[Mgs] alternator wiring quick question

PaulHunt73 paulhunt73 at virginmedia.com
Thu Aug 13 01:31:27 MDT 2015

Firstly I don't really see why a temp sender or gauge problem should make it run rough under load, but if it's OK now then it's water under the bridge.

"But there’s a single heavy brown positive lead that has a large double black connector (made for two heavy wire leads) but only has one female connector in it?"

This sounds like a PO mod to me, there is nothing like that round any MGB alternator that I'm aware of.  Does it carry 12v all the time?

>From what I have been able to determine from 69 to 71 a 16ACR alternator was used that had three connections close together on the back.  It seems that the middle one of these was a large spade and is the output terminal.  To one side of that is a standard spade which is the warning light terminal, and the third terminal looks like a standard spade but has chamfered corners as in http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/altpm2.jpg.  This was the battery sensing terminal which had a standard gauge brown wire going back to the solenoid battery cable terminal, as well as the heavy gauge output wire.  This means that under heavy loads the alternator maintains a better system and battery voltage as it is able to ignore losses in the output wire between alternator and solenoid.

That was replaced by a different system, also three pin, but with two large spades and one standard as here http://www.paulhunt73.webspace.virginmedia.com/mgb-stuff/images/altmetro.jpg.  The two large spades are connected together i.e. are both outputs, although at first only one was used i.e. only two wires to the alt.  On later models a second heavy gauge brown was used to reduce losses under heavy current conditions and maintain higher battery and system voltage.

Most MGBs probably have different alts now to what they came out of the factory with, and with the four wiring systems (the first alts had two plugs and up to five wires) it is anyone's guess how they have been modified.

If the warning light comes on with the ignition, and goes off when the engine is started and revved over 1000 rpm, and if you then see about 14v on the brown at the fusebox, it is charging OK.

If your mystery brown does have battery voltage all the time, and you have an unused large output terminal on the alt, then you could connect it to that.  But unless you have added some large loads to the electrics it shouldn't be needed.  In which case carefully insulate the end and tie it in to the harness somewhere so it can't short to anything.


----- Original Message ----- 
  . I got the three prong plug back on, the temp sensor lead and the oil press transmitter lead all put back on ok.  But there’s a single heavy brown positive lead that has a large double black connector (made for two heavy wire leads) but only has one female connector in it?
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