MGB Register Forum

Dave,
auto electrics isn't my strongest suite, but I could suggest the following:
1. Is the alternator producing any output, or is it just the red"idiot light" that's not working? I have a simple voltmeter which plugs into the cigarette lighter, and it reads the battery voltage with the engine off and the alternator output with the engine running. Cheap from eBay. Would confirm a couple of things.
2. Perhaps start checking for continuity both to the red light and from the red light. Apply a small current and see what lights the bulb💡. Perhaps start with the wire from the alternator. Wiring diagram invaluable here.
3. Substitute a known good alternator!!

Ian F

Ian

It's definitely the alternator - I can get the ignition light on by grounding the wire that's connected to the alternator.

Search engines say it's probably the brushes or rotor but they both seem OK !

Before I get a replacement I wondered if a faulty regulator may cause it ?

it's not producing a current (tested with a multi-meter while running)

Cheers

Dave G

Dave, at this stage I think I would have to resort to a circuit diagram and proceed with substitution of components. Starting with the cheapest!

Others with better knowledge of electrons behaviour than me may be able to suggest alternative approaches (Vic?)

Ian F

You have to decide whether to keep replacing bits of the existing alternator or get a reconditioned one. Check the alternator plug wiring first though..

Vic

The plug wiring looks OK.

I'm curious so might remove the voltage regulator and test it on the bench - there's probably a U tube demonstration to help !

Thanks

Dave G

More likely to be a diode.
I don't bother with trying to repair alternators. You can get an uprated one outright from MGOC. It's 55 amps and comes complete with pulley and fan for £54.95 plus carriage.

Hi Dave
I think the problem lies with the fact you have found one of the brushes disconnected and as a result the rectifier bridge has been damaged. You used to be able buy these separately from Lucas and could replace just that part if the rest of the alternator is in good condition. Unfortunately, most people don't now list this as a separate item, but I have found this one:
http://www.leacyclassics.com/catalogsea ... ifier&cat=
so a call to Leacy Classics might be worthwhile. Make sure it is correct for the type of alternator you have fitted. Remember that in 1969 cars with an alternator were quite unusual and the alternators were pretty unreliable (and expensive) so it might have been changed for the cheaper 16ACR which has lower output than the 18ACR you should have according to the book.
You will be able to get a new/reconditioned alternator from any of our well known parts suppliers. At the forthcoming MG and Triumph spares day at Stoneleigh Moss normally have a VAT paid sale on the day and there is usually a trader who has genuine Lucas alternator and starters there as well. The show is on the 9th February and advance tickets offer a saving over door prices.

If I remember correctly early alternator equipped MGB's were fitted with a 16AC alternator with a separate control box. I had to fit an ACR type to replace the original AC type many years ago for someone and had to do some minor rewiring to bypass the control box.

Peter Cresswell wrote:
> Hi Dave
> I think the problem lies with the fact you have found one of the brushes
> disconnected and as a result the rectifier bridge has been damaged. You
> used to be able buy these separately from Lucas and could replace just that
> part if the rest of the alternator is in good condition. Unfortunately,
> most people don't now list this as a separate item, but I have found this
> one:
>
> http://www.leacyclassics.com/catalogsea ... ifier&cat=
> so a call to Leacy Classics might be worthwhile. Make sure it is correct
> for the type of alternator you have fitted. Remember that in 1969 cars with
> an alternator were quite unusual and the alternators were pretty unreliable
> (and expensive) so it might have been changed for the cheaper 16ACR which
> has lower output than the 18ACR you should have according to the book.
> You will be able to get a new/reconditioned alternator from any of our well
> known parts suppliers. At the forthcoming MG and Triumph spares day at
> Stoneleigh Moss normally have a VAT paid sale on the day and there is
> usually a trader who has genuine Lucas alternator and starters there as
> well. The show is on the 9th February and advance tickets offer a saving
> over door prices.

Thanks Peter

Very informative, and I believe you and Vic are correct, probably a diode on the rectifier.
Strangely there are no markings on the alternator (except for the regulator which is 14 TR) so not sure what ACR it is, but it doesn't look 50 years old !
Looked at testing the regulator but not as simple as I thought so I gave up !
Cheers

Dave G

There is a "band" in the alternator body. If it's three quarters of an inch wide it's an 18ACR. The narrower the band the lower the output of the alternator. It goes down to 17ACR and 16ACR.
Half an inch is a 17, quarter is a 16.