MG Car Repair: Iginition light staying on, battery voltage test, alligator clip

Question
QUESTION: Hi.

After I start my 1972 MG midget 1275 the iginition light on dash stays on.  Any idea why it doesn't go off?

Thanks and appreciate the answers.

Regards

Padraig


ANSWER: Hi Padraig,
The "Ign" light is not an ignition light, it is a charge light. Put a volt meter across your battery posts and read battery voltage. Then start the engine and set the idle at a fast idle (1500 RPM) and read the battery voltage. It should read 13.8 to 14.5 volts. If not, the charging system is not working.
Howard

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QUESTION: Hi Howard

It looks like its not charging.  I tried a different battery too and no difference.  If I start the car and disconnect the battery the car cuts out.  If I remove the wires from the dynamo the charge light goes out which I think would be correct as my understanding is that that wire (brown/yellow I think) provides the earth to the charge light until dynamo is turning.  I check all the wires and all looks ok.  Is it possible to easily test the dynamo, should there be a positive charge out of either connector when car running?  

Thanks for your help

Regards

Padraig

ANSWER: Padraig,
When you have confirmed that you have a "No Charge" condition as the battery voltage test I gave you. You then need to isolate the problem to either the generator or regulator. You can not test a regulator with a generator that is not working but you can test the generator. Do this:

Make up a long test lead of heavy gauge wire (12ga.) On one end attach either a large flat spade connector (if available) or an alligator clip and also attach a short wire to the same end with a small flat spade connector on it (or another alligator clip)

Put a volt meter across the battery posts and read battery voltage. (12.x volts) disconnect the two wires from the back of the generator. Connect your test lead, large wire to the large post and small lead to the small post. Now you have the small lead (field) and the large (armature) leads connected together and a long test lead. Start the engine and set the idle to a fast idle (1500 RPM) and touch the end of you test lead to the hot post of the battery and watch the volt meter. It should go to about 15 volts. DO NOT leave it connected for more than a few seconds. If you seen the 15v then you have either a regulator or wiring problem. If you didn't get about 15v then the generator is bad. A Generator and regulator can be bad at the same time but you have to have a good generator to check a regulator.

If you replace the generator be sure to polarize the new or rebuilt generator before running it. You can use the same test lead to polarize that you just made. If you are not familiar with the procedure go to my web site and read that section under the "Tech Dept" in the main menu. http://mg-tri-jag.net


---------- FOLLOW-UP ----------

QUESTION: Hi Howard

Thanks for the reply.  The generator appears to be fine.  How can I test the voltage regulator?

Thanks a lot

Padraig

Answer
If the generator is good and you have no charge rate, then all you can do is trace each of the other wires on the regulator to see that they all do what they are suppose to do.
"E" should be black wires that are a good ground.
"F" should be the small field wire on the generator.
"A" should be all brown hot wires. (many regulators have 2 "A")
"WL" should be the warning light on the instrument.
"D" should be the large wire on the generator.

If each wire is correct and the generator is good and the grounds of engine to frame and frame to battery are good, and there is no charge rate, then the regulator is bad.
Howard