993 - A/C Diagnostics

From pcarworkshop

Air Conditioning Diagnostics

Check 1

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 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!

Original Article Credit : Chris (ala C993K

Check 2

Normal operation:

• Recirc: Vacuum pressure pulls opens inside air source / Servo Closes fresh air source
• Fresh: Vacuum releases & spring loaded inside air source closes / Servo Opens fresh air source

The problem is that the electric servo could die anywhere - fresh air source open, closed, or somewhere in between. So the only real thing we are guaranteed to know is if the vacuum fails.

If vacuum fails, and servo still operates:

• Fresh air selection: servo opens fresh source, inside air source closed. NORMAL
• Recirc selection: servo closes fresh source and inside air remains closed. NO AIR FLOW.

If vacuum works and servo fails with fresh air closed:

• Fresh air selection: servo remains closed, inside air closed. NO AIR FLOW.
• Recirc selection: servo closed, inside air opens. NORMAL.

If vacuum works and servo fails with fresh air open:

• Fresh air selection: servo remains open, inside air closed. NORMAL.
• Recirc selection: servo remains open, inside air opens. MAY APPEAR NORMAL, but can't shut off outside air.

So to sum up:

• No air flow/Wheeze on recirc - you have a vacuum leak somewhere:
• No air flow/Wheeze on fresh - your fresh air electric servo has died in the closed position.

To verify electric servo:

• Remove black plastic trim under front hood nearest to the base of windshield and locate center electric servo. (There are 3 servos on a 993, one to open/close footwell output, one to open close windshield output, one to open close fresh air input.) 964's have 2 more to control temp mixers. 993 temp mixer servos are in cabin under door sill trim area.
• Operating the corresponding a/c control should show servos moving back and forth.

To verify vacuum -

• Main vacuum "source" from engine routes to same location as the 3 servos, into an electric solenoid. When the recirc button is pressed, the electric solenoid opens the valve, passing vacuum pressure to output. With engine idling:
• Disconnect vacuum source on solenoid to verify vacuum pressure. If not present, problem is vacuum leak in engine area. (Good luck)
• If vacuum present, press recirc, disconnect output side of solenoid and verify vacuum pressure.
• If not, solenoid not getting signal or failed.
• If present, look under dash, under ccu for rubber elbow connecting vacuum line to flap mechanism - it would most likely be unplugged due to stereo installer or other inside work bumping it free.

Original Article Credit : Brian Smith