MTFCA Forum

I'm trying to solve some electrical problems on a car that has a 12 volt Texas T alternator. I don't pretend to know much about electricity so I need some help. At anything above an idle the ammeter is pegged at 20 amps. Someone told me that because I am using an original 6 volt ammeter with a 12 volt system the reading is double--a 20 amp reading is actually only 10 amps but even 10 amps is still too high. My limited knowledge tells me that volts and amps are different. How can I tame my ammeter? Do I need a different meter? Is the alternator at fault?

Sam, if you have an alternator, you can expect more amps than Ford ever thought of your battery voltage should rise to about 14.3 volts and then the Amps will slow down sounds nornal so far, but I don't know if a T ammeter can handle it. Daver in Bellingham

Sam,

Current is separate from voltage, your issue will not be anything to do with voltage per se, but it may be related to the alternator and/or its regulator.

Initially I'd suggest using another test meter (perhaps a current clamp) to determine if your system actually is charging at a high current, or not. It is possible you have a faulty ammeter, it would be a pain to spend a lot of time trying to rectify a problem that doesn't exist...

IF it proves that the ammeter is ok then it is most likely the alternator/regulator that may need attention, or it could be that your battery is the root cause of the issue. In any event I suggest you do the first test and report back.

Luke.

Voltage is not an issue with an ammeter. The problem with a stock type T ammeter is that it only reads to 20 amps +-, and your alternator is probably capable of 40 amps or more. Higher current probably won't hurt the ammeter, within reason, but you need a ammeter that can read higher current if you want to know what is going on in the system with any accuracy. A battery that is not fully charged may draw substantial charging current for awhile, until it regains charge. A properly regulated alternator will allow the charge rate to taper off to nearly zero, assuming no accessories are in use and the battery is in good condition. When starting the car, the heavy current draw of the starter will draw down the battery somewhat, and the rate of charge will go up substantially for at least a few minutes after starting, then it should taper off again. As mentioned above, a 12 volt alternator system with a good, fully charged battery will typically show around 14.3 VOLTS with the engine running at idle or a little above.

Your current ammeter, (no pun intended), should be fine. The question is, why is your alternator pumping out such a high charge rate? Is your battery particularly discharged/dead, such that the regulator in the alternator is allowing such a high charge rate?

As Pat says, your Ford ammeter will only read up to 20A, even if your alternator generates more, which it probably is.
And as he says, you need to find out why.
Modern alternators can produce over 100A, and even small ones manage 30A or 40A.
With that sort of current, something somewhere must be getting hot. You need to find it by touch before you find it with smoke.

However, it is possible that the ammeter is grossly over-reading. Take care if using the current setting on a multimeter in series with your alternator. They will usually only read to 10A. More will blow a fuse, or the meter if no fuse. A non-contacting device which fits round a wire would be good.

Its is my understanding that the amperage output of an alternator is dependent upon its rated current output, rotational speed & the demand placed upon it from the cars electrical system. In other words it would not output more amps then placed upon it. So some calculation can be made battery, coils, lights, That's about 10amps or less (LEDs) what else? - The starter uses the battery so not directly involved, indirectly in using battery power. Don't forget the fuse!

Curious to know some specs on the alternator: what is its low amperage output & at what RPM; same for its high amperage. Had a memeber whose alternator low RPM speed was higher than 30mph in a T

So the answer to the question of going 12volts in a Model T is ...

There is a reason that new cars went to Voltmeters at the same time they went to Alternators.

The heavier current was probably part of it.

I used to install ammeters in cars with alternators for other people when alternators first came out in 1963 on GM cars. Unfortunately, I didn't have enough money to buy a car that was that new. I had to be satisfied with a 57 Chevy belaire 2 door hardtop with an overdrive transmission. Wish I still had it.

Art Mirtes

An alternator rated at 50 amps (for instance) can deliver 50 amps for some period of time without damage. Most alternators can deliver about 1/4 or more of their rated output at engine idle. If you start a car with a partially discharged battery and turn on the lights and heater, the charge rate might reach 40 amps or more until the battery charge comes up. A well regulated system will show a fairly high charge rate right after starting the car, and then taper off to a very low charge rate after a few minutes of running, perhaps as low as 1 or 2 amps. Turning on accessories with the engine running will not raise the indicated charge current much, if any, although the alternator output goes up enough to meet the added load. Just over 14 volts is a typical reading in a 12 volt system when the engine is running and the system is stabilized with the battery at near full charge.

Henry Ford evidently made 2 Inches the standard instrument gauge.

You can buy the adapter ring used to replace the larger ammeter space used or 1919 to 1925 Model T Fords.

Then enlarge the inside are to a 2 inch circle and insert any gage you wish, ammeter, voltmeter, etc.

I have done several and inserted the Ford Tractor Ammeter and other larger 60 0 60 ammeters.

They work for 6 Volts, 12, Volts or whatever you wish.

Stewart Warner has a lot of gauges you can use.

The firewall hole behind the 1926 - 27 smaller dash panel is laege enough so you can do the same for that panel.

This is a 60 0 60 I did recently.