MTFCA Forum

I picked up this 1926 roadster last week. It came without a battery. I’ve read here that model T is negative ground. The battery clamps however suggest that mine is postive ground (one connector is slightly bigger).
I hooked it up as negative ground. Engine starts, if I switch on the headlights, amperes go into minus, so it seems that I have done it correctly. I cannot imagine that an engine would start in opposite direction?
Is there another way to check?
Thanks!

pons0030 wrote: ↑

Sun Aug 02, 2020 10:19 am

I cannot imagine that an engine would start in opposite direction?

If you are only familiar with permanent magnet type DC motors, then you would be right. In a permanent magnet motor, when you reverse the polarity of the armature, the magnetic flux in the armature changes direction. Since the field is permanent magnets, they keep their previous direction of magnetic flux, and the motor then runs backwards. But this is a series would motor,it has electromagnets for both the field, and the armature. That means that when you reverse the polarity, you will make the magnetic flux go the opposite direction, in both the field, and the armature. Since they are both reversed, the result is that the motor always turns the same direction.

Negative ground for the Model T.

Why didn't you swap the the contacts on the ammeter too? Have you checked the cut-out polarity?

I did not do anything to the car. I am just wondering if there was another way of checking. Cutout? I do think that if the ammeter goes into negative when I switch on the lights polarity is correct, or am I wrong? Plus, what damage can it do?

Don't swap anything! The ammeter goes to negative with the lights on. That is correct. If your generator is working and the lights are off, the ammeter should go to positive. If so, everything is OK. With the lights on, your and the engine running, the lights on would go to negative at idle, and then go toward zero with the engine running faster. How far would depend on the adjustment of the generator. If you do most of your driving with the lights off, set it to less than 5 amps, but if you drive with the lights on, then adjust to where it is just above zero with the lights on and going about 20 mph. Note, if you have it adjusted for driving with lights on, than when you drive for distances over a mile or two with lights off, turn them on so you do not overcharge the battery. Some T's use an after market voltage regulator. If so, it should take care of the change from lights off to lights on by itself. I don't use one because I very rarely drive after dark if I can help it because of the slow speed and lack of visibility to other drivers. I live in a rural area so there are dark winding roads here, but if you live in a city with street lights and you wish to go to dinner or other night driving, it would be safe to do so.
Norm

pons0030 wrote: ↑

Sun Aug 02, 2020 10:19 am

.... The battery clamps however suggest that mine is postive ground (one connector is slightly bigger).
I hooked it up as negative ground. Engine starts, if I switch on the headlights, amperes go into minus, so it ...

Sorry, I misread what you said. Since everything appears to function correctly its likely that the wrong cable was used and that is causing all the questions. The cut-out sits on top of the generator and its purpose is to disconnect the circuit to the battery when the engine is off so the battery doesn't run the generator as a motor.

Henry Ford (actually Edsel was in charge by then) switched the Model A to a Positive + Ground and the polarity remained that way for quite a few years.

No one has ever found or published a reason for the change.

The supposed reason to use the positive ground was to reduce the corrosion at the battery terminals.
This is the story I was told, personally I think it’s a load of nonsense. JMHO

Had to do with the current (no pun intended) electrical theory at the time. The thought was (for a while) that juice went from Negative to Positive, so they were still putting the swith on the supply side by having positive, not negative, grounded.

I also heard from an old british iron mechanic that it created more corrosion issues at the switch. Not sure how or why, but I so know + earthed Lucas electrical systems tend to leak more smoke than their American counterparts, and do have connector and switch issues.

Anyway... A T came from the factory as a negative ground car. True, you can flash the generator and change the cutout (or regulator) to be positive ground (like a Model A - maybe they did this to use the newer parts in a T, I don't know), but you have the problem that, by modern convention, you go to jump the car and you pop and fry stuff by hooking it up backwards.

The person who gave You the jump is not happy either. You can reverse charge a battery. AX Me how I know.