I wanted to start by saying, thank you for all of the information!
My 1999 Mk4 Golf 2.0 has been having some issues with AC lately. Up until about a month ago the AC worked just fine. Then randomly it stopped working, generally. After driving for 15-30 minutes the AC would randomly kick on but the AC compressor made a horrible noise. The car has almost 240k miles, so I figured it was time to replace some parts. I started with the FCM, with a used eBay FCM but that didn't remedy the problem. I figured the compressor was dead, since it would kick on randomly. I replaced everything a few days ago and recharged the system at a local shop. After the system was tested and refilled the AC still did not work when I tried to turn it on. While driving home, 10-15 mins in, the AC kicked on randomly. I proceeded to dig in deeper and debug, but I'm stumped!
What I've done so far:
- replaced FCM with a used OEM unit off eBay (though I'm not sure my old FCM was even broken)
- replaced expansion valve, drier, and compressor; the compressor was a reman unit with a new refrigerant control valve, clutch, clutch coil, etc
- replaced AC pressure switch
- flushed the system with compressed air
- flushed the compressor with PAG oil and refilled with 5.0oz
- tested for leaks, put system under vacuum, and refilled with 750g of refrigerant
I've got through the "
Mk IV Manual AC electrical & cooling fan troubleshooting and repair" mega thread. Here are the
A4 fan troubleshooting tests I've run:
Fan response to Air Conditioner
The car failed when performing the first test:
Remember both fans should come on in slow speed any time the key is on and the AC
is on – if the temperature is above about 32 F (0 C). If one fan runs, so should the other
– they do not sequence on, or anything fancy...
Low refrigerant pressure will lock out both fans and the AC clutch.
High refrigerant pressure will put both fans in Fast speed.
I couldn't find any information what actually causes this failure. I need to double check the pressure, but I JUST had the system filled so I doubt that's the problem.
Next, I continued with some more tests, so I would have more comprehensive data:
- Verified S180 (fuse 8, 30 amps, green plastic, above battery) are OK
- Verified S164 (fuse 3, 40 amps, metal strip, above battery) are OK
- Verified 12V at all the connections and strips on top of the battery
- Verified fuses 5, 16, and 25 in cabin are OK and the FCM has power
- I removed the airbox/battery and checked for frayed wires or shorts and found nothing
Fan Response to Temperature Signals
- Verified slow speed fans from the radiator thermoswitch by shorting pin 1 to pin 2 with the key off; both fans ran at slow speed
- Verified high speed fans from the radiator thermoswitch by shorting pin 2 to pin 3 with the key on; both fans ran at slow speed
- Verified the resistance on both fans were within spec.
Am I missing something really simple? Thanks!