Every time I started the car, there seemed to be delay before the AC compressor engages. The scenario is, starting the car, turning AC on, check compressor, and the clutch is not engaged. After driving around for a while it would kick in and the vent blows cold. Today is really hot and I did several test, sometimes the compressor kicked in immediately after starting the car, sometimes after 10 minutes. At one time I drove the car for 20 minutes and it did not kick in at all. Once it did kick in and if I did not turn the car off, it'll keep working till I turn the car off.
I tried to check the pressure at low pressure port (the one with 'Max 6NM', am I right?). The pressure seems to be between 30 and 45psi (the pressure gauge was a really funky one, no markings between 24 and 45 psi) with engine running but compressor unengaged. Is this within the spec, or should I measure that when the compressor is running? The refregent has green dye added in it and made quite a mess when I checking the pressure.
Thanks in advance.
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1994 944NA 100K miles

This could be your clutch cycling switch going bad. On a 240 & 740 it is mounted on the receiver/drier. It screws onto the drier and has a plug going into it. When the low pressure drops to around 18-22 it kicks the clutch on. As pressure rises to around 45 it disengages the clutch.

You can test by removing the plug and jumpering the contacts in it with a piece of wire. If your clutch kicks in and stays on then the switch is probably bad.

You can get one at almost any parts store for around $15.


Good luck
Danny Mac

Thanks Danny. My car is a 900, but it seems to have the same set up. Today I unplugged the plug from the low pressure switch on drier and short the two terminals for several times. Every time the compressor kicked in. Provided the refrigent pressure is within spec, does this give a pretty confirming diagnosis of bad switch?
I browsed through the website of Advanced Auto, and found they have low pressure switches of differnet colors. Mine is black, but the metal body is grey, while the black switch on their website has a brass body. Are switches between different colors interchangable?
And the last question, how to replace the switch? Is it a simple screw off, screw on, or there are more things I should be aware of?
Thanks a lot.
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1994 944NA 100K miles

From what you describe, I can't see how you determined your charge is where it should be. All you've done is override the clutch cycling switch. If the charge is low, you could be forcing the compressor to come on when it shouldn't and ruin it because a low charge = low/no oil circulation. Put a gauge on the low side and see what the pressure is. If it never gets to 23 psi or so, the switch shouldn't throw and the clutch shouldn't come on.

Yes I would say your switch is bad. From what you described it sounds like your pressures are okay.

Don't know about all the different colors, probably just manufacturer specific. Changing it is very simple... like you said just unscrew the old one & screw on the new one. The fitting on your accumulator has a valve to keep refrigerant from escaping so don't worry about that.

About a five minute job! Hope this helps!

Good Luck
Danny Mac

Thanks Danny. Today I changed the switch with one found on a 93 940 from junkyard. The 'odds' of the compressor engaging is higher now (5 out 7 times it engaged right away, and the other two times it engaged after I pushed the AC button on and off a couple times). Now sure if it's the fix at all as the one from junkyard could well be malfunctioning, too. I'll keep an eye on it for a couple days and see how it acts. Thanks again.
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1994 944NA 100K miles