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MG MGB Technical - Ignition Warning Light

Just replaced Alternator on my 73 B last year. Ignition Warning Light coming back on... Car idles OK. Any quick check information would be appreciated. Also, I'm a real novice at this so any comments/steps I would need to do with that in mind would be much appreciated.
J Kennedy

Check the voltage at the battery, it should be 12.5V engine off and 14.5V engine running at 1000revs. Next step, check the the connection into the back of the alternator for looseness, dirt and frayed wires. Make sure you disconnect the earth at the battery while doing this.
c cummins

You say "coming back on", what do you mean by that? Intermittently? All the time? Just when idling? Is it on at full brightness i.e. the same as when turning on the ignition to start, or dimmer?

If the idle revs drop to about 600rpm or so the alternator *will* stop charging and the light come on, and should go out again when revved to about 900rpm. If the light is on at any other time when the engine is running then the alternator is probably faulty.

If the brown/yellow connection in the alternator plug is not making good contact then the light wouldn't come on at all, even when first turning on the ignition. If it is the brown wire that is at fault then the warning light would operate normally, but the alternator wouldn't be charging the battery, which would eventually go flat.

Check the voltage on each wire at the alternator plug with the ignition off and engine stopped, be careful not to short any to earth. You should see battery voltage i.e. about 12.5v on the brown(s) and nothing on the brown/yellow.

With the ignition on but not started it should be much the same, maybe a tenth or two lower on the browns and maybe a tenth or so on the brown/yellow.

With the engine started and revving at 1000rpm or so you should see 14v or so on *all* the wires. If you have 14v on the browns but much less on the brown/yellow, then there is a problem inside the alternator, but it is charging, and you can continue to use the car for the time being. However you may see quite a bit more than 14v, especially as it is revved, and in this case the voltage regulator is over-charging the batteries and you shouldn't use the car until it is fixed.
PaulH Solihull

This thread was discussed between 01/08/2010 and 02/08/2010

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