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MG MGB Technical - Alternator Charging?

I have a 78 roadster. I recently replaced the alternator with a rebuilt Lucas one. I have a digital voltmeter picking up its input across the battery. The ignition warning light glows before the engine is started and then goes out once the engine has started. While driving the voltmeter shows 13.5 volts or so and then occasionaly drops back to 11 volts or so. Then after a while it will pick back up to 13.5. The ignition warning light never comes on while this is occurring. The reason I replaced the alternator is the old one had lost its regulation - going from 11 volts to 18+ depending on the engine speed. Is the alternator working? or do I have some other problem with the electrical system?
G.E. Bulwinkle

This isn't right, it should be charging at a minimum of 14.3v up to a max of 14.7v. It should only drop to about 12v if you slow the engine right down so that the ignition warning light comes on again.

From the symptoms I'd suspect a bad connection in the thick brown wire(s) from the alternator output to the solenoid battery cable stud. You can monitor this by connecting the voltmeter +ve to the alternator output terminal along with its wires, and the meter -ve to the brown at the fusebox. In theory these two should always stay the same and hence the meter won't show anything. In practice as the loading on the alternator goes up and hence the current goes up you will get a small volt-drop developing between the two, but it shouldn't be more than a couple of tenths of a volt at any time. If you always see half a volt or more you have a constant bad connection, if it jumps up to several volts from time to time then it is going fully open-circuit.

However if the problem is inside the alternator then you are unlikely so see much on the meter, so connect the meter beween the output terminal on the alternator and an engine ground. If that goes between 13.5v and 11v also the problem is in the alternator.

Really you need two meters, one to show the system voltage at the same points all along, like at your battery, and the other moved from place to place so you can compare readings when the battery meter is going up and down.
Paul Hunt


I do have two digital voltmeters, one installed picking up the signal at the battery and another hand held. Also I've read through the electrical section of Bee and Vee. I will take your advice as soon as I get the car running.

G.E. Bulwinkle

Make sure your battery is in good condition, a shorted cell can hold down the system voltage.
John H

And would also significantly affect cranking performance, which is usually the first thing you would notice.
Paul Hunt

Also check that the big brown wire from the starter solenoid to the alternator is not split/frayed.

This thread was discussed between 28/01/2009 and 29/01/2009

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