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MG MGB Technical - Alternator light

I have a 67 MGB that has an Delco alternator. I converted the car to negative earth and installed the alternator. The alternator light in the tach is bright red when I turn on the key. After starting the car the light appears to be out. However, when driving the other night I noticed that the light has a slight glow. It stayed the same with or without the lights on. With the engine off I get 12 volts at the battery. With the car running I get 14.77 volts. If the alternator is charging, why does the light still have a slight glow?
Jim Lema

Jim

Others with more electical knowledge should be able to give you the specifics, but you probably have a wire that is either not connected correctly or is grounding. Did you use the old voltage regulator as a junction box for the brown and brown and white wire? If so, disconnect the black ground wire.
BEC Cunha

The light glows when you're not getting enough current to the battery. While you're getting sufficient voltage to the battery, the current is too low. How old is your alternator? It sounds like a recent instal? If so, what amperage is it? If the amperage is too low, or if the pully is too big, then you will not be getting sufficient current from it. You could also be getting some slippage of the alternator belt.

If it's an older alternator and you've only recently started to have problems with it, then consider having hte alternator tested or you may have to rebuild it.

I would first check to ensure the belt is not too loose. Just because it doesn't squeal doesn't meen it's not slipping. Then I would check the diameter of the pulley on the delco alternator vs the one on your lucas alternator. Generally, your delco part should hopefully be more modern and should supply more than enough current to beat out your original lucas one.

Actually, I just read that you've changed from a generator?. No worries there. I'll see if I can check the diameter of my lucas pulley and let you know so that you have something to compare it to. I too did the swap to a delco alternator.I haven't had any problems with it at all.

Let us know what you find out.

~Dave
Dave Ryzuk

I removed the regulator when I did the conversion. I just joined the wires together using a plastic terminal strip. Don't think the belt is lose, but will check that again tomorrow.

Jim Lema

I purchased a new Chevrolet Impala. It had a Delco alternator. For the next 180.000 miles the light would glow dimly . This was only visible at night. The alternator never failed and the battery never showed any inclination of undercharging or overcharging. The dealer said that this was normal.
Sandy Sanders
Sandy Sanders

Jim,

I purchased a vehicle some years ago and it had the alternator light dimly lit from brand new.

It did not bother me at the time because it was not causing any problems. When I did eventually get around to looking for the cause, I found a short circuit field diode. I replaced it and it fixed the "permanent dim glow".

However I would follow all of the other sugestions before you pull the alternator apart.

regards,

peter
peterl

If a white wire is grounding it will burn the harness.

There are generally two reasons for a dim glow from the warning light, one is one or more of the diodes faulty, the other is bad connections between the brown at the alternator and the white from the ignition switch.

The warning light acts as a pair of balance scales between alternator voltage and system voltage, which in a perfect world will be exactly the same as there should be a copper path between them with the ignition switched on. But if there are bad connections anywhere in the circuit you will get a higher voltage at the alternator than on the ignition, and so the light will glow.

Measure the voltage on the brown at the alternator and the white at the ignition switch with the alternator charging i.e. the engine running at a fast idle. A lower voltage on the white than the brown indicated bad connections, measure at the solenoid, fusebox, and brown at the ignition switch to track down where. If they are much the same, then measure the brown/yellow at the alternator. If that is noticeably higher or lower than the brown at the alternator the alternator is faulty.
Paul Hunt 2

This thread was discussed between 12/05/2007 and 15/05/2007

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