Bosch alternator installed, but not charging.

Okay, so the car has been not moving for a while now. but today I got the right sized fanbelt and a alternator bracket from a midget (works well enough for now. Will replace later.). So the alternator is physically installed correctly now.

But... it won't charge. Put a voltmeter on the battery and the alt and you see no rise in voltage above 12.whatever.

My layout is as per onefastmgb.com

Just to make things clear the layout is like this.
Thick brown (ny on wiring diagram) to battery post on alt.
blue wire (red in onefastmgb's harness) to battery post - Believed to be field wire.
white wire (brown in onefastmgb's harness) to brown/yellow striped wire. Leads to alt light.

The only difference I can tell from onefastmgb's setup is that I am using a 4 wire setup. But, after looking at a 3 wire setup, I can see that the extra wire isn't important anyways - it's in the centre, where one of the wires isn't used on the 3wire harness anyways.

What am I doing wrong here?

Maybe this can help, per Bob Munch.

Useful. But it doesn't apply in this case I don't think.

What i'd really love to find is a wiring diagram for bosch AL8657N alternators. Then I could figure everything out by myself. But it seems bosch doesn't want to give them out.

How do you have 4 wires?

Sorry. The alternator plug has four wires coming out of it. The name of the game is really where do these wires go?

The original Lucas plug or a plug you bought with the Alternator? MGB's with alternators only came with 3 wire and 5 wire plugs. Is this the Bosch 55 amp or the Saturn/Bosch 90+ amp?

Steve,
Did you try writing to the author of onefastMGB.com?? I am sure he is much more familiar with his layout than I am. One thing you mention that throws me, is what is this Blue wire?? You say you think it is the Field wire. Have you traced it out to find where it exists in the OE harness as per your car's wiring diagram? Or are you refering to the "blue" wire in onefastmgb's photos on his site? The only blue wire I see on his site appears to go to the water temp sending unit.

Have you traced it out to find where it exists in the OE harness as per your car's wiring diagram? Or are you refering to the "blue" wire in onefastmgb's photos on his site? The only blue wire I see on his site appears to go to the water temp sending unit

No, see lookup table provided:

Thick brown (ny on wiring diagram) to battery post on alt. (from cars wiring harness)
The rest of these wires are from the alternator plug.
blue wire (red in onefastmgb's harness) to battery post - Believed to be field wire.
white wire (brown in onefastmgb's harness) to brown/yellow striped wire. Leads to alt light.



Talking with people in the mg club tonight, we suspect it might be ground. I'm going to try grounding the chassis of the alt to the car chassis tomorrow and see what happens.

Steve On my 78 bs alternator conversion , i also had a 4 wire harness ,the alternator a 95 saturn replacement, wiring hook up went like this brown/yellow stripe from solinoid to thin brown from harrness. one black not necessary not used , just tape it up . the rest went to battery post on alternator. Hope this may be of help.

Ron-c