Changing from Positive Ground to Negative Ground on 1939 Ford

Ramblin Reck

Ramblin Reck

Senior Member
My 1939 Ford had already been converted to 12 Volts but it was left as a Positive Ground. Why is anybody's guess. The generator has been replaced with a rebuilt generator and she starts and runs just fine with no problems. This is a fairly simple process which I may have over complicated in my head, After talking with many folks about this process I believe I have down to a few simple steps.



Step 1 Disconnect the battery and swap the wires from positive to negative. In other words connect the Negative cable to the chassis/engine block and connect the Positive cable to the starting solenoid.


Step 2 Disconnect both wires from the Coil. Change the wire that connects the Coil to The Distributor to the other post on the coil and reconnect the wire coming from the switch to the other empty post. In other words, reverse the coil from the way it is now.



Step 3 Polarize the Generator by Flashing the Generator. Engine is off during all of these steps. Remove the "F" or Field wire from the Voltage Regulator and momentarily touch it to the "B" Terminal on the Voltage Regulator so that it sparks. Replace "F" to where it had been connected. The Generator is now "POLARIZED"!



Step 4 Change the bulbs to 12volt if not already done.


Step 5 Change the gauges to 12 volt or mechanical type gauges, or install a Voltage Reducer to make the 6 volt gauges work.



Any comments, particularly those of affirmation, will be appreciated.
 
A couple of thoughts- I didnt know you could easily change the polarization of a generator, thats interesting. Can you do the same with a starter? Seems like it would try to go the wrong way if the current were reversed. And to use original gauges if you just used a voltage regulator you would be creating +6V with respect to ground, you need -6V. I have been meaning to see whats out there in an isolated DC/DC converter with some current capability which is what you would need. May have to design my own...
 
BlueSpark said:
Can you do the same with a starter? Seems like it would try to go the wrong way if the current were reversed.
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As a rule, starters are not polarity sensitive. They work by the interaction of the magnetic fields from two windings. When you switch the polarity of the incoming power, the fields both switch polarity and the starter still turns the same direction.

Coop

.
 
Not an electrical wizard, but if you do it in the steps you list -- I can see sparks from where I am at.
Suggest that Step 1 is just disconnect the battery cables and then switch them from chassis and solenoid, with no mention of reconnecting them to the battery. And then create a Step 6 -- reconnect the battery cables to the battery.

I know that you probably will be doing that, but there are going to be someone that will read it and do exactly as you say.
 
I've never tried reversing the polarity of the field pole shoes. I've always had access to an AC coil to demagnetise parts. This is common in the magnaflux process. A person might have to give them a reverse shot of DC current several times to get the job done. If the generator works, then you'll know it took.

Battery terminal clamps might have to be changed to fit the different diameter battery posts. Starters are shunt wound for high torque so they aren't polarity sensitive.
 
Ramblin Reck said:
My 1939 Ford had already been converted to 12 Volts but it was left as a Positive Ground. Why is anybody's guess. The generator has been replaced with a rebuilt generator and she starts and runs just fine with no problems. This is a fairly simple process which I may have over complicated in my head, After talking with many folks about this process I believe I have down to a few simple steps.



Step 1 Disconnect the battery and swap the wires from positive to negative. In other words connect the Negative cable to the chassis/engine block and connect the Positive cable to the starting solenoid.


Step 2 Disconnect both wires from the Coil. Change the wire that connects the Coil to The Distributor to the other post on the coil and reconnect the wire coming from the switch to the other empty post. In other words, reverse the coil from the way it is now.



Step 3 Polarize the Generator by Flashing the Generator. Engine is off during all of these steps. Remove the "F" or Field wire from the Voltage Regulator and momentarily touch it to the "B" Terminal on the Voltage Regulator so that it sparks. Replace "F" to where it had been connected. The Generator is now "POLARIZED"!



Step 4 Change the bulbs to 12volt if not already done.


Step 5 Change the gauges to 12 volt or mechanical type gauges, or install a Voltage Reducer to make the 6 volt gauges work.



Any comments, particularly those of affirmation, will be appreciated.
Click to expand...
Great to see a simple plan. Thanks for putting in the effort to inform us.
Obviously you can't please everyone!
 
Not sure why you would want to change any Mechanic familia with old Fords would likely end up with the Battery switched , Ted
 
V8COOPMAN said:
As a rule, starters are not polarity sensitive. They work by the interaction of the magnetic fields from two windings. When you switch the polarity of the incoming power, the fields both switch polarity and the starter still turns the same direction.

Coop

.
Click to expand...


Coop
Dead on Balls Accurate. This was my chief concern until I researched the subject. Thanks for the comment!
 
nelsb01 said:
Not an electrical wizard, but if you do it in the steps you list -- I can see sparks from where I am at.
Suggest that Step 1 is just disconnect the battery cables and then switch them from chassis and solenoid, with no mention of reconnecting them to the battery. And then create a Step 6 -- reconnect the battery cables to the battery.

I know that you probably will be doing that, but there are going to be someone that will read it and do exactly as you say.
Click to expand...



nelsb01
Anyone doing this, is doing it at their own risk and peril. You are absolutely correct that "sparks" will fly when I polarize the generator by touching the VR's "F" Field wire to the "B" post on the VR. That is how the Ginny gets polarized. Or re-polarized is maybe the correct term. This advice came from a man who rebuilds Vintage Generators for a living. In fact he told me I did not need to remove the "F" wire but simply use a jumper wire to get the momentary spark needed to polarize the Ginny. Believe he said, do it a couple of times. Thanks for your concern. I haven't worked up the courage to perform this task but will advise the results when I do it.
 
Ramblin sounds like your right on it your right that it may take several goes to get the polarization to take due to residual magnetic fields in the gen that's why the will work when dead flat [an alternator wont] . Only thing to add is the output will be low to start but will increase with usage as the polarization takes effect properly, generally start and run for 5 mins about 3 times then should be all good
 
I was thinking about this a bit more and wonder about the generator cutout. Mine was replaced by the generator rebuilder and looks to be potted in so not something that can be opened up. Are these specific to a particular polarity? Are they still a relay or are newer ones solid state? If the latter they will certainly be polarity sensitive.
 
The generator control unit needs to match the generator. The 12-volt units made for Fords were all negative ground. Most early ones are 35-amp units but there were 40 & more amp units depending on how the vehicle it came from was electrically equipped.
 
aussie merc said:
Ramblin sounds like your right on it your right that it may take several goes to get the polarization to take due to residual magnetic fields in the gen that's why the will work when dead flat [an alternator wont] . Only thing to add is the output will be low to start but will increase with usage as the polarization takes effect properly, generally start and run for 5 mins about 3 times then should be all good
Click to expand...


aussie merc
Thank you so much for this positive feedback. I will update the instructions to my self. Your advice is truly appreciated.
 
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