Car fault - alternator or battery?

I can't see a battery doing that, even a faulty one. So, on balance I'd say the alternator is at fault.

Ideally though, slot in a temporary battery Just to see... It's the easiest to change, but don't *buy* one to try this!

If the battery has good charge and you have a means of jump starting if needed (another vehicle or battery charger or jumpstart pack) disconnect the alternator, start the car on the drive and see if the problem goes away.

Good ideas, thanks both. Not sure I can source a spare battery, but disconnecting the alternator temporarily is a good plan.

Near me is a place that reconditions alternators and starters for sale to trade and public (Rotec at Sutton in Ashfield 01623 453512). They have rectifiers, brushes etc on the shelf and can test your part to tell what's wrong with it and fix it there and then. Hopefully you will have someone similar near you.

Mowgli
Free Member
Good ideas, thanks both. Not sure I can source a spare battery, but disconnecting the alternator temporarily is a good plan.

Daffy's idea is far more logical/easier to implement, wish I'd thought of it.😁

Depending on the alternator you should be able to remove the regulator as it’s a bolt on item and easily replaced, it may be worth your while if you’re mechanically sympathetic to strip and clean the alternator inside with electrical contact cleaner.

edit : Fill in your car details Here and choose the correct regulator

I agree it sounds like the rectifier. However, how that rectifier is controlled varies (I had an import where the bloody ecu had a hand in it) so I'd dispute that they're usually easily replaced tbh. More often than not they're part of the alt itself IME. However, it could well be the wiring or an earthing fault.

As mentioned above there are pieces that can test the alternator itself. I know one in Coventry if that's any use! After that I'd be getting an auto sparky on the case.

I've rebuilt a couple and they were pretty fiddly TBH, no experience of VAG ones though.

I agree it sounds like the rectifier. However, how that rectifier is controlled varies (I had an import where the bloody ecu had a hand in it) so I’d dispute that they’re usually easily replaced tbh. More often than not they’re part of the alt itself IME. However, it could well be the wiring or an earthing fault.

Some cars do increase the voltage as the battery ages. Whether a 2009 Passat does I don’t know. I guess it depends if it’s one of those cars you have to tell you have fitted a new battery.

Conventional alternator should be around 14.5v and as you switch on electrical loads it should stay pretty steady.
I’ve changed the regulator/brush pack on older Bosch alternators before. Two bolts and done.

I’ve rebuilt a couple and they were pretty fiddly TBH, no experience of VAG ones though.

That really needs a gynaecologist.

Sorry.

I went to try disconnecting the alternator this evening, but the bloody thing is working fine again. Not sure if that means the alternator is ok and it's a loose connection somewhere, or if the diodes/rectifier could give an intermittent fault and still worth replacing?

If it’s an easy job and cheap part/conventional setup I’d do it. Might save you a battery/breakdown, I paid less than £20 iirc last time I did one.

@somafunk and others - finally got round to replacing the regulator on the back of the alternator. £20 part seems to have sorted it - thanks for the tip!