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MG MGB Technical - Alternator question

I was told running the alternator without wires connected to it will damage the alternator. That happened to me and I would like to know if there is a simple test, that a laymen can do, that will confirm that my alternator in malfunctioning. Thanks in advance
Peter Murray

Peter. Battery voltage, on a fully charged battery, is in the 12.5 to 13.0 volt range. The alternator, when it is working, produces 13.5 to 14.5 volts.

So, a simple check is to measure the battery voltage, start the engine, run it up to over 1,000 rpm and check the system voltage to see if it is higher than battery voltage. You can get a couple of the spring clips, as used in battery chargers, at Radio Shack, hook them up to jumper wires, add an alligator clip on the other end of each wire, then connect the alligator clips to the appropriate probes on your volt meter, the battery clips to the battery and make the test from the driver's seat.

Make sure that the clip attached to the positive terminal of the battery cannot ground out against the frame when performing this test. You will find that, over the years, the battery to volt meter jumper wires will find use.

Les Bengtson

Thanks Les, will check it tonight!!!

You won't damage an alternator by running the engine with it disconnected, or at least I didn't when I forgot to plug the harness back in after changing a head gasket, but I wouldn't recommend doing it. When the engine started to splutter and die some 30 miles later I instantly realised what I had done. Pulled in to a convenient petrol station and plugged it back in, but the battery was flat of course. The attendant took some convincing to help me push start it, but of course it started very easily, and the alternator was none the worse.

However you *can* damage it and blow bulbs and other electronic components if you run the engine with the *battery* disconnected, as the voltage can get very much higher than 14.5v.
Paul Hunt

This thread was discussed between 02/12/2008 and 03/12/2008

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