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MG MGB Technical - Alternator question

I've been noticing that my alternator light has been glowing faintly. I haven't been worried about it because I haven't seen any dimming of my headlights or other problems; but starting yesterday, when I accelerate heavily and hit around 4000 rpm, my alternator light flashes on very brightly a couple of times then goes back to being dim again. Does it sound like I need to rebuild my alternator or does it sound like maybe I've got something else going on.

I'm doing another endurance rally in two weeks so I don't want to get stuck in the middle of nowhere!

Robert Rushing

Yep, its time for a rebuild/replacement. Mine did the same thing earlier this summer. It won't be long and all you'll get is the bright red light.

Jeff Schlemmer

Have it checked and it may be that it needs new brushes installed. That can usually be done where you have it checked.
Dale & Barb Mast

Having somewhat similar experience. Relatively new Ford Fiesta replacement that occasionally glows faintly but never goes beyond that and next time out works fine. Just a bad re-man maybe?

J.T. Bamford


Unsure if you have Advance Auto Parts in your town. Ours offers free checks of alternators. Other auto parts stores may also offer this service.
Bruce Cunha

Hello Robert,
I had a similar problem over the last couple of weeks with the generator on my '66 not generating properly causing the light to glow faintly. It turned out to be a loose connection and some corrosion..before you invest $$ just spend a couple of minutes to pull the wires off and put them back on again just in case.

Robert, I should also mention that my light also was dim and under certain conditions it turned on fully.

Since it is time to change altenators, you might want to upgrade to a Bosch 70 amp altenator. Has two wires on the back, you will need to make a two inch spacer at the mount, but the pulley will align with the crank pulley. And you will have delicious extra amperage for all those nice juicy upgrades!

I forgot to mention that when that happened to me, the rectifier was bad. When I removed the alternator, it had a nice rattle, like it was full of rocks. Apparently when it got hot, the solder melted out of it.
Jeff Schlemmer

B-nine - it's a Bosch and it's been very nice. I recommend the conversion for everyone!

I checked the wires Jeremy, but all was well. I'm pretty sure that it's about time it was looked at anyway because I have no idea when it was last touched before I put it on my car. I pulled it off tonight and will have it checked out tomorrow.

Thanks for the advice everyone!

Robert Rushing

If you decide to replace the alternator, look at the archives. There are a couple alternators that are bolt in subsitutes for the Lucas, which are cheaper, more reliable and have higher output.
Barry Parkinson


I have a'64 mgb roadster with a newly installed alternator kit from Moss. My charge light stays on most if not all the time when I turn off the key. I can pull off the single white wire on the ignition switch and the light goes out. If I wait a few minutes,I can put the wire back back on and the light stays off. I can take it off and put it back on immediatly and it will come on. As far as I know the alternator charges normally. What is wrong?


Charles Campbell

I'd suspect the alternator is incorrectly wired, although I can't see why pulling the white off the ignition switch causes the light to go out (unless some other wire is left on that terminal of the switch) nor why it only stays out if it is disconnected for a few minutes.

The dynamo should originally have had brown/green (field) and brown/yellow (output) wires. A typical conversion will:

1. Discard the brown/green between dynamo and control box and run a new heavy gauge brown from the alternator output terminal (large spade(s))to the battery cable at the solenoid.

2. The brown/yellow at the dynamo is connected to the indicator terminal at the alternator (usually smaller than the output spade(s), and the two brown/yellows at the control box disconnected from the control box and connected together.

However it all depends in which alternator you use, some have three terminals - output, sense and indicator - other three terminals - two output terminals and one indicator - some two terminals - output and indicator - and some only a single output terminal and no indicator. These last disable the indicator so you need another indication of charging such as a voltmeter.
Paul Hunt 2

Both the Bosch and the Saturn/Delco are nice upgrades and quite interchangeable with the Lucas.

The indicator light simply bridges the brown/yellow indicator lead from the alternator to the white ignition circuit. Any imbalance between the two wires will cause the light to illuminate. When you first turn on the key before start-up, the light is on because the full battery output comes down the white wire and nothing is coming out of the alternator's brown/yellow. After start-up, the light is intended to tell you if your alternator output is less than your battery output - in which case your battery is draining. The light can also tell you that the alternator output is in excess of the battery output.

I added a voltmeter to my system, and it's been one of the more useful things I've done to this car. For example, I discovered that too many accessories on the ignition circuit will lower the alternator output voltage below the battery level, no matter how many amps the alternator has. If you wish to control high-draw electrical devices through the ignition switch, use a relay with the primary side on the ignition circuit and the secondary feed straight from the brown wire (with a line fuse, please!).

Bruce mentioned Advance Auto parts. They carry a number of rebuilt parts for MGB - and often at about half the prices we're used to paying the major suppliers. If you simply want to replace your Lucas, Advance has them - and with a good warranty. No, I don't work for Advance Auto!

Allen Bachelder

The indicator on the brown/yellow also acts as a pump-primer giving excitation to allow the alt to start charging at about 900 rpm engine speed. Without this circuit the alts on both my cars do still charge but they only start charging once revved to about 3k. After that they charge normlly down to about 600 rpm. A brand-new alt out of the box may not charge at all without a warning light connected if it has no residual magnetism.

Two things will cause increasing load to cause ignition voltage to drop below battery voltage - one is if the alternator doesn't output enough amps to carry *all* the load plus a bit more, the more usual cause and certainly the case with an alternator that is capable of delivering more current than the load is demanding is that volt-drops die to bad connections in the circuit are causing ignition voltage to drop below battery voltage. Whilst this can be in the wiring and connectors between alternator and solenoid it is more likely between solenoid and through the ignition switch and the fusebox, and you could well find that the alternator *is* still charging the battery even though a voltmeter on a white or (even more so) green circuit implies that it isn't.
Paul Hunt 2

This thread was discussed between 27/09/2005 and 06/10/2005

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