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MG MGB Technical - What's this 3rd alternator wire?

Hello all,
On my '73 B, in trying to track down why the ignition warning light does not come on I found an extra wire to the alternator. The wiring diagram just shows 2.

I have the expected large brown and small brown/yellow. The mystery wire is large and attached to a large spade. I can't find it's insulation color. It seems to be wrapped in black tape.

It was wound together with the other alternator wires in the original (looking) blue tape, which I wound back 12 inches or more and still just see this black tape wrapping. This wire is hot without switch being on, just like the large brown wire.

question 1)
What is this wire?

question 2)
I grounded the small brown/yellow wire and the light works, should I assume I have a bad alternator?

Thanks! David
David Steverson

Sometimes there are two wires to the alternator and sometimes three. When there are three the 'extra' one is brown, and can either be standard gauge (i.e. same as the brown/yellow indicator wire) or the same gauge as the main brown output wire. In the first case the extra wire was used with 'remote sensing' alternators which controlled the voltage at the solenoid rather than at the alternator output, the alternators probably had one large spade and two standard. In the second case it is an extra output wire to increase current carrying capacity and reduce volt-drop on later cars, these alternatoes had two large output spades and one small for the brown/yellow. Even when there were only two factory wires the alternators had three spades - two large output and one standard indicator.

If you have anything else it is probably a PO mod, possibly utilising the 'spare' output spade to feed something, I've done that myself on my V8. I can't imagine the factory wrapping a single wire in black then wrapping it with the others in blue, since wrapping tape is available to owners it is still almost certainly a PO mod. The brown wires at the alternator are always at 12v (which is what I presume you mean and not 'hot to the touch') as they are connected to the battery cable at the solenoid. If both are at 12v with the alternator plug removed, and they are separate spades i.e. not connected together, then the black wrapped wire must also be connected to the main battery cable, probably down at the solenoid and is an additional output wire for some reason. If the black wire isn't at 12v when unplugged then it is feeding something, just see what no longer works!

If grounding the brown/yellow at the alternator with the ignition switched on causes the warning light to glow then the warning light circuit is working. If it doesn't glow when plugged in to the alternator then either the circuitry in the alternator is faulty, or the brown/yellow is connected to it incorrectly somehow.
Paul Hunt

Hi David Steverson, British Leyland wiring diagrams for MGB North America negative earth: 1971-72 with seat belt warning,Diagram 13 car numbers GHN5-267580 to294240-Tourer.Alternator or Dynamo shows two wires,Brown & Brown with Yellow tracer.
1972-73 Diagram 15 Car Nos.GHN5-294241 to 328100 Tourer, Alternator shows three wires,Brown, Brown, Brown/Yellow. 1973-74 Diagram 17 Car Nos.GHN5-328101 onwards-Tourer shows three wires,Brown, Brown,Brown/Yellow.
My Jan. 73 (UK Reg)has thin brown lower,thick brown middle,thin brown/yellow upper, conections.
Black with White tracer,same vicinity to Distributor.
Hope this helps. Johnny C.
JG Cook

All the diagrams I have for a '73 have two brown wires from the starter going to the alternator. A missing wire might be causing your problem.
There are after market alternators that internally bridge the two contacts for the brown wires.
Easy to read wiring diagrams are available at http://www.advanceautowire.com click on stock schematics.
Kimberly

Thank you all for your comments! I am going to jackthe car up and trace wires from the starter to the alternator just to see what I really have.

David
David Steverson

Whether there should be two browns between the alternator and the solenoid and there is only one, or there should only be one and there are two, is *not* the cause of why the ignition warning light doesn't work.

Where there should be two in the case of the thick output wire and the thin sense wire, a missing sense wire might cause the warning light to stay on when the engine is started and it should be off, but not the other way round.
Paul Hunt

Paul,
I think I'll replace the alternator, I can pick up a rebuilt unit from the local auto parts store for around $62.
David

PS Some friends from over your way are coming for a visit soon. I'm having them bring me a load of Bassett's Jelly Babies. We can't get the originals in the US anymore.
David Steverson

I dunno, political correctness gone mad ... :o)
Paul Hunt

This thread was discussed between 17/08/2008 and 21/08/2008

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