Well i have a 91 VW diesel Golf. At high RPM....which is not that high since its Diesel, anyway the batt light comes on and the lights go dim and all that crap. Then when I let off the gas everything is fine. Also at highway speed around 110 Km/h the light comes on and off every 10 seconds or so. All is fine when Im driving at low speed and RPM. Any ideas? Im thinking it may be the Voltage regulator. Or should I just swap alternators. By the way the battery is new.

Thanks

 

I suspect the voltage regulator too. It's almost as if it is having trouble maintaining the voltage when the alternator is spun up faster.

Is it possible that a loose belt is slipping at high engine speeds - I realize it's a bit of a stretch (no pun intended) but worth checking before you start paying out.

EDIT - okay, I just realized it's a '91 and likely has a tensioner on the belt...

Is it possible that the lights are dimming from the computer firing the glow plugs?

 

he's right check the belt first. I have seen brushes do that often. When they wear down low enough as the alt rpm's go up they begin to arc and lose contact. Only way to be sure is if it has an external volt reg full field the alt(most books describe how)and then measure the voltage at higher rpm. If it goes up and the symptom goes away it most likely a reg. If it charges worse or not at all which it will if the brushes are shot then its up to you to rebuild or replace.

 

Would place my bets it's the voltage regulator & or brushes.