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MG MGB Technical - Altenator to Starter wiring question :)

Can one of you bright sparks solve my queerie please?
There is a large brown wire from the Starter solenoid to Altenator and a thin brown one.
Why? Would it be better to have the large wire only and fit a link at the Altenator end?
Dave
D M Tetlow

I can't answer your question, but maybe my comments will spark an answer.

When I purchased my '64 B, it had a non-original alternator. There was one large wire from the battery terminal on the starter solenoid to the alternator. There was also a wire going from the starter solenoid to the control box, and a smaller wire from the alternator to the warning light lug of the control box.

I assume the smaller wire on your set-up is for the warning light.

On the '76 B we used to have, there was a plug at the end of the harness. I can't tell you what the wires went to.
Bill Barge

The thick brown wire carries the charging current, and the thin brown wire measures the system voltage, at least that was "The Plan" as I remember it. (The voltage at the alternator is slightly higher, due to the voltage drop down the cable.)

Since the two connections are common inside most alternators, (they are now machine sensed) I think this is redundant, but will carry some of the charging current.

The brown wire with a coloured trace is the warning light feed.
Martin Layton

Thanks Martin.
Sounds sensible to me. I will replace both wires as per the book just to be on the safe side.
Dave
D M Tetlow

Originally it was as Martin said, the alternator sensed the voltage at the solenoid and not at its own output terminals. This meant that under high current conditions the earlier alternators could compensate for any volt-drop in the connection between the alternator and the solenoid, so maintaining a more constant system and battery voltage.

With later alternators Lucas put the sensing link inside the alt, and only one brown wire (thick) to the solenoid. This meant that under high currents you do get a slight system voltage drop that you wouldn't before, but but saved a few groats.

Later still there were two thick browns from alternator to solenoid, which reduces the volt-drop again but via a different method.

If you look at the internal wiring to the alternator spades you should be able to work out whether the two non-indicator spades are connected together directly i.e. they are both output spades. But if in doubt conenct both, just be sure you have the tick one on the output and the thin on the sense.
Paul Hunt 2

Thanks Paul
Dave
D M Tetlow

This thread was discussed between 29/07/2007 and 30/07/2007

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