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MG MGB Technical - Ignition light and Saturn alternator

I looking for help with a problem I am having with my 1979 MGB. First the background information. The car is being run for the first time following the rewiring of the car with an Advance Autowire harness install and a new Saturn Alternator(x2). The battery is an Optima redtop and has been checked and found to be in good condition. With the switch in the on position but the car not running the ignition light illuminates. With the car running the light stays on and does not extinguish as it should. I can disconnect the brown/yellow wire from the alternator and the light will go off indicating that the wiring to the alternator is intact and the ground for the light(Alternator) is lost(by disconnecting the wire).
The alternator is working as with the car running I am getting 14.3 V at the battery. Does anyone have any suggestions. I have included the link to the new wiring diagram to my car below.

http://www.advanceautowire.com/latemgb.pdf

Thanks!
Frank Mooring

Sounds like the alternator is faulty, if you get the warning light still on *and* 14.3v at the fusebox brown at the same time. When starting the engine what do you have to rev the engine to to get the voltage to rise from 12v to 14v? Ordinarily with a Lucas alt (other makes may differ) this should be about 900rpm, and the light goes out at this point. If you have to rev higher than that before the voltage steps up (it's around 3k on a Lucas) then the current from the warning light isn't exciting the alternator field as it should. But even if you voltage steps up as it should it could still be a problem with the alternator internals, or you are connecting the brown/yellow to the wrong point on the alt.
PaulH Solihull

Paul,
I too thought it was going to be the alternator so I switched it out for a second new one I had for another project and got the same problem. I have it wired as per the instructions that came with the alternator(see link below). One thing I haven't tried is a different harness...this may be next unless you can think of something else. I will get back down to the shop and see where I get the 14.3 v and report back on this. Thanks for your response.
Frank

http://mgccars.com/instructions-for-cs130-inst.html
Frank Mooring

Can't be the harness if the light goes out when the brown/yellow is disconnected from the alternator. If the second new alternator was exactly the same as the first one i.e. make, model and source then they *both* could be faulty, or the instructions are incorrect. Really you need to try a completely different alternator that you know works i.e. has been borrowed from another car.
PaulH Solihull

This thread was discussed between 06/11/2010 and 08/11/2010

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