Alternator not charging, but works!

[mattk918]

Hi All. First post on here and I have a 2007 Polo 9N3 1.4TDi (BNV) which I am hoping for a little help with!! I have searched this and other forums and I can't find quite the same problem anywhere.

The short (ish) version is I let the car go flat over Christmas due to lack of use. Ended up doing this a couple of times previously and have always just jump-started it and then driven it for a bit. This time I decided to re-con the battery using a ctek charger and after doing this I had a permanent battery light on the dash. Whether this is co-incidence or I managed to short something as I connected it, who knows!? So it runs fine, but alternator isn't charging. However if I put a (unregulated!) voltage onto the activator/exciter wire it starts charging and the battery light goes out, but then runs like a bag of spanners (voltage maybe feeding back into the J519 module on the output to the alternator?) and I get powersteering fault if I actually drive it. So I'm assuming alternator is OK

I've fitted a new battery, no difference. Checked all the dash fuses and engine bay fuses, all seem fine. Nothing of note on VCDS.

Digging a bit deeper it seems I am only getting about 3.5v on the output from the J519 module to the alternator (Blue wire, Pin 9 of the brown 18 pin plug). Convinced myself it must be the module at fault so picked up one off ebay which I fitted today....... no change! I guess it is possible it has the same fault, but that would be pretty unlucky!

The only thing I wondered about trying, but haven't yet is the Terminal 30 relay (109), wondering if it had partially failed, but I thought that the car wouldn't have run if that was the case? Could the power steering fault when forcing the alternator on indicate that the power steering is also seeing low voltage from the J519 (haven't checked that, don't have wiring diagrams to hand anymore)

So throwing it out to the forum, any other bright ideas on what to try next or what I'm missing??

Thanks in advance

[Johntheo1]

Presume you have read all the threads like this one?. even though for a much older version.
viewtopic.php?t=41114

[mattk918]

I have looked at that one, and other. Most of the examples I've found are predominantly issues with a broken signal wire, rather than low voltage on the signal wire.

However I am now wondering if it actually is just an alternator fault and i've been mis-interpreting how it should work. After a bit of playing this evening I measured 12v (ish) on the signal wire with just the ignition on, dropping down to 3v (ish) once the engine is running.

[Johntheo1]

Cpould you pick up a alternator in a scrap yard, there should be plenty of them around now, some of them will take it back if it doesn't work.

[mattk918]

Obviously I'd prefer not to just keep randomly throwing parts at it until something fixes it, but I guess if there are no other suggestions that might be the route i have to go down....

[mattk918]

For those who like closure on these things, an alternator has indeed cured it!

I was originally convinced it wasn't the alternator since it did charge when I forced it on. I had checked the earths and the alternator was earthed fine to the block and I was measuring only 3-3.5v on the activator/excitor wire. However the breakthrough was when I pulled the wire and realised the voltage on the wire jumped back up to battery voltage. Therefore ultimately there must've been an internal earth fault in the alternator. Kicking myself I didn't start with that. Thanks to those who advised....