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

This is for my MGB GT - I'm posting here as I know there are a number of electrickery experts whose opinions I value.

When driving my alternator light is very very faint, but still on. If I put on the windscreen wipers or washers it starts to glow more. However the alternator is still putting power into the battery - I have a plug in voltmeter and it's putting in around 13.8 - 14.4V. The voltage reading doesn't change when the wipers and washers are on.

I replaced the alternator last year and there has been no change.

In the last few days the alternator light has come on for a period of time - again, no effect on the charging, and still putting in 13.8 - 14.4V. Sometimes it turns itself off, other times, switching the ignition on and off cures the problem.

Any thoughts at where I should be looking? I'm assuming the problem would be with the lead from the light to the alternator? Broken or poor connection? But not sure why the light comes on with the wiper!

The car fuse box has been (badly) replaced and rewired at some point, and is in my fix list so may be a problem there?

That is sounding like a poor earth to me.

Thinking in type.....

Or could it be an issue with the 'hot' feed from the ignition to the lamp? It's dropping voltage or finding earth somewhere which is causing the lamp to light. The 'hot' feed to the alternator lamp shares an ignition feed with the wiper motor.

Yeah, I'm think it may be a poor earth on one of the devices that is 'linked' to the hot feed on the light, so it's not getting the full 12V, so current is flowing from the alternator to earth through the light?

My (very basic and possibly incorrect) understanding is that there is 12V getting to the lamp, and it initially earths through the alternator. When the alternator fires up, it also sends a voltage to the light and cuts the earth so the light goes out. It seems that the lamp is finding an alternative path to earth.

A short lead with a croc clip either end comes in handy for this sort of thing.

Or, what alternator did you fit? Some of the cheap ones are very poor and although they deliver the ‘voltage’ they can’t deliver much current.

But if you never need to charge the battery then the alternator must be providing adequate current and a dodgy earth may be the fault.

Final thought, check your lights. In the dark run the engine and keep the lights off, look carefully for a dimly glowing bulb. If you find one you’ve also found the problem. Incorrectly wired and earthing through the bulb.

Alternator is a fairly standard MGOC one - does the job OK. Due to the dodgy fusebox, the heated rear window switch is permanently live - to my embarrassment on numerous occasions I've left it on overnight and the battery is dead as a Dodo in the morning (not even enough juice to light the LED interior lamp). After a jumpstart, by the time I've got to work (25 miles) the battery is fully charged and back to strength. I usually have a 'routine' when I stop, where I check all the switches but on occasions I get distracted and forget!

I'll check the lights thing in the morning.

In a bid to help out until John provides the answer... (-: I think your thinking in type...Is perhaps not far off in that as best I understand it the ign light is made live from the ign switch when switched on, the ign light picking it`s earth up at the alternator. As you suggest once the alternator is spinning the terminal which was once supplying the ign light with an earth becomes live (hot) thus the ign light deprived of an earth no longer illuminates.

Again it seems you may be correct in thinking that with the alternator now making the former earth that it supplied live (hot) the circuit is acting in a reverse direction with the ign light perhaps finding a route to earth through some other of a few ignition switched circuits..?

I guess there may be only one wire (White..?) at the ignition switch..? Which may then go off to the fuse box to supply all ign switched kit, a partial earth being found through any one of the circuits there, that switching the wipers on apparently increases the earth potential at the ign lamp is intriguing indeed, and might cause me to disconnect the wire providing power to the wiper switch, then fire up the engine to check the difference in any on the ign light...

I do remember stripping my B`s switches and cleaning the internal contacts which were pretty well fouled up with verdigris, the hazard switch in particular caused all sorts of strange anomolies which disappeared once cleaned... As best I can remember the switches could be opened up by moving the rocker which pivots on two pins which fit into holes on either side of the switch casing. Deflecting the side of the rocker enough to disengage the pin will enable the rocker to be removed... and the internal spring to fly of into the distance never to be seen again...though as luck would have it... that did not happen when I stripped my switches..

Hopefully by the time I have finished typing this John will have provided the answer...(-:

I have plenty of experience of a dim charge light which glows brighter under load. In 1976 my Dolomite did this and alternators were £25. A lot then, so a piece of black tape over the light fixed the problem until I could afford a new alternator.

Those Lucas ACR units do vary slightly but I was told sometime later at an auto electrical shop, that the charging circuit is distinct from the warning light circuit. Both are diode controlled, and I recall he said that some charge circuits had three and the warning lamp circuit two. So it was quite possible to have charge and also have a glowing light which glows brighter under load when the diode has failed.

I have no idea if this is still the case. Maybe now a circuit board which replicates the old set up, but it's the "diode" in the warning lamp circuit, almost certainly.


Oh and I've just (partially) remembered that those B's with a seat belt warning light have a diode in the circuit, and if that fails, something weird happens, but I can't remember what.

My only concern with this theory is that the old alternator had this problem, so I replaced it with brand new and the problem continued straight away.