Another Air Con query
 

Porsche 928s (86.5). My air con just keeps blowing warm air. If I drive it when the outside temp is cool it seems to work OK (for a while anyway), however if I go for a drive when it's warm it just blows warm air (it might cool air for the first minute or two).

A bit of background:
1. I have replaced all vacuum actuators and tested them.
2. I recently replaced the compressor and had the system regassed.
3. I have actually removed the heater valve and plugged the vacuum and pipe in the engine bay so no hot water should be getting into the system.
4. I took it to the air con guy this morning to recheck the gas levels and it was perfect. I put my hand on the air con pipe in the engine bay and it was very cold.
5. The HVAC seems to be fine.

Am I missing something?

On my 84 if I had the temp slider all the way to the cold side mine would do the same thing. If it wasn't all the way to the cold side it seemed 50/50.

What it turned out to be in my situation was the micro switch on the full cold side had died. I ordered a new micro switch and soldered it in. I also cleaned the tracks while I was in there. But if you removed the heater valve this might not be your issue.

Out of curiosity why did you remove the heater valve

I wanted to discount the heater valve altogether. I have a new one to install once I resolve the issue.

If you still have the hoses connected to the heater core it will be hot all the time.

I wasn't sure if you bypassed it from you post. Just seemed like you removed the valve and left it to flow water all the time.

Another thing that I had happen to me is that the vacuum solenoids under the hvac box can fail and become a constant vacuum leak. Check them to see if they are leaking or not.

You don't mention any testing/investigation of the mixing motor, which could be one possible cause of problem if you still have hot water to heater matrix.

In connection with this, you should also do a resistance test of the entire sensor/controller chain. Very simple as can be accessed on two of the pins plugged into mixing motor.

Is the compressor turning?

Is the AC line cold past the expansion valve. I have seen valves mess up and let cool air happen at random times.
When you replaced the compressor did you replace the drier and all sensors?

Thanks for the replies. A quick update. I drove it to work today, about 30 kilometres. It worked fine. Yesterday afternoon I drove it (the cabin was very warm as it was a warm day) and it just blew warm air. Also following is a response to some of the suggestions. Thanks again for the advice.

1. I bypassed the heater core.
2. I also tested the solenoids when I installed the new actuators.
3. The compressor is working fine.
4. Hmm, not heard of a mixing motor so I will investigate that. Where is it located?
5. Haven't looked past the expansion valve so I will also check this.

Mixing motor is to the left of and behind the center console, when sitting in the driver's seat...it basically moves a flap to allow warm air to mix with cooled air. Not sure how much impact this would have if you bypassed the heater core? Some I suppose...

Thanks, I assume I can access this by removing the centre console?

No need to remove centre console - if it mirrors my RHD car it’s above your right knee when in seat. Access better by removing parcel shelf (or your car may have knee protector?).

But you can do basic test without anything but ears. In quiet spot turn ignition on, move temp control to full hot. Do you hear motor travel to stop position? Move slider back to full cold. Do you hear motor travel to opposite stop?

But as earlier poster said, mixing motor role dubious if heater core bypassed.

Mixing motor wont do jack squat in your 87 as it controls a pair of doors in front of the heater core allowing air to pass through it.

With no water going through the heater core it wont affect anything.

Have you checked the freeze switch? They can fail over time and lead to issues as well.

Another common issue is having too much compressor oil in the system. It will cause poor cooling capacity but do no real long term harm if it isn't severely overfilled.

Agreed, I don't think it's the mixing motor as there is no water going through the core. The compressor engages so should I assume that the freeze switch is OK. I suppose they are easy to test so I might do that anyway and report back. Thanks again.

You mentioned that you checked all the little vacuum diaphragms, did you specifically verify that your recirculation flap is closed when set to max cooling? If that is stuck open you will be getting outside air, which reduces cooling capacity quite a lot.

If you look up under and behind the glove box, you will see it. If the flap is closed, it will be flush with the grate and you can feel it with your fingers...if not, that grate area will be open.

There's a table somewhere that shows when it is supposed to be in the closed state...I don't remember all the conditions, but certainly with everything set to max cooling it should be closed.