Giantman

Reason I ask is that my compressor clutch disengages after the car is warmed up. It will engage/disengage without any trouble when cold (or off) but when warm it shuts off. I am pretty sure there is no low pressure switch on the SC but if someone can correct me I would appreciate it.

Thanks for your help

theiceman

it may not have anything to do wit the car warming up per se...

My Audi has a thermal sensor right on the evaporator ( i don't have A/C on my S/C ) if it senses the evaporator freezing it disengages the clutch on the compressor for obvious reasons. Not sure if the Porsche is the same but may be worth looking at.

Brads911sc

Check your low and high side pressures with a gauge set... Iceman is correct... The expansion Valve will shut things down if the evap starts to freeze...

Giantman

But then wouldn't it come back on again once the evaporator 'thawed' out some?

Thanks!

Brads911sc

Yes. It would. A few areas to check... the front condenser blower... I have taken apart two of these (one on my current 83 SC ) and both had defective motors. That can really make a huge difference. When you open up a system you are to change your drier and the expansion valve. Perhaps yours need changed. the drier is about 60 and the expansion valve can be found for less. check the high and low side pressures, maybe you are overcharged which can cause the freeze and system shut down. are you running 134 or 12? with R134 you need low side pressure to be 18-20 as opposed to 28 or so with R12. Im not an expert, but just changed out my entire system. The only thing i reused was the evap blower. Maybe be some experts who chime in... check Pelican boards. there are a few really useful AC posts with lots of insight...

griffiths

Does the SC have a low pressure A/C switch? Not from the factory, however someone could have added a "switch", such as:
A) as a binary switch which would have a low and high side cut out; these are typically added in line between the compressor outlet and condenser inlet, or
B) either just a low or high switch in the same position as "A" above, or
C) a high temperature switch attached to the receiver drier which would sense the temperature which correlates to pressure; check your drier and see if you have a two wire switch attached to the blind hole on the top; and check the compressor to condenser hose for a switch.

Normally on a factory air car what turns on and off the compressor with is the thermostat switch (Left side on center console) which has a capillary tube inserted in the evaporator, depending upon where you set the thermostat as the evaporator gets cold or warm the thermostat controls the clutch.

Other things that can cause the compressor clutch to work and not work are:
faulty ac relay (located in smugglers box on RH side rear)
compressor coil
ac fan speed switch on its last leg killing the circuit
ac fuse the typical corrosion in its socket

This is what needs clarification "Reason I ask is that my compressor clutch disengages after the car is warmed up. It will engage/disengage without any trouble when cold (or off) but when warm it shuts off". By "after the car is warmed up" I don't know if you mean cockpit or engine however we can assume engine. Saying "It will engage/disengage without any trouble when cold (or off) but when warm it shuts off" could mean cockpit, outside ambient or engine temps. We'll assume you mean engine again. The ac system circuit can be live (turning on and off the compressor) when the engine is off however the ignition key position is in the accessory or on position; you turn on the fan speed control to either of the three fan speeds and then turn the thermostat fully clockwise for the coldest setting which would mean the compressor clutch would be engaged until such time the capillary sensor senses the cut out point.

If you were running your engine, ac system is on, compressor was engaged and then it was disengaged it could simply be because the thermostat has met its set point. You can read about the thermostat function here at this hyperlink:
http://www.griffiths.com/achelp/achelp2.html

PM if you need more help.

Giantman

Thanks guys for the replies. I guess my question was a little ambiguous. Griffiths in answer to your question, yes I meant when the engine gets warm. Basically with the ingnition on (engine off) the clutch will engage as it should. After driving with the A/C on for a while (about 15 mins) I can feel the air from the vents getting warm and I know then the clutch has disengaged again. It will not come on again until after the engine has been off for a couple of hours and cooled down.

I understand the thermostat is meant to do this kind of thing under ideal cirumstances but I just don't think the a/c ever really blows cold enough for anything to freeze up. I have been suspecting the thermostat switch itself might be faulty somehow but they are not exactly easy to open up and look into.

I have not checked the relay so I will do that. The receiver drier is only a year old and there is still pressure/gas (R12) in the system. I will also have the low and high side pressures checked to rule out the expansion valve closing off.

Thank you

griffiths

After driving with the A/C on for a while (about 15 mins) I can feel the air from the vents getting warm and I know then the clutch has disengaged again. It will not come on again until after the engine has been off for a couple of hours and cooled down.

If the ac blows cold but gets warmer, and if you turn the system off, let it sit for awhile and then blows cold again..... that tends to be a symptom of the evaporator icing up; one clue would be a rather large puddle of water under the front tub from the melt down. To verify if the compressor clutch is still engaged when you sense the vent temps rise the for sure check is to open the engine deck lid and see if the out clutch hub is spinning with the clutch pulley (means the clutch is engaged).


I understand the thermostat is meant to do this kind of thing under ideal cirumstances but I just don't think the a/c ever really blows cold enough for anything to freeze up. I have been suspecting the thermostat switch itself might be faulty somehow but they are not exactly easy to open up and look into.
Maybe and maybe not. Could me moisture in the system freezing inside the expansion valve, but ideally you want to check for clutch engagement and check system pressures and temperatures with tools.


Thank you
Nada.

theiceman

Regardless this is a good one Giant .. keep us posted..

Edgy01

Evaporators icing up is symptomatic of driving in high humidity areas with the windows down as you are conditioning the air. All that humid air, when super-cooled, will accumulate there and then you wind up defeating the cooling.