A/C Electrical Diagnosis

[katies99]

A/C Compressor is not kicking on. I disconnected the two wire connector and put a test light on the harness side and nothing. I put 12V to the compressor side and the clutch engaged. I verified the 10A fuse for the compressor is in place and not blown. Where do I go from here?

[katies99]

OK, I have been doing some serious research online and I found a site where a guy has the same symptoms as mine but when he revs the car above 2000 rpm the ac kicks on. Tried it and so does mine. Is this normal? Some sort of 'car must be moving let's keep it from overheating safety feature'?

[Hackster]

Well, first, the A/C has nothing to do with keeping the car from overheating (except for the driver and passengers...)

If the refrigerant level gets too low, resulting in low system pressure, most systems have protection switches that will prevent the clutch from kicking in, in case the compressor oil has leaked out along with the refrigerant, to keep the compressor from failing. It may be that there's just enough freon left in your system that the pressure builds enough at higher RPM to cut off that safety switch and let the clutch engage.

It may well be something else, but the place to start is to have the refrigerant level checked by a reputable shop. Good luck with it.

[MyOtherRidesAVette]

Originally Posted by Hackster

Well, first, the A/C has nothing to do with keeping the car from overheating (except for the driver and passengers...)

If the refrigerant level gets too low, resulting in low system pressure, most systems have protection switches that will prevent the clutch from kicking in, in case the compressor oil has leaked out along with the refrigerant, to keep the compressor from failing. It may be that there's just enough freon left in your system that the pressure builds enough at higher RPM to cut off that safety switch and let the clutch engage.

It may well be something else, but the place to start is to have the refrigerant level checked by a reputable shop. Good luck with it.

Actually, some cars have controls that force the a/c to shut down and keep the engine cooler, under certain events. I had a Nissan 300 ZX years ago, that when climbing certain very step mountain passes would shut down the a/c. And if your car is overheating, shutting down the a/c is the first thing you want to do. Then turn on the heat, high!

Yes, and btw, you won't have any pressure building in the system without the clutch engaged and the compressor running, the compressor is what pressurizes the system.

MORAV

[katies99]

Originally Posted by Hackster

Well, first, the A/C has nothing to do with keeping the car from overheating (except for the driver and passengers...)

If the refrigerant level gets too low, resulting in low system pressure, most systems have protection switches that will prevent the clutch from kicking in, in case the compressor oil has leaked out along with the refrigerant, to keep the compressor from failing. It may be that there's just enough freon left in your system that the pressure builds enough at higher RPM to cut off that safety switch and let the clutch engage.

It may well be something else, but the place to start is to have the refrigerant level checked by a reputable shop. Good luck with it.

I checked the refrigerant and it is fully charged. I am assuming that this is a safety feature that VW has put in place to protect the engine from having to power the AC when the car is at an idle (thus preventing overheating). The AC blows very cold when I rev up the motor with the transmission in neutral or when driving. When I allow the rpms to drop then the compressor shuts off.

[MyOtherRidesAVette]

Originally Posted by katies99

I checked the refrigerant and it is fully charged. I am assuming that this is a safety feature that VW has put in place to protect the engine from having to power the AC when the car is at an idle (thus preventing overheating). The AC blows very cold when I rev up the motor with the transmission in neutral or when driving. When I allow the rpms to drop then the compressor shuts off.

See post above yours. Actually the cooling fans should take care of this. This should also be able to turn the cooling fans on by turning the a/c fan to position "4".

M.

[katies99]

Originally Posted by MyOtherRidesAVette

Actually, some cars have controls that force the a/c to shut down and keep the engine cooler, under certain events. I had a Nissan 300 ZX years ago, that when climbing certain very step mountain passes would shut down the a/c. And if your car is overheating, shutting down the a/c is the first thing you want to do. Then turn on the heat, high!

Yes, and btw, you won't have any pressure building in the system without the clutch engaged and the compressor running, the compressor is what pressurizes the system.

MORAV

But is my 99 beetle one of them or do I have a defect? I am in the Pacific Northwest and outside air temperature was 66F today. Not exactly blistering heat but I want to get this to work properly.

[Hackster]

Originally Posted by MyOtherRidesAVette

Yes, and btw, you won't have any pressure building in the system without the clutch engaged and the compressor running, the compressor is what pressurizes the system.

MORAV

Not quite correct. Yes, the pressure isn't "building" without the compressor running; however, the freon charge itself creates a small amount of pressure in the system at rest. The low pressure switch is designed to prevent the clutch from engaging at all if the charge is too low.

[MyOtherRidesAVette]

I don't know if VW engineering built this in. Seems to me it may be a problem with the car running hotter than it should, but possibly not hot enough to trip the overheat warning light. Hate these cars not having proper guages. At 66° you should not be having problems like if it were 100°. Are you experiencing this same thing when starting it cold? Or does it just start doing it after the engine has come to operating temperature?

M.

[MyOtherRidesAVette]

Originally Posted by Hackster

Not quite correct. Yes, the pressure isn't "building" without the compressor running; however, the freon charge itself creates a small amount of pressure in the system at rest. The low pressure switch is designed to prevent the clutch from engaging at all if the charge is too low.

Yes, I understand the refrigerant itself is under low pressure in the system. But without the compressor running, how would any pressure build? Btw, the refrigerant is not Freon. "Freon" is a Dupont trade name for R12. Thought you might like to know that.

M.

[Hackster]

Originally Posted by MyOtherRidesAVette

Yes, I understand the refrigerant itself is under low pressure in the system. But without the compressor running, how would any pressure build? Btw, the refrigerant is not Freon. "Freon" is a Dupont trade name for R12. Thought you might like to know that.

M.

Yep... I used the word "freon" like some folks use the word "kleenex". I should have specified R-134 to ensure that I wasn't confusing anyone. Thanks for clarifying that (and if you hold any financial interest in the "freon" trademark, please don't sue me, okay? ).

And, again... I'm not arguing that the compressor must be running to "build" pressure. Sorry if I didn't make that clear. My point was simply that there is a switch present in the system that will disable the clutch if the system isn't already pressurized (charged) with... R-134.

But since the OP states that the system is fully charged, it's a moot point anyway, yes?

Cheers!

[katies99]

Haven't tried it cold. The fans both come on when the ac is turned on but if the car is not running above 2000 rpms then the compressor does not kick in/shuts down.

BTW, I was an ac & refrigeration tech back in the navy days so I am thoroughly familiar with the refrigeration cycle including when there is pressure or not. This refrigeration system is mechanically functioning perfectly. I am, however, not familiar with the specific operating characteristics of the electrical controls on a 1999 VW Beetle and that is what I am asking about.

[Hackster]

Originally Posted by katies99

Haven't tried it cold. The fans both come on when the ac is turned on but if the car is not running above 2000 rpms then the compressor does not kick in/shuts down.

BTW, I was an ac & refrigeration tech back in the navy days so I am thoroughly familiar with the refrigeration cycle including when there is pressure or not. This refrigeration system is mechanically functioning perfectly. I am, however, not familiar with the specific operating characteristics of the electrical controls on a 1999 VW Beetle and that is what I am asking about.

Thank you for your service. I have two stepsons in the Navy- one in Afghanistan, and another in school at Great Lakes. Sorry I wasn't able to help with your problem. Good luck with it.