Porsche Enthusiasts Club Forum

'89 944T recently had the aircon compressor replaced and system charged so the aircon works well when the car is ticking over or cruising.

The issues comes when you put on some throttle, the airflow goes to full heat. Back off the power to steady throttle and the aircon blows cold again.

Any thoughts?

Does the issue only occur with the AC on?

Yes correct, only happens when aircon is on.

It goes from cold to hot almost immediately, and its straight to max heat so it feels like a switch or value changes position, triggered when putting pressing the accelerator from a cruising mode

Could be the valves that are supposed to close and redirect air from the aircon (rather than the heater matrix under the bonnet).

Try putting the heater control to cold (i.e. closing the HCV) and see if it still does that. If it doesn't do it anymore, then it definitely is the valves.

Thanks for the comment leepox.

The heater control is at max cold and airflow still goes hot only when you accelerate (say to pass a car) and returns to cold when you have a steady throttle/road speed, so its not the valves stuck open (or it would be hot all the time right?).

I believe something is triggering the valves to open so that points to the controller unit behaving strangely?

I think it must be electrical. Either the heater control unit or a crossed wire or bad earth somewhere. I just re - commissioned my a/c and it does not do this! I would try swapping out the control unit first if you can borrow a known good one. 10 minutes work.

With the air con off, and dial set to cold, do you still get this issue?

Stuart

hi have you checked the clutch on the compressor? does it drop out if you rev? if it does it could be over charged with refrigerant or even wrong gas used to charge.the original gas on 944 would have been the old cfc R12 up to 1988/89 then changed to R134A A HCFC gas now also not allowed to be used and the so called drop in replacements don't work well in aircon. if the compressor drops out the gas will flow back to compressor and not condenser causing evapourator to heat. to use one of the new refrigerants the expansion should be changed. I have also come across cars with isobutain refrigerant used again not allowed in cars(big bang) these have a code R600 and R600a and are just pure butane, hope some of this helps

there is a one way valve on the vacuum system in the engine bay. its possible it has failed allowing boost pressure into the vacuum lines and forcing the heater valve open. have a look for a blue / black valve somewhere near the bulkhead, you should only be able to blow through it one way.

Thanks for the comments back.

It does not appear to do this when the aircon is off.

Will check the one way valve, the aircon clutch and will need to find another owner with an aircon unit to test - hopefully its one of those.

cheers