Assessing AC issue 70 series - 100psi at AC low pressure valve

Post by **SonicAdventure** » 26 Jul 2018, 18:54

Hi I guess with it being summer here in the US, there will be a usual round of AC questions.

My AC compressor and condenser are only 3 years old. I have paperwork from the PO that it was done. However, the AC never really worked. When I first bought the car 3 years ago, I noticed that whenever I turned the fan on the AC compressor would engage regardless of if the AC button was pressed. I figured it must be a problem with the switch, until I pulled the AC relay one day and noticed that someone had jumped some connections which made it so if the fan was on the AC compressor was on.

See attached photo. I removed the jumper wire and then noticed that while the compressor turned sometimes, sometimes it didn’t. It would run 50% of the time. Someone looked at the clutch gap a while ago and said the gap looked ok.

So, I guess I never really let it bother me as I only have a short commute and it is what it is. I decided to buy one of those refil bottles today with the removable pressure gauge. I turned the car on and switched it to max high and recirculate. I could tell if the compressor was on, but I got a steady 100 psi at the low pressure valve, so I didn’t bother putting any more refrigeration in the system, as red was the “danger” zone according to the instructions.

So, I put the jumper wire back under the AC relay and did the test again. Compressor kicked on, pressure at the low pressure valve read in the “blue” zone roughly 35-40psi, indicating it did not need Freon. I cut the compressor off, pressure went back to 100.

So, it seems like my compressor has Freon and it act like everything is in spec from the pressure gauge, however the jumper wire needs to be in there and it’s still not blowing really cold air. If I drive for a while when the AC is on, when I come out later there is a puddle of water under the car so I know that it’s pulling moisture out of the cabin air and draining properly.

Any ideas?

Post by **abscate** » 27 Jul 2018, 03:53

The condensates under the car is good news, it means the charge and system integrity are good, nominally

The pressure reading with clutch disengaged is essentially meaningless. It just measures vapor pressure of refrigerant stuck in the system, which is high. 28 psi on low side with clutch engaged is nominally ok.

Is the hard AC line going to the dashboard sweating water ? That’s good.

I assume you have Electronic climate control. The compressor should be cycling about 50 % on a hot day. Running it full time is dangerous as it will ice up, and if you are really unlucky, develop liquid into the compressor which really get s ugly.

I believe the 1998 system commands the AC relay to close via the control unit in the dash. If we can find that wiring diagram we can troubleshoot and get rid of thst jumper. It sounds like you have gotten rid of that jumper and put the relay back in?

I Confirmed that the 1998 ECC control unit grounds the AC relay which controls voltage to the AC clutchvia low pressure switch in series.

You have to measure that clutch gap carefully. Most people can’t tell the difference between a gap that will or won’t work without getting in touch with their feelers

Post by **oragex** » 27 Jul 2018, 08:02

Does P1 have an evaporator temp sensor? Maybe it's faulty and stops the compressor too early?

Post by **SonicAdventure** » 27 Jul 2018, 17:37

Thanks all for the replies so far.

Abscate, my car has the manual climate control, so I’m not sure if knowing that now would change your assessment of the situation.

After running it around today, I think I can confirm that in general the AC does work, but the compressor will not stay on consistently, with or without the jumper wire. It will basically turn on and off. When it’s on, it’s shooting cool air when it kicks off the cool air stops.

Post by **abscate** » 27 Jul 2018, 20:24

You could just be low on charge. You can monitor the low side with a gauge, and the high side in software, and see if you are getting 4-5x pressure increase from the compressor.

The other way is knock the building down and build it to specs. Evacuate the system , pump dry for 2 hours, then fill by weight to spec of 750g charge ( check your spec)

Post by **abscate** » 28 Jul 2018, 06:00

Sonic...that should be much easier to troubleshoot. The AC is commanded by the dashswitch and I see inhibitedby one of three things, low pressure switch, high pressure switch, or the thermal switch in the compressor ( not always present)

I think you are floating in the 80-90 temp range right now? The AC should be cycling something like 33% duty cycle? Each minute But should be cool the whole time.

Caveat..I’ve never worked on an MCC so let one of the Euro guys comment here too.

How’s the clutch Gap in the AC? Needs to be 0.4 mm or so all the way around