Talk Morgan Forums Thread Like Summary - 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?

To (hopefully) draw all this to a close

For about a week I had days of redlight on. redlight off so changed the alternator. Light now off, and charging consistently at 14.2-14.4 (the old one was fluctuating all over the place). Problem 2 solved.

Removed the old fuse box and refitted an original, and a couple of independent fuses as per wiring diagram. Everything that should turn off with the ignition now does!

Wiper motor no longer causes a red light glow, but still getting a very very faint one with the washer motor so need to try and find the earth point for that circuit and clean it up.

However, all in all an overall success.

Thanks again for all the help and encouragement.

Sorry George but if I've understood your suggestion correctly you do NOT want to be shorting any part of the ignition to warning light circuit to earth/chassis. The warning light works on potential difference across its contacts not at all on any direct ground connection. To work as TBM describes it can only be receiving a faulty reverse current.

Richard, no need whatsoever to apologise relative to advising where I might get things wrong...Always pleased to learn, thanks.

I think I understand the illumination of the ign lamp depends upon potential difference, and that there should be none once the alternator is generating voltage..

My thinking on the alternator end of the warning light circuit was once it had been disconnected from the alternator was to then to ground it, was more as an "extra" check o confirm that it`s switched live side of the ign light circuit was not picking up some sort of grounding via the many paths available to it along the lines that Dave suggested i.e. through a failing seat belt diode..

Always greatly appreciate your input Richard.

Thanks Rich, I have a good stock, and will probably go for the same black cover style as the originals