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MG MGA - My alternator conversion...

A couple years ago I installed an alternator on my 1958 MGA when I rebuilt the motor (1500). I followed the instructions to the letter. The engine runs just fine. However, in my limited mechanical experience I thought that when the battery is disconnected while the engine is running the alternator would keep the engine running. To my surprise the engine died when I disconnected the battery (the negative cable). Of course I only thought to perform the test recently!

Is my alternator working properly?
Is the alternator charging the battery?
Is there something in addition to the instructions I should have done?

gerard hutchinson

Im not an expert on auto-electrics Gerard but I believe the alternator will not run if you disconnect the battery.
To my knowledge the alternator needs a power supply from the ignition switch to run, I think it is known as the "exciter" circuit. If you are fitting a Lucas type alternator this wire will be the thinner of the two wires,normally yellow green. So if you cut the battery connection then you are also cutting this exciter circuit.
The thicker wire is the power output wire to the battery and is usually yellow.

If you have been running it for a couple of years and it has been running ok, ie battery not going flat and ignition light going out as soon as the engine is running then I wouldnt worry.

To put your mind at rest, check the voltage at the battery when the engine is stopped and also when it is running. You will find that the reading will probably be 12 something when stopped and 13 something when running if the alternator is charging ok.

Colyn Firth

Running an alternator with the battery disconnected can blow the alternator diodes.

M F Anderson

Not sure how to give you an answer to this Gerard but would like to note the following

The engine should run with the battery disconnected, however as noted this can damage the rectifier if the battery is disconnected whilst the engine is reving at high speed. (plenty of amps) if the engine were idling then it ought not to be a problem but is not good practice.

The first thing I would do in this situation is measure the voltage across the battery/ies this should be above 14 volts?
Bob Turbo Midget England

Thank you gentlemen. This board is always a huge help!
gerard hutchinson

This thread was discussed between 01/06/2011 and 08/06/2011

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