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Discussion Starter #1
Good Evening,
I'm trying to troubleshoot an A/C oddity. I've got a '91 running the original R-12 system. Upon startup, the A/C kicks on, compressor engages, and blows nice and cold through the vents. However, after about 20min of driving in the heat, suddenly the A/C stops blowing cold. Popping the hood confirms that the compressor is still turning, but appears to be noiser than normal. If the car is left to park for 10-15 minutes, the A/C is cold again, but only for a few minutes. Then it reverts to the warm air.
Could this be a sign of a worn Compressor Clutch? If so, can it be replaced without changing out the compressor, or is it all one unit? Thanks!
-Lloyd
 

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Worth checking the heater valve pack isn't sticking open before
you dive into the AC circuit looking for faults...

I suggest you run the car on a coolish day with the AC off, and heater
set to COLD....and see if the vents blow warm air after 15-20 mins.

A rise in temperature from the vents indicates the heater valves aren't fully
closed...which will lead you to chase an AC fault.

Start here, as you've already said the compressor is running as it should
even after 15-20mins.

The only state thats really changed after 20mins is coolant temp.

Good luck,

Regards,

Alan.
 

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A quick check for the air-con side would be, find the high and low pressure pipes in the engine bay, if all is well with the air-con one should be ice cold with condensation forming on it and the other should be hot, maybe even too hot to touch. If this is not the case it would suggest a fault somewhere within the air-con system.
Worth also checking that the electric fan is running whenever the compressor is engaged as this is critical to get the heat out of the air-con circuit.
Is the compressor cycling on and off or is it running continually? If it is on continually then this could be a sign that you are low on gas. I think it is a pressure switch that cycles the compressor so if you don`t have enough gas the pressure can`t be achieved to switch it off.
These thoughts are based on my car which has 134a but an r12 system is basically the same. ( I think??!!)
 

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Does the air flow decrease with time? If your charge is low your evaporator will form ice, air flow will decrease and air will feel warm.

After your experience this problem pay attention to the evaporator drain that ports condensation out of the evap housing. If the charge is low and evap is freezing up you will see a much larger than normal puddle of water as the ice melts.

If this is the case you have a freo leak somewhere that must be repaired.

Not real familiar with the M5 AC, mine works fine so I haven't got into the system. Other sensor can cause this same problem

If you have AC gauges you can see what your high side pressure is. A GENERAL RULE is the pressure on the high side should be 2.3X the ambient temperature taken at the grill. eg 90deg F would be 207 psi high side. If is much lower than this you are low on freon. If it shows a good charge other sensors or mechanical parts may be at fault.

Without gauges it is very difficult to troubleshoot AC problems.

Good Luck
 

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Also, a worn compressor clutch will let you know by a horrible noise as the bearing fails. If it is engaged and howls, squeals or other wise make an aweful noise it usually be the compressor getting ready to fail. Compressor won't spin freely and clutch will drag under extra compressor rotational force due to impending failure. Neither case is good.
 

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Discussion Starter #6 (Edited)
Don't know how many of you saw my most recent posting. Regardless, I ended up getting a gauge set to test pressures (quite suprisingly the "R134A" test set actually had R12 connectors which then linked to the R134a disconnects, so I was able to use the same gauge set up without switching over my ports).
Anyway, the "bad" news is that High side is running right at 150psi, while the low side starts at around 50psi, but over 10min gradually reduces to 0psi. I have to assume this means I need more R12. I last had it filled in 2004, so I'm hoping this minor amount is due to normal leak-off. If not, I'll probably end up stripping the system and switching either to R134a or R12a. We'll see. At least now I know the cause of the problem!
Thanks for all the troubleshooting help!
-Lloyd
 
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