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MG MGB Technical - Alternator light gets brighter with revs?

Out for an evening drive with a friend of mine in his '73BGT. He starts the engine, and I see that the lights for the Temp gauge and rev counter have fused, and the ignition light is glowing fairly brightly. Having replaced the alternator on this car for a used unit 12 months ago, I was wondering if this was a problem. He then tapped the accelerator pedal and the ignition light got brighter. Does anybody have any ideas what this might be? Might it be related to the blown fuse?

Hi---------Sounds like a brush in the alternator. You might have a shop that works on them around you that can check it for you.
Dale & Barb Mast

Dave ,

HAve a multimeter on hand and check the voltage outputs
at different rpm. Might indicate either a brush or a
voltage regulator problem. Voltage fluctuations may be
within the 2,5 - 3,5V range.

Dave - For troubleshooting help in this area, go to Paul Hunt's web site, The Pages of Bee and Vee at: Click on Spanner ont he left side of the screen, then on Electrics and finally on Alternator/dynamo. Paul explains the operation of the alternator and how to troubleshoot various faults, including the symptom you are describing.
Good luck - Dave
David DuBois

just went thru this, replaced the Lucas unit with a remanufactured one from moss motors, which in 4 months went south ..., i fortunately added a ammeter and a voltmeter to the circuit. i witnessed the demise of the second unit. the regulator stopped tegulating and was putting in excess of 15 volts and more than 35 amperes ( the limits of my meters) into a fully charged battery WHAT WAS THE IDIOT IGNITION LIGHT DOING ALL THIS TIME ... NOT A THING ... if i did have the voltmeter and ammeter who can guess the damage to the audio system, battery, ignition system
i now have a ford fiesta bosch unit in , it was a drop in conversion, no shims, nothing, the plug was also the same ... go figure .... go with the prince of darkness if you wish but the bosch is my advice
the people who say the idiot light is all you need are suffering
the light came on after the alternator self destructed
if i had been watching i might have been able to disconnect it before it died ... but what could i do
with a piece of crap
rocco grillo

No relation with a blown fuse, and the lights in the temp gauge and rev counter aren't fused anyway.

Warning light brightening with revs is one of the symptoms of a faulty alternator - an open-circuit live side output diode. This can damage the rotor windings, regulator output stage, and brush boxes as well as the warning light. Because of that even though the diode pack is a replaceable unit you would be better off getting a guaranteed replacement alternator.

An ammeter adds more problems that it removes. If you must fit something fit a voltmeter.
Paul Hunt

Silly question, then, but what would make those two lights go out?

Blown bulbs? Disconnected wire? That is if the other instrument panel lamps still work. If none of them work it is probably the rheostat, although that usually fails dim ... that is, even dimmer than usual.
Paul Hunt

This thread was discussed between 21/11/2004 and 24/11/2004

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