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MG MGB Technical - Alternator indicator light off, WHY???


I made a battery reverse polarity connection when spring arrived this year.

Result: I had to rebuilt alternator. Now everything work fine except for dashboard alt. pilot lamp which stays always off. Any idea was wrong. May be a burnt bulb...

Thanks all,

Jean Guy Catford

The bulb is the first place to check.

Jean Guy. Kimberly is correct that a bad bulb is the first suspect. Wires are the second.

You need to remove the bulb holder from the dash and get it out where you have access to the bulb and the wires. The bulb can be removed and two wires, connected to the battery, used to test the function of the bulb. If the bulb lights, you have a working bulb. If not, replace it and see what happens.

While you have the bulb holder out, use a test light to see if power is flowing. There should be a brown/yellow wire, coming from the alternator. Do not remember the color of the wire to the ignition circuit but, the wiring diagram should give you that information. Turn the ignition switch to the run position and check for voltage on the wire to the ignition circuit. You should either see the test light illuminate, or, if using a volt meter, the battery voltage.

Start the engine and test for voltage on the brown/yellow line. You should see either a light, or, if using the meter, about 14.5V present.

One quick check of the bulb is to remove the connector from the alternator and jumper the brown/yellow wire to a ground source. I made up a wire having a battery charger style clamp on one end and a male spade terminal on the other. The clamp is fastened to the ground and the male clip inserted into the female connection, to the brown/yellow wire, on the alternator plug in connector. With the brown/yellow wire grounded, turn on the ignition switch and see if the light illuminates. If it does, you know the bulb is good. If not, you need to remove the system and check it out after you re-attach the plug in connector to the alternator.

Les Bengtson

Thanks all,

I'll proceed as you suggested.

Jean Guy Catford

White from the ignition goes to the other side of the bulb.

As a quick test of the bulb, wiring and all connections outside the alternator unplug the wiring from the alternator, turn on the ignition, and you should have 12v on both the brown (output/sense, could be one thick or two one thick and one thin or both thick) and brown/yellow wires (indicator, thin). If you conenct a ground to the brown/yellow (be careful NOT to connect it to the browns) the warning light sghould glow. With the ignition off there should be 12v on the brown(s) only. If that's OK, then either the alternator rebuild is bad, or the connections between the alternator electronics and the pins are incorrect, or possibly the indicator just isn't making a connection.

Without a working indicator bulb the alternator generally won't start charging until revved to about 3k rpm, as it has to rely on residual magnetism rather than the 'pump priming' function of the indicator circuit. Once it starts it will charge normally down to about 600 rpm. Measure the voltage on a brown say at the fusebox. With the ignition off and a good battery you should see about 12.7v. with the ignition on about 12.5v. Cranking about 10v, and charging about 14.5v.
Paul Hunt 2

Well guys,

I checked as suggested and the problem is a lack of current to the indicator light. I consulted wiring diagram in Haynes and as usual, it need 6 Ph. D. degrees to understand it. In Intereurope manual is was more clear. But the guy reponsible for that was on the sado-maso side of the force. I could not figure why the ignition lamp #44 was connected to heater blower switch #168.

Any way, I understood and deducted it was supposed to be fed from ignition switch.

Thanks again,

Jean Guy Catford

There are easy to read wiring diagrams available at click on stock schematics

Thank you kimberly,

In fact it is a lot more clear that way,

Jean Guy Catford

This thread was discussed between 14/05/2007 and 16/05/2007

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