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MG MGB Technical - Alternator/starter Wiring

My new alternator doesn't seem to be charging the battery, as I found out on a night drive home from work as my headlights dimmed to near nothing...
Anyway, I know what to check out on the alt. and all from looking through the archives (do it when I get off work) but I have 1 question from something I read.
Everything says the large brown wire from the alternator attaches to the starter selonoid where the battery connection is. As I sit here at work pondering my problem, I don't think mine is connected there but goes straight to the battery.
I already checked the wiring and re-spliced/cleaned up bare areas, etc. from alternator to the battery and no shorts, etc.
Could the fact the large brown wire from the alternator is not connected to the starter be a possibe cause of the non-charging alternator (providing the alt. checks out ok)?
Thanks.
Joe Fabian

Joe, It shouldn't make any difference other than you would have a longer wire running back to the bettery with some small loss due to the wire length. I don't know of any standard MGB wiring setup that had the thick brown wire going to the battery. Clifton
Clifton Gordon

Hi Joe.

The fact that the wiring has been modified suggests that a previous owner has had problems in the past, which may be re-appearing now.

Where is the alternator sense wire (the other brown wire) connected ?.

Don
Don

Hello Joe

I am rebuilding a 'B' and at the moment I have just wired the alternator. This is how I see it, and I am glad to share it with you. The heavy wire from the battery goes to the starter along with a large dia. brown and a small dia. brown. The alternator has a thick brown and a brown and yellow.The brown from the starter goes to a fuse from here to the ignition and onto the alternator. Can I recommend www.mgcars.org.uk/electrical/body_schem-3.html.

Good luck.
G Heath

Just want to add to this that when I start the engine the alternator light on the dashcomes on and goes off after rpms increase.
Is it possible that some of the alternator is bad and the other good, i.e. the prong the power to the ign. light comes off is good and the prong where the heavy brown wire comes off , goes to solenoid, is bad? I got this rebuild from Moss only a few weeks ago and the bench test they sent said it was good to go.
Any more ideas? I will test the output of the alt. (heavier brown wire)when I get the chance.
Like I said earlier, I already checked and re-spliced/ replaced bad wires etc. So to me this means the alternator is bad contrary to what the test Moss sent said.
Joe
Joe Fabian

Just pointing out that the fuse that G Heath mentions is a mod, you won't find one in factory wiring. If you *do* fuse the browns there should be a second fuse in the alternator output wire(s) to fully protect the wiring against burning in the case of a short.

Alternators used either one or two browns over the years, and the two browns could either be one thick and one thin or two thick, and that was just the ones with the internal regulators. The thin wire *was* a sense wire, the two thick wires both being output wires but giving double the current carrying capacity. But in all cases the two browns go back to the same place, electrically speaking, on the solenoid. This could have either been large lugs on the same stud as the battery cable, or spades that were connected to it. There was always another brown at this point - the feed up to the ignition, lights etc.

When you turn on the ignition the ignition warning light (or more correctly 'no charge' light) should glow, and once the engine is atarted and revved to about 1000 rpm or above it should go out, and stay out unless the revs drop below about 600 rpm or the ignition is turned off. In this latter case it should flicker and die as the engine spins down.

If it doesn't glow with the ignition then remove the plug from the alt and connect a ground to the brown/yellow (NOT the browns!) wire in the plug. This should make the warning light glow. If it doesn't the alternator is faulty, or maybe your plug wiring is not compatible with the alt.
Paul Hunt

On the other hand, I installed a Delco unit, removed the old regulator, jumped all the brown leads together and attached to the b+ on the alternator and ran the field wire (indicator light) to the ind lug.

No problems, no worries.

Mike!
mike!

Tahnks for the info. I'm still working the problem. My indicator light does come on when I start the car and goes off after rpms hit about 1000 (give or take). But still alternator not charging the battery, etc. I checked wires and only the larger brown wire from the alternator is attached to the stater solenoid. The other smaller brown wire(from the alt.) is connected to others (about 3) and spliced to the wire/cable heading to the battery. The IND ligt wire on the alternator is connected correctly and works.
Could the fact the smaller brown wire is not attached to the starter like the larger one be the cause of the non-charging?
Just an idea.
Joe
Joe Fabian

"My indicator light does come on when I start the car and goes off after rpms hit about 1000 "

This is not correct, but it may simply be the choice of words. The warning light should glow when you turn on the ignition, not when you start the car. When you first start the car it should still be glowing, until it is revved to about 1000 rpm or higher.

As long as both browns are electrically connected to the battery cable it doesn't matter exactly which physical connection they go to. You should have 12v on both browns and the brown/yellow in the alt plug with the plug removed and the igniton on.

If you still see on 12v on the browns with the plug back in and the engine running at a fast idle i.e. 1500 rpm then the alternator is faulty, pure and simple.

Paul Hunt

This thread was discussed between 08/08/2004 and 12/08/2004

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