MTFCA Forum

Though I still have tons to learn I have rebuilt/refurbished 3 Model T’s so some stuff I have done many times.

The original ammeter on the ‘24 Coupe was not working when I bought the T and someone had wired across the rear terminals in order for the engine to run. I replaced with a repro ammeter today and it registers backwards though it is wired properly. The wire lengths to the back of the ammeter are obvious and I also looked at the MTFCA electric book to refresh my memory.

When the headlights are turned on with the motor off the needle goes to the “charge” side, not discharge.

What am I obviously missing here?

I think some are, mine registered backwards when I got the car. Maybe designed for another, positive ground car?

Yes, same issue with my 26 T. Fitted new one and needle worked in reverse. Just swapped the connections over and all good. These are cheap and rubbish and would love
to buy a quality version that fit the 26/27
Alan in Western Australia

Reverse the wires on the ammeter. Your car should be 6 volt, negative ground. You may have a Model A ammeter, or a generic ammeter. The Model A and later Fords used positive ground.

Yes, I am wired 6v, negative ground and have switched the wires. Though much more expensive and for the Model A I wonder if this brand is more reliable and worth the price?
https://www.westach.com/1-5-inch
The tractor guys appear to like this brand.
With enough interest maybe one for the Model T?

I know the model A Fords were positive ground (I don't know if early V8s were or not?). That is where the old misinformation came from that mechanics fifty years ago thought ALL antique Fords were positive ground! That is why for half a century, many people buy their model T toy and find everything wired backwards. Not really a big deal. But a lot of model T owners in the past half century have had to correct their cars.
The ammeter itself doesn't know the difference, or care. It simply reads the electron's direction and to some extent strength of flow. And yes, a positive ground ammeter is wired backwards of a negative ground ammeter (or the printing on the face is reversed?). Swapping the wires should be fine.

Also, there used to not be any standardized direction for the ammeter to read. While most cars may have fit the usual - and + position? There were many exceptions, and many replacement ammeters, that were reversed. When driving a model T you are not familiar with? Take a close look to be sure you are reading it right. When I drive an unfamiliar car? I usually turn on the headlamps for a second just to see that the ammeter reads the right direction, as Michael P mentioned in his OP. I also do that because years ago I met a model T owner that wired his ammeter backwards because his generator didn't work. And he wanted people that looked into his car with the engine running to think the generator did work!

An ammeter typically measures the amount of current flow from the device being measured connected in series to a current load placed upon it. It is not a grounded device per se. For it to work properly the connections to it must be attached in the direction of current flow. Having said that, we normally associate current flow from negative to positive, however in the case of a Model T the ammeter is measuring the load from the positive output of the generator to the positive connection to the battery and the load placed upon that circuit. It can be assumed that the terminals in the back of the ammeter correspond to the direction of the current flow. So rather than refer to positive and negative think about the current source to the load when wiring and "assume" that the connections in the back correspond the the meter scale on the front. Remember the ammeters, vintage or repro, are "indicators" and not "scientific instruments" measuring current flow. Even though there are different types of ammeters the basic connections and purpose are the same.
Moving-coil - D'Arsonval galvanometer is a moving coil ammeter. It uses magnetic deflection, where current passing through a coil placed in the magnetic field of a permanent magnet causes the coil to move. These provide steady pointer movement.
Moving magnet ammeters operate on essentially the same principle as moving coil, except that the coil is mounted in the meter case, and a permanent magnet moves the needle. Used for large amperage loads
Electrodynamic ammeter uses an electromagnet instead of the permanent magnet of the d'Arsonval movement. This instrument can respond to both alternating and direct current[4] and also indicates true RMS for AC. Might be used in the Mag ammeter
Moving iron ammeters use a piece of iron which moves when acted upon by the electromagnetic force of a fixed coil of wire. Not sure if this is what is used in the vintage ammeters

There is no electrical coil as such in my ammeter, just a flat u-shaped brass conductor between the terminals. There is a short football-shaped piece at the bottom of the indicator needle shaft that I assume is magnetized like a compass needle, which tends to align itself with the magnetic field around the brass conductor. There is a hair spring that returns the indicator to zero when there’s no current flowing.

When Fun Projects was in business they made a quality Ammeter. Don’t know if the folks who bought them out is still doing it. Information for what it’s worth

Here is an original 1926-‘27 Ford ammeter with the proper bright nickel plated escutcheon plate in the original box that I will be posting on EBay tonight for anyone who wants to try out an original Ford ammeter, or just put it on display. Jim Patrick

The last one I saw on Ebay similar to that one sold for $125.

T bezel was brass and not shiny like that one.

Hoyt made it.

I still have it in the box.