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MG MGA - alternator charging light

I am in the process of restoring my ‘60 A and the rolling chassis, engine and drive train are ready to test. I have converted to an alternator and, before starting the engine, I noticed what I think is a problem.

The wiring harness is not being used. I wired the coil, the fuel pump and a small 12 volt bulb (so simulate the charging light) to the battery + (neg. ground) through a switch. The alternator (Lucas) has three terminals. The two larger terminals are connected to the battery + (at the starter switch). The smaller terminal is connected to the other side of the light bulb. Pretty simple. When I turn on the “ignition” switch, the fuel pump starts running, but the charging lamp does not light. Shouldn’t it? I didn’t want to start the engine and spin the alternator until I felt more comfortable.

More info – With the alternator not running, I expected to see 12 volts between the alternator small terminal and battery +. Nothing there. (I unplugged the wiring connector from the alternator and measured directly from the Batt + and the small terminal.)

If this is not right, any ideas on what could be the problem? As I see it, it must be the alternator. I’m not sure about the history of the alternator as it was part of the “engine rebuild”. It looks new. Any tests I can do on the alternator itself before I go back the engine shop?

Frank Bruns
FJ Bruns

Hi Frank (how's hot lips? LOL)(jokE)

Seriously if I understand you correctly then the test ought to work. The ignition lamp should be wired to the battery plus (normally via the ignition sw) then onto the small terminal of the alternator which is exactly as you have it.

So it should light up when you switch on, therefore either the alternaotr is faulty, you can check the brushes initially, or perhaps the alternator i not earthed correctly. Sometimes the alternator does no earth to the engine very well especially when someone paints the mounting brackets. Chech this by fitting a flying lead to the earth (battery neg) and the casing of he alternator.
Robert (Bob) Midget Turbo

I get the MASH reference a lot. I only wish Hot Lips was in my past!

Thanks for the confirmation. I checked the alternator again today and, yep, its bad. To fully check it, i attached a flying lead to the small terminal in the cable (detached from the alternator) touching the other end of the lead to the car chassis -- the bulb lit. Then I touched it to the alternator frame -- the bulb lit. Now touching it to the small terminal on the alternator -- no light. The alterantor is now removed and going back to the rebuilder.

Thanks again.
FJ Bruns

This thread was discussed on 22/01/2012

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