Please give this another analytical look folks -- I really could use the diagnostic help!
Spent the entire day starting from scratch on this. Forget what's been writen prior. Here's what I know:
2004 Golf BEW TDI
Problem: S180 fuse blows (fuse block atop of battery) when AC is activated (AC button pushed and cabin blower on) but only with the engine running – it cannot be made to happen with the engine off. This problem is intermittent but frequent. I can go days between fuse blowing episodes or blow a dozen fuses in a row. Also, it occurs only when activating the AC – it does not happen once the AC has already made it through being activated, however if I turn off the engine, it will next occur only during AC activation at some point in the future. On those occasions when I turn on the AC and the fuse does not blow, the AC is nice and cold. So essentially, everything works as it should except that frequently, when the engine is running and the AC is engaged, that S180 fuse will blow immediately upon AC engagement – there is no lag to the fuse blowing -- happens right away on compressor activation.
The clutch coil shows about 3.5 ohms of resistance measured at the harness connector.
I can find no shorts in the wiring between a) fuse S180 and the coolant thermo switch, b) in either of the two cooling fan harnesses, c) in either of the two fan control module harnesses, or d) in the compressor clutch coil harness. I used the instructions from “A4 Coolant Fan testing, MANUAL Air Conditioning - REV 7 6/18/2009 referenced in the “check your AC fan function sticky” as the methodology.
I have replaced the fan control module with no change.
Both cooling fans are about 2 years old. I have tested them by jumping and with direct power. Both are functioning perfectly on both low and high speeds. Both fans show between 0.7 – 0.9 ohms when measuring across the pins at the fan motor harnesses (pin 1 to 2; 1 to 3; 2 to 3).
Both fans come on at low speed with engine off / AC button on / cabin blower on – as they should.
The battery top fuse box was installed in May after the (commonly reported) melting of the box at the S180 fuse slot. I also replaced the alternator cable to the top fuse box at that time with a larger gauge over-sized cable.
The alternator is functioning as it should.
I believe that I have essentially eliminated:
- the fans and their harnesses
- wiring (I can’t find any suspect wiring, dead shorts, or make anything short to ground with wiggling)
- the FMC
- the coolant thermo switch
My impression is that this problem must be related to either an intermittent short along the clutch coil cable or at its plug connection, or perhaps an intermittent clutch or coil problem causing a resistance spike on compressor start up. This fuse-blowing problem requires two things to occur – the engine must be running and the AC must be activated (cabin blower on with AC switch on) and it then either blows immediately or, if it doesn't blow, won't while the AC is running once it makes it through compressor start up. I don’t have the diagnostic tools/know how to troubleshoot the compressor (AC high/low pressure), etc., if that’s even a possibility of being the problem. It just seems to me as though there is some source of resistance that sometimes rises to the level of blowing the fuse on engaging the AC compressor.
Could a failing clutch or coil cause this and if yes, intermittently?
Where would you go from here? What have I overlooked?
I am prepared to take it in to a shop 10 days from now and if I can't find the problem, want to at least help them by focusing on a particular area.
Thanks very much for the assistance -- this is a bugger!
