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MG MGB Technical - ignition light revisited

at my last post i did some intense trouble shooting and received some excellent information.
i got a 15 voltmeter at radioshack and epoxied it to a cigar lighter plug and inserted it into the socket. it read 12.5 v static, dropped to 10 v when cranking and 14v at 2000 rpm. ignition light still glowed brighter as rpm increased. being too old not to know what was going on, i got a 30-0-30 ammeter from jc whitney for 6.00 sans shipping. cut the two heavy brown leads from the alternator, and inserted the ammeter (using only one wire) i adjusted the rpm to 2000 and started adding loads, the lights, fan, wipers, brake lights, radio, flashers etc. as the loads were added the output from the alternator kept increasing. it maxed out at 35 amperes. end of story
i don't care what the ignition light does, the battery is charging and everything is fine in MUDVILLE tonight

there may be something wrong with the alternator, but so what as long as its doing what it supposede to do

you can be sure that the ammeter will be in the circuit soon

rocky
rocco grillo

My memory may be off but I think I remember taking my dash apart once and then when I put it back together I got the bulb holders in the back of the instrument for the ignition and the high beam lights muddled up (on a 70 GT with a steel dash). The ignition one is isolated and the high beam one is earthed to the case of the instrument I think. Now I think getting them wrong meant my ignition light was on all the time too. This was a few years ago so I can't remember exactly what happened. I do know getting them in the wrong way round caused something funny to happen with those warning lights. The way I worked it out was by pulling the lights and holders out of the back of the instrument so it stopped earthing to the case.

Simon
Simon Jansen

I would check out the battery and battery ground.

If I remember what I read sometime back somewhere on the web right, if light glows brighter with rpm then it is your battery that is the problem.
Kevin Pearce

Running with the ignition warning light glowing all the time is like taping over your temp and oil pressure gauges - you won't know anything is going wrong until you grind to a halt at the road-side. If the light is glowing when the engine is running at over 1000 rpm it is because the white (ignition) and brown/yellow (alt indicator) wires are 'seeing' different voltages when they should be the same - about 14.5v. Bad connections in the white and brown circuits can cause the white to be lower than its should, and faults in the alternator can cause the brown/yellow to be either higher or lower than it should. Whilst a voltmeter tells you much the same as the warning light with no drawbacks an ammeter is not a good idea, it introduces several points of failure, possible wiring fires, unwanted resistance and volt-drops.

Glowing brighter with increasing revs is one of several symptoms for a failing alternator documented in the Workshop Manual. Some years *need* both browns top be connected for correct functioning of the voltage regulator. On other years they are used to increase current carrying capacity, disconnecting one of them limits the ability of the alternator to carry the load it should.
Paul Hunt

Sounds like your rectifier is going in your alt. Take it by your local alt shop and have it checked. Bob
Bob Thompson

This thread was discussed between 28/08/2004 and 01/09/2004

MG MGB Technical index

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