Talk Morgan Forums Community Vehicle Talk (non-Morgan) MGB Alternator Light Issues.....

All replaced over summer, and extra one put in (no braided cables)

This is the trouble with my understanding of electrickery. I know the basics, but no idea about connecting a diode in series. I don't think I have one in my shed

I found this:
http://www.mgb-stuff.org.uk/sb.htm

I think when the seat belt diode fails the ignition light stays on permanently, as I recall.

Yes - Paul's website is legend in the MG world - like GOMOG but more accurate. I've messaged the MG club to see if anyone has a photo of the location of the module so I can get to it easily.

I think because you have already replaced your alternator Phil with zero change you can rule out the diodes - it has to be an existing earthing fault imho.

Diodes can degrade as well as just full-on fail .... that might explain the variations you're seeing ... instead of simple 'fail to short circuit' it may be just failing and noisy on it's way out. Could give similar symptoms to a dirty connection.

K

Ps I'd look for a poor connection first ... if you've tried the wires, have you had the bulb out and cleaned it and its holder .. it is a series part of the circuit!

I might be inclined to disconnect the ign light connection at the alternator and use a test lamp one end connected to perhaps an unswitched live at the fuse box, the other end connected to the alternator ign light connection.... That way you will see if the temporary light, acts in a similar way to the original ign light in the variety of circumstances you describe, and might save you opening up your alternator to perform the diode checks...?

As ever just thinking in type and well past my sell by date on MGB thinking and much else..

That's a plan.

Unlike the old Morgan dash, the MGB is pretty enclosed and there's a fair bit of dismantling to do so that technique would see if that's necessary or not.

Just a thought, for Richard V6 to give his wisdom

As the OP has severely discharged the battery a few times beyond zero V, could this have failed a least one cell ? Under charge, alternator keeps systems running but never sees correct voltage pushing back so charge light glows ?

Also I would use a temp ammeter (or clip on version) to check if the alternator is putting out a good amperage

Richard ??

TBM not trying to muddy the waters further but I did have a similar issue on my narrowboat driving a high current alternator. The weight of keys on the ignition switch had turned it off. The alternator then started charging intermittently and given it's size (160 amp) I could hear the change of load on the vintage diesel driving it. There was a random cycle lasting a minute or two before it would cut back in again. The warning light followed this sequence as expected.

Point I'm making is the randomness of your issue could be down to intermittent and/or resistive feed from ignition switch to warning similar to my accidentally turned off ignition switch, the results of which are unpredictable as alternator appears to self energise for while then drop out again with sequence repeating.

Suggest therefore following up on your suspicions of this feed to warning light or just jury rigging a cable from live side of battery to warning light, remember to remove it though when you want engine to stop though. As your findings develop I'm more inclined to think this may be the issue rather than a failed diode. The ignition switch side of warning light feed receiving a varying voltage directly affecting its degree of illumination as the alternator maintains a more constant higher voltage to battery and other side of warning light. This would explain why it's brighter when you add load such as wipers as ignition side of warning light gets an even lower voltage.