My AC was working perfectly until Friday, when it worked fine in the early morning, then when I left my breakfast stop, I noticed that although the air flow was as before, it wasn't cool.

When I arrived home, I checked and found that the belt was in place, but the compressor clutch would not engage. I checked the fuse and it was good, pulled the relay and it tested good.

I took it by my favorite shop and left it for a few hours. They were super busy, so didn't really have time to give it their best shot. They reported that the compressor clutch was bad (later I found this to be a false reading), and that they tried to put in some refrigerant, but "it wouldn't take much because we couldn't get the compressor to stay on".

After I got back home, I did some more testing myself. When I apply power directly to the compressor clutch it engages. I then tried jumping terminals 30 and 87 in the compressor relay socket, and the clutch engages. With the jumper in place, the AC blows cold.

The troubleshooting information I found on this forum is pointing to the 3-way pressure switch, perhaps triggered by low system pressure.

Any other thoughts on this, or how to troubleshoot it further?

I think my next step will be to take it back to the shop, have them evacuate the system, and refill with the prescribed quantity of refrigerant.

 

Bob,

You say you tested the A/C relay and that it checks out OK. You then bypassed the relay and the A/C clutch engaged.

To be sure, check if the signal that activates the A/C relay is high or low. I would assume low based on the above. It could be low pressure or a break in the wire.

I'd get or borrow some A/C gauges and verify the pressure. Bypassing the pressure switch will confirm that the wiring is OK. Could be the switch, pressure or signal to the switch.

 

Hi Alex,

Glad you are online--thanks for such a quick reply.

Could you provide specifics on how the check the signal from the pressure switch? I'm assuming I would check the relay socket terminals with a DVM, but I'm not certain which terminals or what to look for...

I read in a troubleshooting guide posted by Brian Smith "3-Level Pressure Switch: This switch, inline to the evaporator under the front hood on the driver’s side near the windshield, tells the CCU when the refrigerant high side pressure is either too high or too low. Between 2.5 and 27 bar (37 to 390 psi) the high/low switch passes current to the compressor. Anything outside this reading will disengage the compressor. As noted, this switch is downstream of the CCU, meaning the CCU has no real knowledge of the shutoff."

 

Bob, I'm at work so unless Tore or another RL member steps in, I'll get back to you later. I have to look at the schematics. I also have someone coming after work who needs their car looked at. Time...

 

There is the inside (evaporator) temp sensor that could do the same thing.
To diagnose with the obd tester
GL Bob

 

Checking actual pressures with a gauge set is a good idea to start

Another common issue are the CCU's AC switches.
The contacts get dirty over time.

You may already know this but you can remove the CCU and clean it, pretty easy DIY.
Easy test is to "fiddle" with the max AC button. Give it a workout...

 

I had the exact same symptoms when my evaporator was leaking. Refrigerant leaked out, compressor wouldn't engage.

 

Thanks, Falcondrivr. Any other symptoms of a leaking evaporator? Maybe oil under the car, or on its bottom?

 

I added 134 with the yellow dye to detect leaks. The yellow dye showed up under the car. The drain for the evaporator is on the bottom of the car, directly below the center black air intake at the top of the hood. It's kind of shaped like grill of a new Kia...about 4 inches across.

 

To the pressure switch: You need to jumper pin 3 (white / orange cable) and pin 4 (black cable). When doing this, do not let the compressor run long because you are basically defeating the low pressure protection that is most likely caused by some kind of leak. It needs recharge and you might need to jumper the switch as described above to get the system going again. This is exactly how it worked for me.

Just print the DIY from use BS911 'COMPLETE A/C DIAGNOSIS' if you want to get into proper trouble shooting.

 

Found a resolution yet?

This is copied from our very own rennlist DIY 993 section, user c993k:

I explained this problem once before but here's the complete scenario.
The clutch on the air compressor stopped working. Not good when it's 100F in South Florida!
Doing quick checks, all the fuses and relays that relate to the compressor clutch in the front and in the engine compartment are fine.
My first though was I must have a Freon leak and the pressure switch cut out the clutch. I had the system cleaned out and refilled.
No clutch. I replaced the pressure switch, no 12V to the clutch yet.
The Air Conditioning shop couldn't figure out the problem either, and they have worked on most of my other Porsches in the past.
They know their job but they know when something is beyond their capabilities.
I traced all the wiring and the only thing I could do is run a wire to the clutch from one of the rear relays.
I chose not to do that and just try to find the problem myself.
I have all the manuals and I'm a broadcast engineer/electronics tech for a few decades so I'm quite capable of making changes or correcting the problem.
I took out the front and rear fuse/relay boxes and started the tracing.
I noticed that the engine compartment relay that delivers the 12v to the clutch was using the same wire as the relay that controls the right fog light!
I checked the right fog light and sure enough it was DEAD!
The relay that controls the right (passenger) fog light is the problem!
It's the one that my finger is pointing too in the front fuse/relay compartment.
NOT DOCUMENTED ANYWHERE.
So now you know that if the front right fog light is out and the air is not working the little relay is the fault.
BTW I took the relay apart and re-soldered the connection to the tab and everything is working perfectly! ~Chris~

 

Bingo!

But it isn't a relay that controls the right (passenger) fog light (those go ON and OFF together when inserting/removing R01).

R01 is the Back-Up Lights / Rear Fog Light relay which feeds R31, the Front Fog Lamp relay. Pulling R01 kills the front fog lamps and back-up lights.

And just as R01 feeds R31 (Front Fog Lamp relay), it also feeds the coil of the AC Compressor relay - pin 86.

 

Alex, you gotta pm.
Glad to help!

 

Thanks O'Neal,

I ended up solving it by finally figuring out what K4 meant which you'll find hanging off the AC Compressor relay at pin 86 on Sheet 3 - Heater-AC.

When I see "K", I think relays, but in this case it was the coordinates on one of the electrical diagram sheets...in this case Sheet 1 - Lights. And that is where I noticed that relay R01 (Back-Up lights - Rear Fog Lights) was a feed to the AC Compressor Relay.

To confirm all of the above, I went into the garage and pulled R01...sure enough, the back-up lights, both front fog lights and AC compressor turned off.

I don't know how/where the person that wrote that article came up with a relay for the left front fog light; one relay activates both front fog lights.

I got a PM back from Bob and he now sees why I was getting 14V at the White/Orange wire at the pressure switch. He is finishing off putting the CCU back in.

Its a PITA working with electrical diagrams on a small laptop screen, scrolling back and forth. I'm going to scan in my diagrams in high-res and print them to large sheets.

 

It will be interesting to hear back from Bob on what his fix was
looking forward to it!