MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Glowing red alternator light

Over the last few months my alternator light began glowing. 1974.5 mgb roadster. It would stay on bright when first started until I reved the engine then would glow faintly when at idle. It would go out completely when being driven. Upon returning from a run it would then glow faintly at idle. No problems starting but I do notice the headlghts get brighter when the engine is reved. I don't have a meter to test (I know I should get one). I don't know how old the battery is. I've owned the car almost 2 years now. Could it be the battery or alternator or a combination of problems.
John

Sounds like the alternator is not putting the right amount of charge out. It's a simple enough job to change it, so give that a go first.

Andrew.
A I McGee

Hi all.

I agree with Andrew, it sounds as if the alternator is on the way out. Re-building one is a nice little winter project...

Don
Don

Yep, if you follow the board, you may have seen this come up about every other week all summer long. The most reliable fix is a Bosch replacement. A fun project would be to restore the original, but yours is on its way out. Double check all the electrical connections, but it sounds like a textbook case.
Jeff Schlemmer

It's normal for any standard MGB alternator to cause the headlights to brighten when the engine is revved so that isn't necessarily an indication of impending failure. The light glows when there is an imbalance in the voltage between the electronics inside the alternator and the white wire just after the ignition switch. The bigger the imbalance the brighter the glow. This can be because of faults inside the alternator, but it can also be because of bad connections somewhere between the two. Personally I'd be looking for those before shelling-out on a replacement alternator. Measure the voltage on the brown-yellow and brown spades of the alternator (wiring connected), the battery cable stud of the solenoid, the battery +ve terminal, the brown at the fusebox, the brown at the ignition switch, and the white at the ignition switch. In a loss-free world these would all be the same, but in the real world there will always be small differences between the alternator brown at one extreme and the white at the other. Also measure the voltages with minimal electrical load i.e. just the ignition and again i.e. with all lights, brake lights etc. switched on. At idle and at about 3k rpm. These differences will increase as the electrical load increases, and as the system voltage increases i.e. when the engine is revved. A difference between the brown and the brown/yellow at the alternator would indicate an alternator problem. Small voltage changes part-way through the chain I describe above, but suddenly becoming noticeably bigger at some point, indicates a bad connection.

With minimal electrical load at 3k rpm you should see about 14.5v, this will reduce with everything switched on, but should still be in excess of 12.8v even at idle (unless the idle speed is very low) in my experience.
Paul Hunt 2

This thread was discussed on 04/12/2005

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.