12V Negative Ground Wiring Diagram

paulizor

paulizor

Junior Member
I bought a sort-of redone '30 CCPU earlier this year. Happier than a kid with a new toy. Over 3 weeks ago, and after a lot of mechanical tinkering to improver performance and braking, she died with repeatedly blown fuses, frayed wiring harness and some really hokey wiring connections. The re-do from the previous owner was a 12V conversion, negative ground and alternator. I searched seemingly everywhere for a wiring diagram, asked this Forum and some vendors and the InterWeb. I got some replies like, should have stayed with Original, and such. But, after a new harness from Snyder's and merging bits and pieces of good info from FordBarn, here is what I put together (and everything works!) Hope this helps the next Junior Member.
 

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Nice job paulizor. thanks drawing up your diagram and for sharing. I notice you run a 20 amp fuse?

I have the same basic setup but I have a 30 amp fuse for (what I refer to) the Main Fuse. I added a second 25 amp fuse on the yellow wires going to the light switch and horn from the alternator. Why? Because when I first bought my car, the first time the family was out after dark and I turned on the lights, it killed the engine (Blew the fuse). So here we are on a busy street with no lights and no engine.

After I got it home I found a short in one of the headlights. So I added the second fuse so if I ever have a short in the lighting circuit again at least it won't kill the engine.
Actually I have rewired the headlights with relays so it should never happen again. Over kill? Yea. Stock? No, but it's all hidden so nobody knows they're there.
 
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I've changed my wiring to go from the switch to the coil rather that coil to the switch. That way, when the key is off their is no power going to the coil. It just made sense to me using 12v.
Brian
 
The best and easiest to follow wiring diagram that I have ever seen for the model A is on page 94 of Brattons catalog . If you used original style color coded wiring harnesses and follow this diagram, it really couldn't be more simple. To change to negative ground you first install the battery negative ground. you reverse the connections at the ammeter and the coil. You will need a ballast if you continue to use the six volt coil, or change to a 3.0 OHM coil that is an internally resisted coil that is suitable for use on a twelve volt system. Probably most of the ammeters offered are 26-27 model T and will register backwards and this will have to be checked out. I also use a 30 amp fuse on my 12 volt applications.
 
Thanks to Les Andrews.
 

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There are three things that change from stock wiring (pos gnd) and neg gnd. The ammeter wires are swapped, the coil wires are swapped and the battery leads are swapped.

The point to point wiring does not care about the voltage (bulbs and coil do, but not the wires). Even the changing of a generator to alternator makes no difference.

You have a great drawing and now a full understanding of how your car is wired and I applaud you for doing so.
 
Purdy Swoft said:
The best and easiest to follow wiring diagram that I have ever seen for the model A is on page 94 of Brattons catalog . If you used original style color coded wiring harnesses and follow this diagram, it really couldn't be more simple. To change to negative ground you first install the battery negative ground. you reverse the connections at the ammeter and the coil. You will need a ballast if you continue to use the six volt coil, or change to a 3.0 OHM coil that is an internally resisted coil that is suitable for use on a twelve volt system. Probably most of the ammeters offered are 26-27 model T and will register backwards and this will have to be checked out. I also use a 30 amp fuse on my 12 volt applications.
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As usual excellent advice Purdy. I copied this page, plasticized it and put it in each of my A's. Wayne
 
daveymc29 said:
Do you have the pop-out ignition switch still? Or is that possible?
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I don't have the pop out and I've gathered from my readings that the pop out may have grounding issues when converted to negative ground. Maybe someone knowledgeable can comment.
 
paulizor said:
I don't have the pop out and I've gathered from my readings that the pop out may have grounding issues when converted to negative ground. Maybe someone knowledgeable can comment.
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The popout can have the inside wire ground out if it's the old original wire with worn out hard insulation, but the popout doesn't care about polarity.
 
You are right about the wiring diagram in the Brattons's Parts catalogue. I am going to see about getting a better copy from them and having it plasticized for future reference.
Gerry Birch Bay WA.
 
I've changed my wiring to go from the switch to the coil rather that coil to the switch. That way, when the key is off their is no power going to the coil. It just made sense to me using 12v.
Brian

Hello, I was wondering if you could elaborate on this.

Any diagrams or reasoning you would care to share.
I am putting in a 12volt one wire alternator and I want to get it right with any possible improvements.
12 volt neg ground system. I understand what you are saying that the coil should not be hot at all times is a good idea,I just want to picture how to do it.
Thanks, Herve Boissonnault
 
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