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MG MGB Technical - Ignition (charge) warning light

Hi all,
Everything was fine this morning when I started the car (78B UK model). Ignition light turned on when I switched the ignition on and turned off after the car started.
It drove like a dream all the way to work. Once I stopped the engine, the ignition warning light came on and remained on even after I removed the key.
I switched the ignition to "start" and the light turned off. The car still started okay. I switch the engine off and the light comes on again. It's like everything is suddenly back-to-front.

After reading a few other posts I narrowed it down to either battery, alternator or charging circuit.

The battery appears fine. It shows approx. 12.5V with everything disconnected. With everything connected (but key in the "off" position) it shows approx. 12.4V. With the engine running it shows 13.3V so I assume the alternator is working and charging the battery.

With everything hooked up and the key in the "off" position, light is on, I remove the plug from the alternator and the light goes out. I pulled this plug apart and connected each of the 3 wires in turn. The small white/brown wire is the culprit. With it connected the light is on. Disconnect the white/brown wire and the light turns off.

I have unwound the loom from the plug as far back as I can looking for broken or short circuits. I can only unwind it to around the area behind the brake cylinder. All the wires look good up to that point.

Any suggestions? Is my alternator kaputski even though it is still managing to charge?

Thanks,
Dave
D O'Brien

Not the wire, that is his job.
Alt is part dead; if you give more detail about exactly what the light does versus engine speed, we could maybe figure it out, but it is still a regulator & rectifier replace job. And the 13.3V is too low, and is indication of a failed diode.

FRM
FR Millmore

It sounds like you have a faulty diode in your alternator. Any repair shop, that deals with these units, can fix it for a reasonable amount of money and in very little time. RAY
rjm RAY

Dave - are you sure that you have a White/Brown wire into the alternator?

If you had said that it was the Brown/Yellow wire that was the culprit, I would understand. But in its standard form (and I am sure this applies to the Australian export market, the White/Brown wire is between the ignition relay and fusebox.

Oddly enough, if you removed this White/Brown wire from the relay, then the light would also go out - the cause being the relay has stuck in the 'on' position... Even if it is the relay stuck on, pulling the plug from the alternator will still cause the light to go out, but the fault of course is not the alternator.

It could be that you have had your car re-wired and someone has used White/Brown to the ignition light, in which case, apologies!
H Adams

Dave - are you sure that you have a White/Brown wire into the alternator?

If you had said that it was the Brown/Yellow wire that was the culprit, I would understand. But in its standard form (and I am sure this applies to the Australian export market, the White/Brown wire is between the ignition relay and fusebox.

Oddly enough, if you removed this White/Brown wire from the relay, then the light COULD also go out - the cause being the relay has stuck in the 'on' position...
H Adams

Thanks all. It was the alternator.
1st, the wire. It's a bit dark under the bonnet and the wire has 30+ years of wear. It was probably brown/yellow now you mention it.

I knew there was a good auto electrician near my place and lucky the MG isn't my solo drive these days. So I pulled the alternator out and took it down to him for a quick bench test.
He didn't bother testing it. He explained straight away that it was 99% the diodes and/or some minor parts inside. He looked up prices for replacement parts and straight away told me he was happy to repair it although with labour and parts I might be better off looking at a new unit, depending on the price. Btw this guy is well-known for his honesty. He's no saleman. He found suppliers and prices for 2 different units, both almost identical to the original Lucas alternator. The more expensive one was over $500 and the cheaper one was $320. He then asked me if I knew of Heritage (I do... I buy almost all my parts there). He checked their price and told me I could get the original from them for $178. Proving he is no salesman, he recommended I do that. No charge for the advice.

All fixed. Now I have my old one on the bench with the back panel removed, wondering if I can attempt fixing it so I have a spare. I can solder and I can use a multimeter. I see 3 diodes. Wish me luck. :)
D O'Brien

Should have fitted any of many well documented conversions to much better and cheaper alts.
But, now you are thinking about fixing this one, better learn how it works, The selenium rectifier stack is cooked, and the regulator with it no doubt.

FRM
FR Millmore

Also, a good time to replace the brushes and clean up the slip ring on the rotor assembly. RAY
rjm RAY

This thread was discussed between 30/08/2012 and 31/08/2012

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