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I want to repair my Volkswagen Passat '07 A/C. I've taken it to two shops (dealer and independent) and they both say the entire a/c system needs to be replaced. The strange thing is, that sometimes it actually comes on and when it does it works great. The rest of the time I can't get it to come on at all.

Is there a way to force the A/C to stay on when I tell it? I've heard that there is a limiter that keeps the compressor off when certain sensors are not reading correctly to protect the compressor from damage. In my case though they're recommending replacing the entire system so I don't see any problems forcing it to work until it can't cool anymore. Also does it make sense that I need a new compressor when it works great sometimes?

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What exactly does the entire system include? So far sounds like you need a third opinion. – Move More Comments Link To Top May 12 '14 at 19:29
    
compressor, Receiver/Dryer, expansion valve, probably the condenser. They said something about metal shavings in the system from a bad compressor. – JMC May 12 '14 at 19:41
    
Also to note, the dealership said they could get it working by just replacing the compressor, but that it would fail again over time if I didn't replace the entire system. – JMC May 12 '14 at 19:48
    
That sounds more reasonable, and if it had an internal compressor failure that's usually the course of action. But if it works sometimes it makes me wonder if it's an internal failure of the compressor. How do they know their is metal in the system? Did they see it, or are they assuming. – Move More Comments Link To Top May 12 '14 at 19:54
    
In reality i'm not sure about how the "know", but they acted as if they looked in the system. I never trust the dealership. The independent shop recharged it and that's when it started working intermittently. – JMC May 12 '14 at 20:00

If the system is either working fine or not at all then the likely failure is in the control gear for the system, the AC clutch would be my first port of call, you'll need an assistant:

Open the engine bay and look for the AC compressor, it is one of the ancillaries at the side of the engine run by a serpentine belts. The compressor pulley has a clutch built in, so that the pulley always spins, but it only drives the compressor when the AC is switched on (activating the clutch).

AC Clutch

You can see if the clutch is working by:

  1. Locating the AC compressor
  2. Starting the engine, with the AC off
  3. Observe that the middle of the pulley is stationary, this means that the clutch is not engaged
  4. Get your assistant to turn the AC on, you should hear a click and observe that the middle of the pulley is now spinning

If it does not spin then you've either got an electrical fault or the clutch has failed. If the middle of the pulley is not spinning you can try to get a multimeter on the electrical connector and read the voltage when the system is turned on

  • If there is 0v you have an electrical fault
  • If there is ~12-15v the clutch has failed and the compressor should be replaced
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