AC compressor(?) failure assessment

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. Suggestions?

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The pop reads like the compressor pressure relief valve activated. The lack of cooling could arise from the compressor control valve reducing the variable displacement pump's displacement because the evaporator pressure is too low. As long as the system is pressure tight, and has the right amount of refrigerant and compressor oil there are (at least) two areas to look at. The receiver/dehydrator which is responsible for removing moisture and dirt from the refrigerant and storing refrigerant to provide for a smoother/more stable supply of refrigerant. The expansion valve which allows the compressed and cooled refrigerant in liquid form to rapidly expand into a low pressure and low temperature fluid. This then flows through the evaporator which has this fluid evaporate into a gas while at the same time taking heat out of the air that the cabin fan blows through the evaporator. If the system has too much moisture (or dirt) this can be interfering with the flow of refrigerant and the because the evaporator pressure is too low reduces the capacity of the compressor to the point while it is spinning it is not compressing any refrigerant. Absent a leak the most common way for moisture (or dirt) to get into the system is if the system is opened up. After being opened and then being buttoned up again, the system should be subjected to a prolonged (hour?) at low pressure (essentially a vacuum) to remove any moisture (along with refrigerant and some oil). Then if the system proves to be leak free the proper amount of refrigerant is added back along with the necessary amount of oil. There is an oil budget part of which is set aside as the amount of oil that ends up coating all the surfaces in the system and the free amount which is tasked with lubricating the pump. If the expansion valve doesn't meter the right amount of fluid through into the evaporator this can have the pressure drop which causes the internal pressure control valve in the compressor to act. There can be a mechanical failure, physical damage, or contamination by moisture or possibly dirt if the amount of dirt overwhelms the receiver/dehydrator. Generally, anything beyond adding refrigerant (which is not as a simple task to do right as one might think) a qualified A/C tech is probably called for. The A/C system controller may be queried for any error codes which may point the problem area. Then if the system has to be opened up it can be properly evacuated to remove the refrigerant mainly, then after it is buttoned up, evacuated again and then refilled with refrigerant and oil.

Is it possible there was a problem with the AC clutch that "broke lose" and then quit functioning, without leaking any refrigerant?

Thanks for the info and assessment Macster, helpful as always. A Durametric scan showed no error codes. I am curious how to further diagnose the compressor to see what happened, and is happening in there. I have a better set of gauges on the way, and will then get a better idea of the charge in the system and whether the compressor is doing anything at all. I want to dig as deep as I can into the failure mode, before I turn it over to a shop. I posted this query over on Renntech and was referred to a copy of "After Sales Training - Climate Control Systems Diagnosis and Repair". http://www.renntech.org/forums/topic...re-assessment/ Stay tuned....

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No error codes is good news, I think. Using Durametric system can you query the system for the various A/C system temperature sensor values? If you feel comfortable trying to trouble shoot this yourself then using the gages to check out the pressures is I guess a good next step. Is the compressor actually being turned? The clutch is an electromagnetic one. There is I think a call out for the distance between the pressure plate and the belt pulley of the compressor with no power supplied to this and then with 12V power supplied.

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That is my next step today - try to confirm if the compressor is actually turning. It is hard to see with the supercharger and intercooler ducting in the way. It looks like the clutch is engaging, but.... Is this info in the shop manual? Will try to find. Edit - Found it - 0.5mm on page 87-32 of shop manual THANKS

The info about the distance between the pressure plate and pulley is in my Turbo's shop/factory manual. I do not have a 996 factory manual handy to refer to.

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.

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. And.... even if the compressor was still freely rotating, it does not appear that a replacement clutch is available for this beast (well, other than one from Porsche for some absurd amount of $$$$, way more than a complete compressor+clutch unit) (I have a parallel thread continuing over on Renntech)

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Well, if you tell me the supercharger is located close to the AC compressor I'd hazard a guess the A/C compressor ran too hot. AFAIK the A/C compressor clutch is not a contact type of clutch like the clutch in the transmission bell housing that, well, you know what it does. The A/C compressor clutch is an electromagnetic one. Any time electrical current is fed to an electrical circuit the circuit heats up. The A/C is designed to withstand this, and shed this heat under normal circumstances so the heat doesn't reach failure level. With a supercharger in close proximity. Not so much. That's all I got.

Thanks, Macster The supercharger is on the other side, by the alternator, but the intercooler and ducting are in front of the compressor area. (I should have a picture somewhere, that would speak 1000 words). Whether this influenced the compressor failure, I don't know. It looks like this clutch is a contact type, based on the pattern on the pulley and the pressure plate (but that could be the result of the failure). Again, a picture will help. 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. Anyone have recommendations for a replacement compressor+clutch? PartsGeek.com has a new Denso for $300 I think. Pelican price is $450+ I recall.

D- Try Rock Auto http://www.rockauto.com/catalog/more...439&cc=1364363

I don't know if you're a used-part kinda guy, but there are a couple of used ones on Amazon currently: Amazon Amazon

from what Ive read when diagnosing ACs on other vehicles, when the compressor takes a crap it dumps shrapnel through the entire system and is a nightmare to get completely cleaned out...Let us know what you find out.

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Well, aren't you just a ray of sunshine? Yup, that is a concern. Pete - already looked up RockAuto, similar prices and options. Used? Contemplating this, given Mr. Ray Of Sunshine's comments, for Round One of the repair. I will soon be able to disassemble supercharger system and R&I AC compressor blindfolded and one handed. Wait, that's how it seems to work, anyway. Stay tuned. Still want to determine nature of compressor failure first, to know how big the project may become.

I have a used compressor I'd give you a good deal on. Let me know.

Curious how this all turned out...

I hate to add to the Mr Ray of Sunshine's comment, But! my Audi Q7's A/C compressor took a dump and that's exactly what my very trusted Indi Mechanic told me. The thing implodes and shoots shrapnel through the entire system, I had to replace everything but the hoses

I'm assuming you still have a functioning AC air valve mounted to the cross brace?

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Wow! Did you happen to notice that this is a thread from 2014? 8 years ago...hmmmm...wonder if they figured it out?

Are you able to safely drive the car with a seized AC compressor? Obviously without using the AC while driving.

As long as the clutch on the pully spins free u can drove it. I would pull the power wire or fuse on the oompressor to make sure it doesnt try to turn on.