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AC compressor(?) failure assessment


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My AC has stopped functioning. Recently I started the car and engaged the AC and it cooled properly. Parked for a few minutes and restarted (I normally switch off the AC before stopping engine, but failed to do it this time). Upon restart, there was a definite "strained" sound from the engine, my first impression was of a locked up PS pump, that lasted for a bit and then a "pop" or release of some kind and the noise went away. No AC cooling after that.
I checked to see if the AC clutch was engaging and it is (and disengages when switched off).
I put a gauge on the low press side in the frunk and it showed high pressure. (it was a cheap AC refill gauge that pegged on pressure, so I quickly disconnected for fear of safety. (but I would expect high pressure when the compressor is not running)
After hearing the pop and lack of AC cooling I expected to see an obvious release of compressor oil in the engine compartment, but nothing. And I obviously still have a charge of freon in the system.
So, the question is where to start diagnosing. It is as if the compressor was pressure locked on the restart, and perhaps there is a relief or bypass that was set. the compressor is hidden under the supercharger and intercooler plumbing, so I will pull that loose to get a better look.

 

2000 996 C4 Millennium Edition w/ VFEngineering supercharger and assorted other goodies

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yes, the condenser fans are running.

When I engage the AC, there is no noticeable load on the engine from the compressor, even though the compressor clutch appears to be engaged.  It would seem the internals to the compressor are gone, or there is a bypass/relief open.

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You can put a set of a/c gauges and measure the low pressure and high pressure side the same time. If the compressor is working, you should see low pressure on the low side (~30PSI) and high pressure (150+ PSI) on the high side. If not, it could be the internal regulating valve inside the variable displacement compressor See pdf page 38 here for more info on it and the a/c system in general http://www.renntech.org/forums/files/file/1663-after-sales-training-climate-control-systems-diagnosis-repairpdf/

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This could be a number of small issues (sensor failures, etc.).  If you do not have the expertise and the proper equipment to evaluate it, I would suggest taking it to a pro.

Another thing worth trying is to scan it with Durametric for any fault codes in the climate control system.

 

Agree with John though that this is probably beyond DIY if you don't find anything obvious.

Edited by Ahsai
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"Sorry you don't have permission for that" when I try to download the pdf (will try again at home, perhaps it is my work network that is blocking me)

The Climate Control System.pdf is likely what I was looking for to lay out the system, so I can start the search for failure modes.

 

I scanned with Durametric last night, and no codes were present for the HVAC.

 

I have a new set of gauges coming, so I can check the Low and High side pressures.  The initial high pressure indication on the Low side told me that the compressor was not performing.

 

Hopefully the Diagnosis guide will help ID the faults before I turn it over to someone else.  At a minimum, I will take it to a shop to recover the freon before I start any disassembly and replacement.

Thanks for the help.

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  • 2 weeks later...

Update:

finally pulled out the supercharger ducting and found the AC clutch totally fried and the compressor seized.

So, I am up for a compressor replacement.  The system is still under full pressure.  Curious for reasons for the compressor failure.  The system had been operating well right up to that point.post-59202-0-39998200-1407118700_thumb.j

Edited by DWS964
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Any speculation on the root cause of the compressor failure and seizure?  To my knowledge, the system had not been opened up previously and has only 60K miles (but 14 years) on it.  I am trying to sort out how much of the system I should change out (receiver/dryer, expansion valve, ...) and other procedures (flushing, etc) to be done. I plan to install the new compressor myself, given the non-standard complexity of the engine compartment with the supercharger install; but will perhaps employ a shop to do the flushing, evacuation, and recharge.  I want to pay special attention to getting the correct amount of oil back into the system.  The shop manual has some very good and specific references on oil quantities per component and recharging procedures.

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Any speculation on the root cause of the compressor failure and seizure?  To my knowledge, the system had not been opened up previously and has only 60K miles (but 14 years) on it.  I am trying to sort out how much of the system I should change out (receiver/dryer, expansion valve, ...) and other procedures (flushing, etc) to be done. I plan to install the new compressor myself, given the non-standard complexity of the engine compartment with the supercharger install; but will perhaps employ a shop to do the flushing, evacuation, and recharge.  I want to pay special attention to getting the correct amount of oil back into the system.  The shop manual has some very good and specific references on oil quantities per component and recharging procedures.

 

The clutch assembly at the front of the compressor has a large bearing in it, which can seize.  I don't think Porsche sells the clutch and bearing separately from the compressor, but you may find it in the after market.

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Any speculation on the root cause of the compressor failure and seizure?  To my knowledge, the system had not been opened up previously and has only 60K miles (but 14 years) on it.  I am trying to sort out how much of the system I should change out (receiver/dryer, expansion valve, ...) and other procedures (flushing, etc) to be done. I plan to install the new compressor myself, given the non-standard complexity of the engine compartment with the supercharger install; but will perhaps employ a shop to do the flushing, evacuation, and recharge.  I want to pay special attention to getting the correct amount of oil back into the system.  The shop manual has some very good and specific references on oil quantities per component and recharging procedures.

 

The clutch assembly at the front of the compressor has a large bearing in it, which can seize.  I don't think Porsche sells the clutch and bearing separately from the compressor, but you may find it in the after market.

 

The pulley still spins freely, so I assume this means that the bearing that would be part of the clutch assembly must still be fine.  I cannot get the compressor shaft to rotate.  So, apparently the compressor seized, while the clutch was engaged, and instead of the belt slipping, the pulley+clutch unit tore loose the rubber isolator built into the pressure plate hub (the melted rubber in the picture above).  The pressure plate hub attaches to the splined shaft of the compressor, and the electromagnet couples the pressure plate hub to the spinning belt driven pulley.  So I think I can still drive the car safely with the clutch pressure plate removed, just no AC.

Need to figure out why the compressor seized, but bottom line is that I need a new compressor+clutch unit.

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Any speculation on the root cause of the compressor failure and seizure?  To my knowledge, the system had not been opened up previously and has only 60K miles (but 14 years) on it.  I am trying to sort out how much of the system I should change out (receiver/dryer, expansion valve, ...) and other procedures (flushing, etc) to be done. I plan to install the new compressor myself, given the non-standard complexity of the engine compartment with the supercharger install; but will perhaps employ a shop to do the flushing, evacuation, and recharge.  I want to pay special attention to getting the correct amount of oil back into the system.  The shop manual has some very good and specific references on oil quantities per component and recharging procedures.

 

The clutch assembly at the front of the compressor has a large bearing in it, which can seize.  I don't think Porsche sells the clutch and bearing separately from the compressor, but you may find it in the after market.

 

The pulley still spins freely, so I assume this means that the bearing that would be part of the clutch assembly must still be fine.  I cannot get the compressor shaft to rotate.  So, apparently the compressor seized, while the clutch was engaged, and instead of the belt slipping, the pulley+clutch unit tore loose the rubber isolator built into the pressure plate hub (the melted rubber in the picture above).  The pressure plate hub attaches to the splined shaft of the compressor, and the electromagnet couples the pressure plate hub to the spinning belt driven pulley.  So I think I can still drive the car safely with the clutch pressure plate removed, just no AC.

Need to figure out why the compressor seized, but bottom line is that I need a new compressor+clutch unit.

 

 

Just be aware that if an internal bearing on the compressor has gone bad, it has probably also filled the entire system with debris, which will need to be completely flushed out before the system can be closed up and recharged, otherwise any residual debris will kill the new compressor.  Probably also be a good idea to toss the dryer and install a new one as you will never get that clean of debris.

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