MTFCA Forum

Thanks to all who responded to my previous post. My 1916 with a 1921 engine is positive ground. To change it to negative ground I plan to:
1) reverse the battery cables
2) polarize the generator by briefly touching B+ to the field terminal
Should I disconnect the generator wire while I polarize the field (and reconnect it before I start the engine)?
3) reverse the wires on the ammeter
4) start the engine and hope it works!
Have I left anything out?

If it was mine I would just reverse the cables and try it.
Then worry about the rest.

Dan has good advice, except your battery cables are at a set length.......It's better to unclamp your battery from a carrier. Pick it up. Rotate the battery 180° reinstall so the negative terminal is closest to the driver side rail of the frame, the connect the " short " negative battery cable... then the longer cable ( positive + post ) that goes to the starter foot switch.

Hope these online pictures are helpful.

Close-up on positive terminal starter switch cable

Martin wrote: ↑

Wed May 14, 2025 2:33 pm

Thanks to all who responded to my previous post. My 1916 with a 1921 engine is positive ground. To change it to negative ground I plan to:
1) reverse the battery cables
2) polarize the generator by briefly touching B+ to the field terminal
Should I disconnect the generator wire while I polarize the field (and reconnect it before I start the engine)?
3) reverse the wires on the ammeter
4) start the engine and hope it works!
Have I left anything out?

Not being an automotive electrical expert in any way shape or form, my first if not only question would be "Why"? What's the advantage? (OK, 2 questions!) I see none. And a good way to ruin a battery charger if out of "habit" you hook it up wrong. Don't ask me how I know!

Both of my newer Schumacher chargers have an "idiot" light that displays if one reverses polarity !

love2T's wrote: ↑

Thu May 15, 2025 8:31 am

Martin wrote: ↑

Wed May 14, 2025 2:33 pm

Thanks to all who responded to my previous post. My 1916 with a 1921 engine is positive ground. To change it to negative ground I plan to:
1) reverse the battery cables
2) polarize the generator by briefly touching B+ to the field terminal
Should I disconnect the generator wire while I polarize the field (and reconnect it before I start the engine)?
3) reverse the wires on the ammeter
4) start the engine and hope it works!
Have I left anything out?

Not being an automotive electrical expert in any way shape or form, my first if not only question would be "Why"? What's the advantage? (OK, 2 questions!) I see none. And a good way to ruin a battery charger if out of "habit" you hook it up wrong. Don't ask me how I know!

I know positive ground veekles rust easier, as you are making the frame a diode, or sumthin like that.

I look at the battery terminals prior to placing a charger on the battery.

I didn't know you could change from + to -

learn sumthin' knew erry day.....

Dan Hatch wrote: ↑

Wed May 14, 2025 2:37 pm

If it was mine I would just reverse the cables and try it.
Then worry about the rest.

Dan,

How silly to think it could be that easy.

Don't forget to change the wires on the coil too. If you don't change them, you loose about 20% of the intensity of the spark at the plug

Tom... Could you give a little bit more help on that... Explain your procedure and supplement with pictures if possible. Thanks.

The Model A ran with a positive ground. The cutout is different. The Model T is negative ground. Don't know whether the starter is wound differently for pos or neg ground, but it might be. You don't want the starter to spin backward. The coils should work either way, however, if you have a distributor, Not sure.Your spark plugs might have a problem? If you are using a radio in the car, it might also cause a problem So be sure why the polarity has been reversed.
Norm

Good morning Norm... I totally agree with you.... The whole basis of this problem reverts back to improper battery installation. I've seen this a few times in the last 40 years where people, on their own, assume... the Model T electrical system worked the same as the later Model. A electrical system being that it was a positive grounded system..... Not purposely ignoring any Model T information but just assuming that it was the same. ..... And for those who experienced these assumed " problems " , installing the battery with the proper negative ground fixed the problems. It is amazing how by following manufacturers guidelines will cure assumptions and ignorance.

Moxie26 wrote: ↑

Thu May 15, 2025 5:31 pm

Tom... Could you give a little bit more help on that... Explain your procedure and supplement with pictures if possible. Thanks.

I believe he is referring to the preference to arrange the high-tension polarity such that electrons flow from the spark plugs central electrode to the grounding electrode, and not the other way around. On a distributor ignition system, it is simple to reverse the low voltage connections to the coil to accomplish this.

With stock timer and trembler coils...it gets way more complicated especially when operating on magneto is considered.

From time to time prolonged discussions on the topic occur here, if you do an external Google search on the site for Spark Plug Polarity you can find several.

Line_Noise..... Your posting reflects Google Artificial Intelligence.... What questions do you have on negative grounding for the Model T electric system that is different from Ron Patterson's diagram ???

WillyR wrote: ↑

Thu May 15, 2025 11:13 am

love2T's wrote: ↑

Thu May 15, 2025 8:31 am

Martin wrote: ↑

Wed May 14, 2025 2:33 pm

Thanks to all who responded to my previous post. My 1916 with a 1921 engine is positive ground. To change it to negative ground I plan to:
1) reverse the battery cables
2) polarize the generator by briefly touching B+ to the field terminal
Should I disconnect the generator wire while I polarize the field (and reconnect it before I start the engine)?
3) reverse the wires on the ammeter
4) start the engine and hope it works!
Have I left anything out?

Not being an automotive electrical expert in any way shape or form, my first if not only question would be "Why"? What's the advantage? (OK, 2 questions!) I see none. And a good way to ruin a battery charger if out of "habit" you hook it up wrong. Don't ask me how I know!

I know positive ground veekles rust easier, as you are making the frame a diode, or sumthin like that.

I look at the battery terminals prior to placing a charger on the battery.

I didn't know you could change from + to -

learn sumthin' knew erry day.....

The problem is that DC electricity actually flows from negative to positive. This is called Electron theory; an Electron is a negatively charged sub-atomic particle. The reason that a battery has a negative post is that is where the Electrons are. In the automotive world we use what is called conventional theory and pretend that Electrons flow from positive to negative. It may be easier to understand and visualize conventional theory, but that ain't what is actually happening.

My post is not AI, I typed it myself and did not copy from or consult any AI tools. Maybe it is just my writing style.

I was attempting to clarify why Tom V. was suggesting the coil wires be reversed.

The "20%" in Tom V's post refers to the potentially weaker spark when the high tension current flow is not in the preferred direction (preferred = electrons from spark plug center electrode to grounding strap).

I do not have any questions about wiring the connections in the car and have no disagreement with Ron's diagram. The connections I am referring to are on the internal high voltage secondary windings inside the coil boxes which are not shown on the diagram.

If Martin's car has a distributor, then Tom's suggestion is easily accomplished. If Martin's car has stock coils and timer, then likely no connections involved were modified when converted to positive ground, so nothing needs to change to revert to negative ground.

Maybe Tom V. knows or assumes Martin's car has a distributor. I don't know, maybe we will find out if one of them revisits this post.

In a stock T ignition system, running on mag, depending on the node, the coil is being fired with either a positive OR negative charge. AC current is in play. Running on Battery, it depends on how the coil is wired internally. I am told ford & kw coils are wired differently.
If you really cared, you could match your power & coil for optimum spark direction. Do it really matter for 99.9% of us - no.
With modern, can coil ignitions, it mattered enough that manufacturers specify + & - wiring.