6V negative ground assistance needed....please

adavis

adavis

Senior Member
First off, my car seems to start/run fine with the way its wired but I would like to converse with someone who really knows model A wiring to make sure its done correctly. With that said, here is my scenario.....
When I bought my '31 coupe last year it was a running, driving car that had been in storage since 1969 but taken out a few weeks before I acquired it and "tuned up". When I got it home I noticed that it was wired NEGATIVE GROUND but honestly I'm more "comfortable" with that style of wiring so I didn't think much about it. I drove it around for a short time and then tore the car apart. Now its all back together and I have about 600 miles on it but I decided I wanted a tachometer so that is where I'm stuck. I have a 1950's Sun tach that I'm going to use and I don't want to screw it up so I started looking closely at the wiring on my car and I just want to make sure its correct.
I'm running a Mallory Dual Point distributor and a standard replacement model A on/off ignition switch. I'm a little confused as to which wire should go to the condenser on the side of the distributor and which wires should go to the coil. Right now, with the ignition off I can touch my test light to BOTH sides of the coil and the light comes on. Is that the way it is supposed to work, because it doesn't make sense to me. Like I said, this thing ran when I bought it and runs just fine now. When I turn the key off it quits. It charges just fine, the headlights work great, and the battery doesn't discharge even after 3 weeks without running. If anyone has a simple wiring schematic showing a negative ground with a "modern" distributor that would be great. Any advice/suggestions would be greatly appreciated. Thanks.
 
Most cars have the ign switch between the battery and coil;
when you test these with the ign switch off, there is no juice at either the coil or points.
Fords As ( for a reason I've not been able to fathom) have the ign swtich between the coil and points so when you test a Ford A with the ign switch off, there is still juice at both coil terminals, but no juice at the ponts. ( assuming of course no-one has rewired it diffferrently from original)
 
Someone may post a wiring diagram or some vendors have one in their catalog. Only difference from yours to original is positive on the orig is NOW negative. HOWEVER, I would be careful with the TACH as most 50's, about 56 and earlier were still POSITIVE ground, and it'd be a shame to blow a good tach. On the other hand it might not be ground sensitive? Others may chime in I'm sure.
Paul in CT
 
I ran my Model A for many years with 6V negative ground, as did about half of the automotive world before 1956. There's a chance that the Sun tach was designed for negative ground.

Steve
 
Thanks for the input. I feel A LOT better knowing that the coil is supposed to have power at both leads with the key off. That seems really odd to me (like it would drain the battery) but I'm no electrical guy either.
The Sun tach I've got is converted to solid state electronics for 6V negative ground. My whole concern is that I had something wrong because the coil had power on both leads, but if that is how its supposed to be I guess I'm good.
 
adavis,

When you say that the tach is converted to solid state electronics, what do you mean ? Was it vacuum tube technology ?

Also, how are you going to wire up the tach ?

Marc
 
If you are running a two post repro type switch instead of the original popout switch, connect the - side of the coil to one of the posts on the switch (doesn't matter which) The other side of the switch would connect to the condenser of the Mallory distributor. When converting to negative ground, you swap the battery cables and the coil wires and that is it for the wiring changes. If you are running a model A generator and cut out, you will need to polarize the generator by jumping a wire accross the posts of the cutout for a second. If you want to run a diode type cutout on negative ground , I have run model A generators negative ground and it can be hard to keep them charging negative ground. If the generator is completely rebuilt with new field coils and armature there should never be a problem. Old generators that have been running positive ground for years will revert back to positive and have to be repolarized on a regular basis.
 
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My original Sun transmitter box was gutted and a circuit board was installed, eliminating the need for obsolete mercury batteries. That was what I meant by "converted".
I guess what it comes down to is I need a 6V power source that is switched on/off by the ignition. In a normal car I could find that on the "ON" side of the ignition switch, but apparently the "ON" side of my model A ignition switch is simply passing power to the points (if I'm understanding what is said above). So then, is there anywhere on the car that is "hot" when the car is on/running and off when the car is off, or will I need to install some sort of switch just for the tach?
Thanks.
 
adavis said:
Thanks for the input. I feel A LOT better knowing that the coil is supposed to have power at both leads with the key off. That seems really odd to me (like it would drain the battery) but I'm no electrical guy either.
The Sun tach I've got is converted to solid state electronics for 6V negative ground. My whole concern is that I had something wrong because the coil had power on both leads, but if that is how its supposed to be I guess I'm good.
Click to expand...

Only one wire of the coil has power to it. What you are reading on the other post is voltage feeding through the coil windings. When the ignition switch is turned on AND the points are closed then you have a completed circuit to the coil. This is when it would run the battery down.
 
Model A wiring-diagram.jpg

I hope the wiring diagram attached
 
"...So then, is there anywhere on the car that is "hot" when the car is on/running and off when the car is off," The answer is -NO! The wire from the ign switch through the armoured cable to the points is the only one that fits this criteria, and since the points ground this wire when closed it'll be no use tapping into it.
"Will I need to install some sort of switch just for the tach?" The answer is Yes ! ( or re-wire with a different ign switch as Purdy suggests, or fit a seperate switch or relay for auxillaries)
 
adavis said:
My original Sun transmitter box was gutted and a circuit board was installed, eliminating the need for obsolete mercury batteries. That was what I meant by "converted".
I guess what it comes down to is I need a 6V power source that is switched on/off by the ignition. In a normal car I could find that on the "ON" side of the ignition switch, but apparently the "ON" side of my model A ignition switch is simply passing power to the points (if I'm understanding what is said above). So then, is there anywhere on the car that is "hot" when the car is on/running and off when the car is off, or will I need to install some sort of switch just for the tach?
Thanks.
Click to expand...

There is a simple change which will give you this option, just as in a modern car. Just run a wire from the ammeter output to the switch (you can use the wire that connects the coil input to the terminal box), and a wire from the switch output to the coil input. The red wire from the coil output can then be bolted to the armored cable behind the instrument panel. Be sure that this junction does not short to the armored cable shield, and is well insulated. The tach, or any other circuit such as turn signals, can then be connected to the switch output. This works with an aftermarket switch and armored cable, not with an original popout switch and cable.
 
OK, so I'm back and still don't have a working tach. I got the ignition switch from Snyder's that has the accessory post. They said the accessory would supply straight 6V power with the key on and it would be off with the key off. Not so. It is the same power that feeds the points, which means it is off and on with the points and varies directly with the rpm's.
So here's where I'm at. Is there anyone on the Barn that can help me draw out a "modern" wiring diagram for my coupe? I realize this is probably going to tick some of you off and you'll tell me to head to the HAMB, but this question is really specific to the Ford model A so I'm asking it here. I want to wire my car so the points are connected to the coil, not the ignition switch. While I'm at it I need to put in a modern ignition switch with a switched 6V power side. I could run a starter solenoid too if its necessary, but the foot lever is fine too. I would like to add a small fuse block to protect the lights, 6V power supply (cigarette lighter) and starter.
If anyone has done this and wouldn't mind walking me through it I'd greatly appreciate it. I'm not great with the electrical side of things so I'd rather have someone with that knowledge tell me what to do rather than screw things up by myself.
Once again, I apologize if this offends any of the purists on the site. My car is my daily driver and I'm not trying to restore it to perfection......just drive and enjoy it.
 
Buy the armored distributor cable that ends at the firewall, so you can run the wire directly to the coil. Then run power from the terminal box to the Snyder's switch you just bought and fron there to the coil. You can connect the tach and other accessories to the switch ACC terminal, or to the same terminal that now feeds power to the coil.

To fuse the lights you can just connect a fuse where the lights pick up the power at the cutout.
 
Paul in Colorado said:
There is a simple change which will give you this option, just as in a modern car. Just run a wire from the ammeter output to the switch (you can use the wire that connects the coil input to the terminal box), and a wire from the switch output to the coil input. The red wire from the coil output can then be bolted to the armored cable behind the instrument panel. Be sure that this junction does not short to the armored cable shield, and is well insulated. The tach, or any other circuit such as turn signals, can then be connected to the switch output. This works with an aftermarket switch and armored cable, not with an original popout switch and cable.
Click to expand...

I once tried to do something like this to install a little light bulb that would alert me to when the ignition switch was on. The problem was, the car refused to start--the points seemed to need all the power themselves. I'm sure the tach takes a lot less power than my light bulb, so maybe it will be ok--just something to watch out for.

I don't understand why the ACC connection on your new ignition switch doesn't do the job.

Steve
 
adavis said:
So I think I may have just found my answer, but HOLY SMOKES that's expensive! Is there another option or is this the only choice I have?

http://macsautoparts.com/model-a-fo...minal-a11575ema/camid/MDA/cp/JS0R3CHL1106108/
Click to expand...


That looks like what I'm using, but I think I've seen that sort of thing on racks at NAPA type stores.

DON'T CHEAP OUT on the ignition switch. I've had to replace 3 or 4 of the cheapies in my career, each time after being driven nearly nuts isolating the intermittent problem.

Steve
 
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