The brief
Dean came in sweating, the aircon in his A4 wasn't cool, and he asked us to check the air conditioning compressor, an expensive part most people assume is the culprit. But aircon faults are easy to misread, so it got a proper look rather than a swap. The compressor pumps the refrigerant round the system, and it's driven by a belt over a set of pulleys. If something else binds in that area, the compressor can't turn freely, so the aircon doesn't do its job, even though the compressor itself is fine. On this car the radiator cooling fan had jammed and was restricting the pulley's movement, so the cure was the radiator fan, not the compressor. A jammed fan doesn't free itself, so it gets replaced.
The diagnosis
On the lift the visual inspection of the compressor area showed the real fault: the radiator cooling fan had jammed and was restricting the pulley's movement, which is why the aircon wasn't cooling. The compressor itself was fine, no need to replace it. That's a radiator fan replacement, not a compressor, so a fraction of what Dean expected to pay.
The work
The jammed radiator cooling fans were removed and new genuine Audi-spec fans fitted, the wiring and the fan switching checked, and the pulley confirmed turning freely again. The aircon was checked over, the system confirmed cooling, and the belt and tension checked. A check at the vents confirmed cold air at idle, holding cold under load, the fans cycling normally, and no warm running.
The outcome
Cold air at the vents at idle and on the move, the pulley turning freely, the fans cycling normally, and the aircon whispering cool again, with the compressor untouched because it was never the problem. Dean got the A4 back with the aircon fixed and a tidy sum saved. Looking properly rather than assuming the compressor meant the part that had actually jammed got replaced, which is the difference between a fans job and a compressor bill.
