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I have a '08 Audi A3.

A month ago I noticed that the air conditioning was performing poorly and since I bought it used, I took it in to recharge the refrigerant, without any improvement.

When the car is in neutral, idling or revving a bit, the A/C fan starts working, and in a couple of minutes I have cold air blowing from the vents, as long as the gear stays in neutral. Using a mercury thermometer, the middle vent temp was around 8-10 C.

However, when the car is driven, the A/C stops blowing cold air instantly, until it goes back to idling. There is no indication on the dashboard that the A/C stopped working or is working on safe mode (or econ), like other cars do.

The mechanic could not find any pending fault codes.

This time we noticed that half of the refrigerant was lost, the low-side pressure was at 3, high-side was at 14 with an outside temperature of 26 C.

We recharged again the system with a UV leak detection kit. There are no leaks visible and I cleaned the AC radiator from the outside.

What could be wrong?

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    If you rev the engine when stationary and in neutral and hold it at high RPM does the AC stay on or shut off? – GdD Jul 6 '18 at 11:28
  • I gave it a try until 3500rpm, was working the whole time. Should I try with a bit more? – marksf Jul 6 '18 at 11:40
  • 2 minutes should do.\ – GdD Jul 6 '18 at 11:50
  • It's an external control variable displacement compressor. Something is making the climate control unit/whatever controls how much the compressor is pumping destroke (reduce the compressor's pistons stroke and therefore the refrigerant flow across the system) too much when the car is being driven. The compressor, among other instances, should be destroked when you load the engine up and the cabin has been cooled enough, to provide a better driving experience, but something is just making the compressor cut the pumping action off too much. An OBD scan would be a great start. – Al_ Jul 6 '18 at 13:29
  • You should get in touch with a mechanic/tech who is able to actually check how much your compressor is pumping at any time (particularly elaborate OBD systems, or electrical equipment which can sample the compressor control signal). The stroke valve and therefore the refrigerant flow is controlled by a PWM signal by the way. – Al_ Jul 6 '18 at 13:31

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