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MG MGB Technical - Alternator Problem?

My alternator light no longer illuminates when I start my '69 B. Additionally, my headlights are dimming and my windshield wipers stop in mid-swipe, which leads me to suspect alternator problems. The bulb is fairly new. The fan belt tension seems okay. Battery terminals are clean. So, if the alternator's bad why doesn't the light illuminate? Any thoughts?
john willbanks

Hi John

Could well be only the bulb has died. The warning light circuit needs to be complete in order for the alternator to start charging.

Replace the bulb before you do anything else.

Cheers
Ian F
Ian Fraser

Re-fettling an alternator the other day, I ended up with *exactly* these symptoms. The problem was that I'd neglected to reconnect the feed wire from brush #2 to plate #3 of the rectifier pack. If changing the bulb doesn't resolve this, try removing the placcy end from the alternator and checking that said wire is intact and connected OK. Vibration and age can cause these cheesy crimped-on connectors to give up.
Tim Cuthill

Check the voltage output before you do anything, you should see about 14v at a fast idle with minimal load. If the warning light has stopped working *and* the voltage is dropping maybe the plug has fallen out the back of the alt. The warning lights acts as a 'pump primer' for the alt and enables it to start charging at about 900-1000 rpm, but in my experience used alts will start to charge once revved to about 3k, after that they work as normal.
Paul Hunt

John, Does the light come on before you start the engine? Key on engine dead some power goes through the lamp to the altinator. Measure for 12V on the small wire at the altinator's connector. If no 12V the bulb or wiring is bad. IF 12V is there either the regulator, brushes or field coil is bad.

Engine dead key on power flows through the bulb, to the altinator. In the altinator it flows to the brushes, through the field coil (the rotating armature) and to the voltage regulator which completes the circuit to ground. Any of those pieces deffective and the lamp never comes on the the battery goes dead.

cheers, Doug
Doug Gordon

Thanks gents. Will explore some of these suggestions this evening.

John
john willbanks

Okay. So last evening I'm thinking I need to check voltage, but instead decide to check the bulb because Christmas ornament boxes are blocking my work bench/tool cabinet and pulling the bulb just seemed logical. I Pull the lens/bulb away from the dash with ignition on. Rubber shroud that covers the connector to the socket has slipped off allowing metal-to-metal contact. The bulb flashes. I'm thinking a bad ground somewhere.
John Willbanks

The ignition warning light doesn't have a ground as such and must be insulated from the case of the gauge which *is* as ground potential. If it were the white side of the bulb that were grounding you would cook the wiring back through the ignition switch. If it were the brown/yellow the light would be on all the time the ignition was on, regardless of whether the alternator were charging or not. You say 'flashes', so it could be an intermittent connection between bulb and holder or a wire broken inside the insulation, or indeed one side of the filament detached from its post but coming into contact when the bulb is vibrated. If you ground the brown/yellow (NOT the brown) at the alternator plug (pulled out) the warning light should glow with the ignition on, which is another non-voltmeter test you can make.
Paul Hunt

This thread was discussed between 09/12/2002 and 13/12/2002

MG MGB Technical index

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