2007 TDI Avant - A/c fault - compressor shear link

After being driven to distraction by the issue (diagnosed by professionals as everything but the actual problem) I'm posting this to give others the "gen". (note: sequences shortened for narrative purposes - total actaul time elapsed = 12 months)

So: first symptom was no cold air / econ light on permanently. OK, so that's likely the high pressure sensor - scanner agrees. Buy cheap replacement on ebay - no change. Read up an discover there are two types - visit to Euro Car Parts gets me (apparently) the right one for my reg - no change.

Decide that a it could be low gas as well take it to local independent - he states "its full". Hmmm.



Take car to local Audi electronics specialist who puts the big computer on it and discovers I still haven't got the right sensor fitted (but it could be the wiring). So buy new Audi genuine sensor and off comes the bumper again. Full check of the wiring = OK. Compressor solenoid resistance OK and getting a feed. Fit new sensor and no change.

Lightbulb moment is discovering that this year of A4 has a clutchless (always spinning) compressor that has a shear link to protect it. Back under the car, and oh look you can spin the compressor shaft with the auxiliary belt still on - shear ink sheared!

So, tracked down new pulley / drive plate (no idea what the problem is at this stage so played safe) - arrived from Germany on Friday.

This is the key bit - the design of these things is such that if enough of the rubber buffers wear and break up, they can jam the compressor shaft enough that the shear link fails. Having fought to get the torn remains of the drive plate off the shaft (which f****r thought a 7mm AF shaft end was good idea?) and the pulley / bearing off the the compressor, its a poor design indeed. New pulley / drive plate fitted in 30 minutes, without even breaking the A/C circuit and hey presto I HAS AIRCON!

It's a really simple check - engine undertray off, start engine, observe shaft end, turn engine off. if it wasn't rotating, try and spin shaft with spanner or hex key (variants within variants) to determine if the compressor is failed (shaft won't turn) or shear link failed (shaft turns reasonably easily). Hope this helps someone else...