enough current, then it's not enough to engage the clutch on the compressor, especially if all the components running in a series.

Could I bypass the delay relay and in-dash switch with a switch of my own directly from the battery, or could something fry down the line? (obviously with an in-line fuse when coming from the battery) If the delay relay is only to not engage the clutch immediately after starting the car, I think this would be OK.

Yup, compressor stays on and I hear the fans kick on after a few seconds, and this answers the high-side pressure switch question if it's working or not.

 

Found the culprit. Bad switch. I bypassed it with another one and everything works as expected. $100 for a replacement! I think my temporary solution will have to stay for a bit.

 

Good work tracing down your issue and getting a good resolution.
I have an AC question that I hope gets replies.

I am trying to resurrect the AC in my 1993 240 B230. A few years ago it was making awful screeching when the motor was started and also when AC was switched on. Belts were adjusted at AC and AC to power steering pulleys and then with pulleys turning there was no more noise. But when AC was switched on there was noise. That told me I fixed one issue, loose belt(s). But the noise on activation of AC told me it was also likely a problem with the AC pulley. Since we did not need the AC I left the condition; just did not switch on the AC.

Now that I have removed the belts to replace the Water Pump and Timing Belt I have been able to hand spin the AC pulley to check it for roughness. Nothing. It spins freely and long on the bearing. The clutch also seems to spin freely in the disengaged position.

Before I pulled the front of the engine apart I did not try the AC with engine running. Frankly, I expected to just replace the compressor. with new pulley and clutch.

Of course, now that I have been reviewing more Forum and YouTube how-to's I have come across bits about testing the clutch and stator as opposed to replacing the compressor.
Both clutch and pulley seem tight on their shafts with no wobble, no looseness, no thrust play, no grinding, roughness, no rumble or stiffness, nothing to indicate bearing wear, just smooth free rotation with a hand-spin.

I have determined that this compressor has one wire to the stator for the clutch and one wire, fused, for power. According to what I have learned, that means the case is the ground. There does not appear to be a bare connection on the compressor for the stator wire to connect a 12v jumper to the battery. The ground is no problem, since a jumper from the negative battery terminal will produce the complete circuit when the bare end is touched to the compressor case.

My question is how do I test the clutch activation using a jumper wire from the battery to the stator wire of the compressor?

Thanks, as always, to the Forum members for your experience and help.

 

Should I separate the power wire at the fuse and connect my 12v battery jumper to the free end of the fuse wire?