Diagnosing A/C system issues
 

The previous owner swapped in an r134a A/C system into the 964 and it hasn't functioned properly since I got the car.
Since it's approximately the same temperature as the surface of the sun in Houston, after a long and sweaty deliberation, I've decided to try and fix the A/C system.

Here is where I am so far:

• Clutch engages with 12v applied directly
• No power to car harness side of wire powering clutch
• If I jumper pins 30 & 87 on the relay in the LR of the engine bay the clutch engages fine
  • Checked power to pin 86, getting constant 12v
  • Applied power to relay between 85 & 86, got contact then checked continuity between 30 & 87 and read 0 ohms
• I've filled the system with coolant and it seems to be holding pressure/cooling the air (need another can of coolant since mine was 3/4 gone)

It looks like the issue is the green wire isn't getting 12V to supply the compressor with power.
Before I tear into the harness/my interior to trace the wire forward are there any tests at the CCU to see if there's power being output?

I tried searching, but there are countless threads with myriad issues, none of which seem to be the same.
Thanks,
Joe

Sorry wrong thread

From other threads, it may be the controller. These can be tested and rebuilt.

I found these helpful https://rennlist.com/forums/993-foru...diagnosis.html - This one lists the reasons for a non running compressor.

https://www.bergvillfx.com/porschehvac2 - This one explains Compressor operation from the CCU

I rented a manifold gauge set and vacuum pump; looks like there is a pretty decent leak somewhere....I think.
I wanted to check the system pressure to ensure it fell in line with the 3-level switch parameters.

The low side had 3 PSI when I hooked the gauges up; I put the pump on and left it running per the instructions for ~60 minutes. The system didn't change, let alone pull any vacuum.
If I'm understanding the A/C system and issues I'm having is this indicative of a significant leak in the system?

I've tried searching on google and here, but this is my first foray into A/C systems and am trying to fail forward.

Forgive my questions as I'm trying to get the obvious steps out of the way. :)

Were the control knobs on the hose ends and on the manifolds open fully? Were the hose connections tight and were the seals in place?

It happened to me recently when I borrowed (rented) gauges and a pump from O'Reilly as my gauges wouldn't get down far enough. In my situation it was a missing seal in the low-pressure hose at the manifold. Fixed that and surprise surprise, had no vacuum leak. But I had to revert to my gauges since the O'Reilly set had a wonky high-side.

The evaporator replacement guide at P-Car Workshop lists the pressure ranges for low and high sides depending on ambient temperatures for an R134a fill.

When you say you were getting chilled air, was that with the clutch relay jumpered?

The Climate Control Unit produce a active low (GND) signal to the compressor clutch relay when you press one of the A/C buttons. This signal is routed through the pressure switch, and will be interrupted if the pressure is too low or high.

The CCU will turn off the signal when the evaporator gets near freezing temp, but this is normal operation, and the compressor clutch will cycle in 15-30 second intervals in 20 deg C ambient temp.

The compressor relay gets +12V from the front fog light circuit.
See this page for details, chapters A/C compressor operation and Three-level pressure switch.
Cheers,
Tore

I’ll have to double check the seals since I’m renting the gauges from O’Reilly, also.

All of the gauge valves were were fully open, yes.

The compressor clutch engaged and I got cooler air than usual when I jumpered the relay in the engine bay.

I checked the relays (engine bay and fog light relay) for function using a 12V source and my multimeter and they both seem to be functioning properly.

Bought a new set of manifold gauges to rule that out, and figured they’re inexpensive and will get used again.

Running into to the same thing. Only now it’s sitting at 0 psi. Could just be an error between the gauges.

Thoughts?

I don't have anything constructive to add to this thread, but I chuckle every time I see the thread title... with three 964s, I feel like half my life is "diagnosing AC system issues" :banghead::roflmao:

:roflmao: Since the car spends the majority of its time on the track I am VERY close to gutting the A/C...but gotta force myself to fix the problem (especially living in hot and humid Houston)...:crying:

I think the next steps are to jumper the relay to get the A/C to cycle, fill the system with dye, find a leak (hopefully), fix that, then vacuum the system and check CCU/3-level switch/etc...

I jumpered the R01 relay (pins 30 & 87) to get the compressor to cycle, I put dye in the low pressure port, ran the car with the A/C (snowflake) button on, blue dot, and fan on 4. I pulled the wheel liners, shark fin, side skirts, and front bumper to have access to all A/C lines and junctions.

I ran the car for about 5 minutes with the A/C "on" then looked for leaks and couldn't find any. I know the smart thing to do would be to take it to a shop that understands A/C systems and ideally 964 systems...but now I'm like a dog with a bone and for my sake I need to understand WHY this isn't working at all, but I'm at a bit of a loss and need to defer to the smarter minds on Rennlist.

I'll be out for a few days for work so can mull things over, but any more ideas, tips, and pointers would be greatly appreciated.

The evap can be suspect in our cars it’s in the suitcase not sure if the dye would come out of the drain under the car be maybe

Thanks for your help and fantastic website, Tore.
After testing the 3-level switch I found some anomalies compared to what I see on your website, which leads me to believe there might be a wiring issue somewhere.

I am getting continuity between pins 2 & 4 with the car off.
This is where the differences lie:
https://cimg6.ibsrv.net/gimg/rennlis...2996c8d4a4.jpg

I tried following along to this bit from the website:
Test the switch like this: Turn A/C ON (by pressing either of the buttons on the CCU) The GND signal shall be present on the sensor connector pin 4 (black wire). If pressure is OK the signal shall also be present on pin 1. (white/orange) You could also disconnect the sensor connector and use an ohmmeter to measure any connection between pin 1 and 4. The condenser cooling fan pressure switch has GND connected to pin 3 and closes to pin 2 (brown/black) if the pressure is over 17,5 bar.

What I ran into was, obviously the wires don't match up to the pins, but the power/ground characteristics don't match up, either.
Could this be an indication that there is a wiring fault at the CCU? The switch is obviously getting power, but it seems to be coming from the incorrect wire.

I tend to be better at the mechanical side of things, chasing down wiring is by far and away not my strong suit. I appreciate everyone's help with this.

It wouldn't have in my car. The drain looked like this when I took it all apart.

https://cimg3.ibsrv.net/gimg/rennlis...d8eb9dc178.jpg