Alternator Wiring

[DJThom]

I was reviewing the wiring diagram for my '76 MGB last night in order to sort out my alternator conversion on the Elan. I noticed the alternator has 2 brown wires going to it that both originate from the same place - the post where the starter solenoid is connected to the battery.

Why the redundant wire?

 

[Froggmann]

I can only guess that one is the battery charge circut and the other is the field wire for the regulator.

 

[Tom_Fisher]

In the 69 MGB alternator, there are three yellow-brown wires that go into the alternator plug. However, if you do some tracing you will discover that one of the yellow-brown wires runs up into the harness and doubles back giving the appearance that there are two wires. So what appears to be three yellow-brown wires really is in fact only only two. My impression is that this wire is used as a jumper in the plug.

Hope this helps.

 

[AlanT]

The thin brown wire is a 'shadow wire' and is needed to make the alternator warning lamp operate correctly. But if the unit has been rebuilt it could have received the later one-size-fits-all rectifier that may or may not need the extra wire.

Alan T

 

[DrEntropy]

This is a Series-2 Elan?

The factory "Workshop Manual" has a piece in "Section M" about alternator conversion.... "M.11"

All ya really need is an excitor wire and an output to battery and system. Don't "OverTech" the conversion!

 

[DJThom]

I agree with Tom, essentially my brown wire doubles back. Some of the other years only have 2 wires to alternator: 1 for battery and one for warning light. They make the jump internally.

I'll check out the reference DOC makes. I think I have it right now and didn't need the extra wire. I redid my harness so I could take off the big dynamo box on the firewall and bring the harness down low to hide it.

 

[DrEntropy]

I kinda like that low-tech lump of a regulator! /bcforum/images/%%GRAEMLIN_URL%%/jester.gif

I've run Oscar Juniors in the grill, Cibie H-4's as headlamps. Never had a voltage issue. I do pay very close attention to ground wires/straps tho. /bcforum/images/%%GRAEMLIN_URL%%/wink.gif

 

[Nunyas]

On a 176 B there should be 3 wires connected to the alternator. The little wire is for the charge light in the dash. The two big wires connect to the same lug on the starter. At the alternator they're connected to two separate lugs. My best guess is that it is so you can take advantage of both sides of the wave form that an alternator makes. Alternators create AC voltage (and current) that then goes through a rectifier to change it to DC voltage and current.

On my car I've connected both big wires to the same connector on the alternator with no 'ill effects'. However, I DO notice that there is a cyclic dip in voltage. Particularly at night, when I can see the light from my headlamps cyclicly (is that even a word?) dim (or would flicker be a better description? hmmm) for less than half a second.

Of course, my assumption here is that the alternator is using a full wave rectifier, and the two heavy wires going from the alternator to the starter serves a dual purpose:
a) Twin take offs from the full wave rectifier/voltage regulator to take full advantage of the complete power production of the alternator.
AND
b) The two heavy gage wires add to the total power carrying capabilities of the electrical system. Particularly in the high current sections of the electrical system; those being the sections between the alternator, starter, and battery.

Also (this is a bit of an after thought), the blade connectors on the alternator can only accept a finite wire gage that is much smaller than the gage of wire that connects the battery to starter (the battery to starter wire is the heaviest gage (possibly 4gage or 2gage) wire in a 76 MGB from what I've seen). So the two connectors allow you to connect two heavy gage wires to the alternator to make up for the limitations of the blade connectors.