850 E31 A/C issues Please Help

I just purchased a 1991 850i with 50,000 original miles. The car is in spectacular condition but the A/C is not functioning. It will not work at all most of the time and then for some unexplanable reason, without touching anything it will start to cool the car. But then, if you shut the car off and start it again it is back to not working most of the time. The freon pressure is at 120lbs so it is not leaking and for sure full and pressurized.

Someone suggested it is the dash climate control panel that has gone bad and also said that the newer style (no turn knobs) will plug right into the older style 1991 harness and work fine and solve my problem. Any input??

Originally Posted by James427

I just purchased a 1991 850i with 50,000 original miles. The car is in spectacular condition but the A/C is not functioning. It will not work at all most of the time and then for some unexplanable reason, without touching anything it will start to cool the car. But then, if you shut the car off and start it again it is back to not working most of the time. The freon pressure is at 120lbs so it is not leaking and for sure full and pressurized.

Someone suggested it is the dash climate control panel that has gone bad and also said that the newer style (no turn knobs) will plug right into the older style 1991 harness and work fine and solve my problem. Any input??

Yes. Stop.
You have a 91 so unless it has been converted you are correct that it has Freon. After 94/95 it would have Klea, R134a, and if you get it recharged it will have to be converted to R134a if it hasn't been. What's in your system?
A pressure of 120psi means absolutely nothing. Got that? Nothing. I assume it is your high side pressure when the system is running and the compressor is engaged. But it could be your Low and High side pressure when the engine is hot and the compressor is NOT engaged.
Do you know which part is the compressor? When the compressor is continually running, or most often running, is the system blowing cold air? When the compressor is NOT running is that when it does NOT blow cold air?

The 120lbs was taken by guage on the "fill" side with the engine off. The compressor does NOT kick on most of the time. When it does kick on it cools the car very well. But no amount of fooling with the knobs will make it come on 95% of the time, then it will just start working without touching the climate control panel. It actually came on and worked well yesterday for a while WHILE THE SNOWFLAKE BUTTON WAS OFF! I dared not touch it and the following morning it would not come on again.

So it appears the malfunction is directly related to the compressor NOT engaging when told to do so by the climate control panel. I went to the junkyard yesterday hoping the 7 series panel would swap out but it is much longer. I did rip on apart to see how it worked though. There doesn't seem like much I could "fix" other than cleaning the contact points off on mine. But the fan switch on mine and for that matter everything else on m,y climate control panel seems to function as it should. The only contact points I see are on the Fan roller knob which seem to work fine, and the driver's side temp roller knob, but that set of points only seems to function when the knob is turned to it's coldest setting, then they close.

Hope this info helps!

Your 120psi on what you call the fill side, which is what we call the vapor or return side, is normal when the engine is off this time of year. If you were to check the other fitting btw, called the liquid side or high pressure side, you would find exactly the same 120psi. Of course this is all changed once the compressor engages!
I'll tell you a quickie we would do in the shop which in your circumstance would help with the diagnostics. Turn the ac on and have the engine running. Open the hood, locate the ac compressor. The front plate of the compressor-not just the pulley-should be turning but it probably won't be or you'd have cold air. Take a broom handle and very carefully, while holding it with two hands, tap sharply on the compressor clutch's front plate. Let me know if it suddenly clicks in and starts spinning. Watch out for the drive belt when you are doing this.

Don't mess with the control head (the thing in the dash) at this point. They are bulletproof. Far more likely the problem is somewhere else. Once you do the broomhandle test you can post your result and we'll go to the next step.

I didn't do the broom stick test, but I did run a jumper wire from a positive terminal to the compressor and the A/C works fine from the dash control with the junper wire. SOOO I am obviously not getting hot juice to the compressor when the A/C is turned on from inside the car. From what I read, it can't be a relay because supposedly the 91 does not have an A/C relay. The dealer couldn't even look one up for that year and model. So the next potential source of the problem would be the pressure switch which is also supposedly problematic on the 91's. Any feedback on this?