What drives A/C Compressor?

Hi,

My A/C stopped working last Friday. It stopped working like this every 18 months as the Fan control module simply burned. Appears to be an electrical problem every time. I replaced the module and A/C worked fine again.

I replaced both fans last year.

This time is different. Both fans still work when I press the A/C button but no cold air comes in. Engine RPM drops slightly. I assume because both fans kick in. Problem is I don't think the compressor kicks in. Any suggestions what the problem might be?

Is the compressor driven by a belt and activated by the clutch? I smelled burnt right before A/C failed. Is it possible that the belt broke?

TDI MK4 Golf 2001.

The same belt also drives your power steering and alternator. You'd have a dummy light as well as a very difficult time turning the steering wheel if the belt broke.

Time to figure out why your AC fan control module keeps burning up!

Thx.

I put a new module that I had bought for $30 and cracked open with a hammer the old module but it wasn't burnt this time so I guess I need to find out why the clutch is not engaging.

I'm looking for some posts on how to manually activate the clutch.,,

Put the +-12V from the battery to the clutch connector and it should engage. But ypu can chwck for lower at the connector too. If there is one, the clutxh is fired. My bet is that it is.

I am betting that the A/C clutch is no good. I normally see cars come in with the small rad fan not working and a bad A/C clutch. If you replaced the fans because they were not working it might have put extra load on the A/C system.

I replaced the coil in the clutch last year because a used coil just happened to come my way. I had the same symptoms - burning smell, no a/c.

It was the coil that burned out causing the clutch to no longer engage.

Anyways the used coil I had put in was iffy so I got a year out of it and now I am going to replace the entire clutch assembly which turns out is pretty inexpensive. The clutch assembly includes a new coil.

The coil is just an electromagnet that engages the clutch magnetically. You can wire a switch and relay to it. I don't know if there an any ill side effects as the car reads throttle position, temp, at the least before allowing the clutch to engage.

You hardwire that AC to a relay and you will fry the system in a short time. There are pressure switches in the system and many other parapmeters that dictates how the system operates. Find the cause of the problem and don't just bypass it.

Not to highjack the thread, but I have a very similar issue….

I checked the low side fitting for the ac and it buried the needle on my cheapo ac filling valve. The clutch is fried on the ac compressor (smelled like burnt lining as well) and there is no resistance at the coil. I know I need a new clutch and coil, but when I turn the compressor it takes high efforts to turn. I have checked a buddies TDI and his is easier to spin. Is it because there is too much pressure in the lines that the compressor is hard to turn or is there debris in the compressor?

I also probed the line going to the compressor coil and it is surging….. There will be a split second of power, cut out for 3-5 seconds, then another split second of power. Does the fan control module send a signal to the coil like that if there is too much pressure in the line? Or does it sense that the coil is an open circuit and fault to this setting?

Any ideas would be obliged.

If the compressor is hard to turn, then it is bad.

DanG144 said:

If the compressor is hard to turn, then it is bad.

Click to expand...

Is there anyhting else that needs to be R&R'd in the ac system when doing a compressor? I dont want shavings if it went bad getting into the new compressor.

Also, should the power feed going to the compressor be surging like that with the symptoms mentioned? Thanks for the help,

Curtis

If there are metal shavings, you need to have a very good flush done.
You may want to replace the condenser, rather than depend on the flush.
You will still need the flush.
You will need a new filter drier.
A professional would also replace the TXV (I usually do not.)

The power to the clutch often cycles when it is bad, apparently a protective feature of the FCM. If the clutch is working and it is cycling off and on, that is abnormal, and is often a sign of a bad High pressure sensor.

DanG144 said:

If there are metal shavings, you need to have a very good flush done.
You may want to replace the condenser, rather than depend on the flush.
You will still need the flush.
You will need a new filter drier.
A professional would also replace the TXV (I usually do not.)

The power to the clutch often cycles when it is bad, apparently a protective feature of the FCM. If the clutch is working and it is cycling off and on, that is abnormal, and is often a sign of a bad High pressure sensor.

Click to expand...

Ok, here's a question for you then Dan. I sit at idle sometimes in my car at lunch time. I keep the A/C on, and do hear and feel it engage and disengage. Should it not cycle at all when the a/c is on? Or is there an acceptable time period that should pass between cycles?
I had a new compressor installed last year, if it is cycling like I think you are saying, I want to fix the problem before it happens again.
Is there a diagnostic for the High pressure sensor besides this?
Thanks

Aaron

You can check the output of the HP sensor by watching a pressure gauge and looking at the duty cycle on a meter.

Make sure that both of your fans are working. Your fans can shift to high, but then pressure should come down.

I have never seen a properly charged, healthy system on a mk IV cycle off due to overpressure.

If your clut h is cycling too often, the pressure is probably too low.

No, usually if the clutch is cycling the pressure (actual or only what the HP sensor sees) is too high. The sensor is on the high side. So if the pressure is high enough to allow operation, the pressure only goes up; it cannot cycle on low pressure.

This is totally different than the systems that are built to cycle on low pressure on the suction side.

When I replaced my ac clutch recently, my mechanic discovered the bracket holding the coil lead connector had come off and the coil lead was rubbing against the pulley, causing a short. It was causing the ac clutch to disengage randomly.

Replacement clutch was fairly inexpensive and came with a new bearing, shims, coil, and clutch assembly. A/C is blowing ice cold again.

If the system has discharged, you have a leak. I have not had to refill the ac system at all.

DanG144 said:

No, usually if the clutch is cycling the pressure (actual or only what the HP sensor sees) is too high. The sensor is on the high side. So if the pressure is high enough to allow operation, the pressure only goes up; it cannot cycle on low pressure.

This is totally different than the systems that are built to cycle on low pressure on the suction side.

Click to expand...

I was talking about the overall system pressure. Low, as not enough refregereant, regardless where the pressure sensor is located, if there isn't enough refregerant, but not completly empty, the system will cycle at a rapid rate.

I cannot see how this can happen. Have you seen it happen on a system like ours?

When the compressor kicks on it moves refrigerant from the low pressure side to the high pressure side, and raises the HP side temperature to boot. How can the pressure drop on the HP side from this?

The variable displacement control valve (Refrigerant control valve) will work to try to maintain about 30 psi of suction pressure, dropping the displacement and lowering the pumping rate. Since the LP side pressure is controlled at a greater pressure than the HP side low pressure cutoff, it is very difficult for me to see the operational mode you are describing.