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MG MGB Technical - Alternator warning light flickers!

A couple days ago something new started happening on my car ('71B) - the red warning light started flickering at idle up to about 2000rpms. The higher the rpm's, the less it would be on. This, by the way, is all intermittent. Besides checking for the typical loose connection, is there anything specific I should be checking for? I have no idea how to test the alternator.
Thanks in advance, Jeff.

Jeff Schlemmer

My '71 'B did this a week ago, aftr sitting unused for about 10 days. I gave it a good 30 mile belt and things returned to normal. I assume it is a (new) battery that needs charging, and this i will do.
Ken R
Ken Rich

Besides the typical loose connections, also check for the typical corrosion of the battery cables. Most of this really happens back up inside the usual moulded on connectors. As stated here before, corrosion can be so bad that only a few strands may be making the actual connection and transmitting the current for the whole system - a less than ideal situation from the battery's point of view and not a happy one for the alternator/generator either. Hope it is something simple.
Bob Muenchausen

Going back to basics the light will come on if the voltage on the white (ignition) is significantly different from that on the brown/yellow (alt indicator). Normally the two are the same except during starting and switch-off. Given the variation with revs I would suspect that on the brown/yellow is dropping (can be tested with a voltmeter - analogue probably better), which could be caused by a number of intermittent things inside the alt such as slip rings, brushes, regulator, diodes, or just connections. There again simple engine vibration could be affecting a connection in the brown/yellow - warning light - white circuit. If the amount of glow changes with load, say headlights on and off, at a given rpm then I would suspect the alt internals.

PaulH.
Paul Hunt

Jeff,

Start with the simple (and cheapest) things first - believe it or not, it could be something as simple as your alternator belt being loose. I've had it happen to me.

I DID have a bad alternator, and just as a matter of course I changed the twin six-volt batteries too. But I was amazed a few days later that the problem I thought I had fixed returned. It did so only because I didn't have enough tension on the belt, causing it to slip and so only make my new alternator charge at higher RPMs...

In the engine bay:

Check the belt - it should deflect no more than a quarter-inch at it's longest run when you push on it with your thumb, check the pig-tail connections on the back of the alternator (WD 40 them for safe measure and plug them back in), check your woven metallic body-ground strap.

In the cockpit:

check your battery terminal posts as Bob suggested (I got into the habbit of cleaning them with baking soda/water mixture and a battery brush a long time ago) and you can WD 40 them too after they're cleaned - it helps keep the corrosion at bay.

If the problem persists, I lean towards the alternator, especially if - as Paul suggested - the problem increases with a load - headlamps, heater blower on, etc...

The good news is the MGB alternator is relatively inexpensive (compared to a new car!) and you can change it out in about 15 minutes - including washing your hands.

Cheers,

Tom
MG Tom

Hi,

You might consider replacing the Lucas with GM Delco one wire system if you find yourself in the "replace the alternator mode" Winter is coming and the Delco produces enough electricity to run the wipers ,lights, stereo, and turn signals- all at the same time in snappy fashion. There is much information about this simple swap in the archives where I originally came across the tip. I did mine for about $23.00 Found an almost new alt in the Pull A Part yard. Easy job to do. Car starts and runs better all around. I think that the battery is more up to full charge and that is where the difference is . Would I do it again?? You Bet.
Alan

Do you lose the warning light with the one-wire system, and if not, how not? If Delco are so good don't they make a two-wire system? And wouldn't that be even better?
Paul Hunt

Hi,

No, the warning light still works as it should. In fact,once the car is running, the rpms have to drop to about 400 before the kight even flickers. Meaning the alternator is charging all of the time. There are several websites that explain how to do this swap.

Hey if the Lucas was so good AND up to date GM would probably contract with the prince of darkness to replace their line of alternators with the fabulous LUCAS 34 amp special. Take the Lucas to any electrical shop and they will look at you curiously and tell you it is junk! Period.
Alan

Alan,
Is there a specific Delco model # that is best for this conversion? Do you have any of the Web sites handy?

Thanks for any info,
Pat
Pat

If you are interested in the delco conversion try this : http://www.mgbexperience.com/service/delco.html

I used the Delco 3 wire and besides having to cut off the molded plug for the alternator and having to fabricate a bracket it is relatively simple. There is a lot of information in the archives about this conversion.

you do lose originality but I have found that the delco has been more reliable than the remanufactured units being offered by most part stores.

If you want to keep original, you might look into having your alternator rebuilt, some people on this site have found that to be a better route than buying a remanufactured alternator.

Jayme
77 MGB
Jayme

Alan - if your alt only has one wire that is presumably the brown. So where does the brown/yellow from the warning light go? Out of interest.

PaulH.
Paul Hunt

This thread was discussed between 04/10/2001 and 10/10/2001

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