A/C gurus confirm or deny please

I have been diagnosing a no a/c situation. I just replaced the FCM due to internal damage. I also have tested the fan operation by jumping the coolant thermos witch. Both low and high speed work.

However, when I turn the power on and push the a/c button (it lights up) but the fans do NOT engage. Also the voltage at the compressor plug jumps from .5 a 3.00volts. The compressor will engage if jumped with my power probe. All fuses on top of battery are intact as well has in the cab. Gauges hooked to service ports displayed about 110 psi low side and 100 high side at idle and 85* outside

This leads me to two options that I have found during my research.

1. low charge
2. Temp sensor in cowl bad

I have found threads with similar symptoms but no definitive answer.

Can anyone confirm the drop in voltage due to a interlock?

Charge is acceptable for running conditions.

Overdrive0902 said:

I have been diagnosing a no a/c situation. I just replaced the FCM due to internal damage. I also have tested the fan operation by jumping the coolant thermos witch. Both low and high speed work.
However, when I turn the power on and push the a/c button (it lights up) but the fans do NOT engage. Also the voltage at the compressor plug jumps from .5 a 3.00volts. The compressor will engage if jumped with my power probe. All fuses on top of battery are intact as well has in the cab. Gauges hooked to service ports displayed about 110 psi low side and 100 high side at idle and 85* outside
This leads me to two options that I have found during my research.
1. low charge
2. Temp sensor in cowl bad
I have found threads with similar symptoms but no definitive answer.
Can anyone confirm the drop in voltage due to a interlock?

Click to expand...

Don't you also have to put your interior fan selection to "2"?

The fan must not be "OFF", but if the AC button lights up it's not in the OFF position.

Correct, I turned the interior fans on as well. The high side port has a small leak coming out of the valve, I add a 12 oz can of R134 with stop leak (lowside is now sealed). I will jump the temp sensor this evening. If I get a new result I will hook the gauges up and go from there.

the voltage drop is puzzling, but I'll will note my findings this evening

Well jumping the ambient temp sensor had no effect. Adding refresher had no effect. Gauges still at 110low -100high. Jumping the compressor dropped the low side to 90 psi.

Still no rad fans. Maybe the coolant thermoswitch is bad. I hate to keep throwing parts at it but maybe it's time to admit defeat and take it to a VW shop

I read above that you had checked the fuse but did you check the connectors under them. My fuses were good on top of the battery but my fans didn't come on. Reinserted them several times and the fans started working and have for 3 years now.

I was testing with a power probe and read over 12 volts for all fuses. I guess I can probe the harness going into the FCM

I JUST got through a very similar (if not the same) situation tonight that took me a long while to diagnose.
My AC light lit up, but NO cooling fans were spinning.

#1- Check your fuses

#2- Are you sure the fans are spinning? Check the fans individually using a jumper wire.
#3- Check the system by jumping the connections in the fan thermal switch connector. The system should spin the fans on low jumping 1 and 2.
Then jumper 2 & 3 and the fans should spin on high when you turn the ignition key on.


#4 You may want to see if you are getting power to the AC high/low pressure switch. If you are getting voltage from the 3 connector switch, then check the white wire coming from it to see if you are getting a duty cycle above 20%. (you will probably have to pierce the wire with a safety pin).
If not, suspect a bad hi/low switch.

You can remove these while the AC system is charged, as there is a schraeder valve under the switch. Even better is to borrow a switch from someone.

#5 Check on the FCM. Swap that out if you can.


Good luck!

Manual AC?

I am under the impression but have to confirm that the compressor clutch 12V comes directly from the FCM.

Can anyone confirm that?

don't forget the ambient temp switch, if faulty it will stop the ac from coming on ..

But the ambient temp switch would result in 0 volts rather than 3 volts.

You need to do the AC troubleshooting in the PDF linked in this post:
http://forums.tdiclub.com/showthread.php?t=242699

Any measurement of the voltage to the compressor clutch coil must be done under load- with the coil hooked up, not unplugged. Any unloaded voltage measurement of the signal to the coil is essentially meaningless.

Is the top of your HP sensor oily? If so this is a sign that it is failing. The best way to tell, though, is with the troubleshooting mentioned above.

VChristian said:

But the ambient temp switch would result in 0 volts rather than 3 volts.

Click to expand...

not necessarily, the ambient switch is not in series with clutch coil