Very strange battery/alternator problem?

[Barn]

ok, guys,
i have oct 2002 TDI 130 sport with the limited edition launch dashboard (extra batt and oil gauges).

So having the batt gauge i have got into the habit of allways watching it when i start the car!

just recently i noticed that after i start the engine, instead of going up from 12v to 14v as normal, it was going down to 10v and staying there!

There is no squealing from the fan belt so i don't think that is slipping and no charge warning light on the dash either!

now,this is the strange part!........
If i rev the engine it won't start charging until i hit exactly 3500 rpm!
As soon as it hits 3500 rpm the gauge shoots up to 14v as it should and then it will stay there, even just ticking over with lights, heater, stereo on etc, it stays at 14v like it should untill i stop the engine! If i restart right away, it's ok, if i leave it a few hours, i have the fault again.

I have tested the battery and it is fine, so the question is, why will it not start charging until i rev it up to 3500 rpm? and why,if there is a fault with the alternator, does it then work as normal once i have reved it to 3500 rpm?
surly an alternator either works, or it doesn't?

Is there something that the engine management does when the engine revs hit 3500 rpm or something?

thanks in advance,
Barn

 

[DEAN0]

Check the wires/plug pointed out in this thread - they seem to cause some odd electrical issues.

http://www.seatcupra.net/forums/showthread.php?t=209472

 

[RUM4MO]

check the wires/plug pointed out in this thread - they seem to cause some odd electrical issues.

http://www.seatcupra.net/forums/showthread.php?t=209472

+1 !

One is the battery feed to get the alternator "excited" so that it will generate power - it is said that at around the engine revs you mentioned, "self excitation" occurs so that is why it works okay then.

 

[Barn]

thanks guys, i'll take a look