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MG MGB Technical - I hate electrical stuff

Ok..... the little red idiot light glows dimly when the car runs and headlights are off... I turn on the heater fan, it gets brighter, I turn on the wipers, brighter still..... I can't move the fan with my bare hands without making the whole works turn, and the electrics shop say my alternator is performing fine... what's the deal here?
David Ryzuk

David:

What kind of RPM are you talking about? Is your battery going dead? Does the light go off at higher RPM? If the battery is charging, I would not pay much attention to Mr Lucas.
Pete

This is usually caused by bad diodes in the alt. Depending on which diode(s) and whether open or short you can get 'normal' output voltage but reduced maximum current and excessive heat and noise.

However it can also be caused by bad connections elsewhere on the car, so when the engine is running at a fast idle, and you have some load on so the warning light is glowing, measure the voltage on the brown/yellow and the brown at the alt, and the whites at the multi-plug by the steering column. You *should* see about 14.5v on all of them, but for the warning light to be glowing the brown/yellow must be higher or lower than the white. Come back with the three voltage readings.
Paul Hunt

I agree with Paul Hunt in that your problem may be a bad connection elsewhere.

You might check the push-on tab connection at the brown wire at the starter soleniod.
Daniel Wong

Where in BC are you?

If you are really fu-shnickered and on the island, I may be able to drop by and give you a hand in the great spirit of MG fellowship! I've been known to havea knack for fixing the electrical issues.

I had a greatg solution to the fruity MG wiring. When I was doing the paint and such, I re-engineered the wiring. (Had to do at least a little, because i have a V6 in mine) It now has 10 fuses and 10 separate sub systems... non of this mystery crap everyone else has to endure...

Drop me an e-mail if you like.
Gord Bestwick

Paul's got it right. The only way to really check alternator dides is to disassemble the alternator. If you can lay your hands on a known good one, swap it out and see what you get. I'd bet that's your problem.
Jim Blackwood

David. I had a similar problem to what you describe. The alternator checked good for voltage, but was only putting out 17 amps which meant the car was running partly on the battery and the light glowed dimly. Since I do not know what your electrical shop checked on your alternator, it is hard to offer advice. If they only checked output voltage, I suspect low amp output. If they checked both, the bad connection theory would be the one to check first. If not, I would go with a new or rebuilt alternator and then see what you get. Les
Les Bengtson

thanks everyone for your replies... I've sor of been avoiding the enevitable cecking of every connection and section until I find a fault.

Gord- Thanks for the offer.. I'm just south of you, in Victoria. I'd hate to inconvenience you like that. I might get in contact with some of the club guys here.

Les- I like your solution a bit better.. I'll slap the ammeter on the thing in the morning.

Anyone wanna tell me what to run when I test the current?... I know the current increases with the load I put on it, but should I just run the engine and comepare that to a number that it's supposed to read.. if so, does anyone know that number?
David Ryzuk

oh.. and I know to put the ammeter in series
David Ryzuk

When load testing your alternator , be very sure NOT to load it down below 12 volts ! this will damage the diodes . this is on any alternator by the way .

-Nate
Nate

If the alt is supplying the correct voltage then the current drawn from it will be appropriate to the load, by which I mean that you will only find out if your alt is putting out a reduced max by loading it up to exceed that max, or by 'fooling' the regulator.

The Workshop Manual states that for an output test you need an ammeter capable of reading 35 amps or greater. Inside the alt bridge the regulator green 'F' connector to ground and with the alt connector replaced and the engine running at 3300rpm you should see 34 amps on the ammeter, which is placed in the main output lead of the alt. This applies to the original Lucas alts, of course.

For a bench test you need a 15 ohm, 35 amp variable resistance as the load. As well as the 34 amp max the alt should maintain between 14.0 and 14.4v at a 10 amp load.
Paul Hunt

This thread was discussed between 31/12/2001 and 03/01/2002

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