ammeter on a 30/31

O

old31

Well-known member
A friend of mine brought over a new running roadster.

When checking the ammeter someone had run a jumper from one post to the other post.

Why? What is the reasoning for this?
 
Possible defunct ammeter. Have you tried removing the jumper to see what effect that has?
 
Maybe someone was trying to bypass the ammeter with part of the current because they were running an alternator (or high out put generator) and didn't want to fry the ammeter? I did the same thing on my '32. Note I used a loop of wire to get the correct resistance. My ammeter now reads half of what is actually passing. Works great, looks original, you just need to mentally adjust the reading.

Charlie Stephens
 

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You could also use a pot (potentiometer), which is a variable resistor, to adjust the bypass current. Put it in series with the ammeter as shown above by Charlie.
 
Get rid of the jumper because it allows all current to bypass the ammeter. If there is an electrical problem, the ammeter is a useful tool to detect it. If the car has an alternator or one of those hi-output faux generators, you will need a 30 amp ammeter.
 
old31 said:
The car does have an alternator that looks like a generator.
Click to expand...
That explains the wire. The alternator puts out enough amps (when charging at max) that it would destroy (either immediately or over time depending on the charge rate) the ammeter if all of the current went through it. Do NOT remove the wire. Ideally you could adjust the length of the wire so the ammeter would read half of the actual charging rate. I would have to study the wiring diagram but you might be able to do this at the junction box on the firewall.

Charlie Stephens
 
Bob Bidonde said:
Get rid of the jumper because it allows all current to bypass the ammeter. If there is an electrical problem, the ammeter is a useful tool to detect it. If the car has an alternator or one of those hi-output faux generators, you will need a 30 amp ammeter.
Click to expand...
The wire only allows part of the current to bypass the ammeter. If the resistance of the wire is equal to the resistance of the ammeter it would split the current to half each. I hate to disagree with you but DO NOT REMOVE THE WIRE. See my previous post for reasons.

Charlie Stephens
 
Our coupe has the expensive generator look-alike alternator on it. I run a 20 amp safety fuse and a $20 30 amp ammeter from the vendors. No problems as of 23,000 miles. The ammeter is your friend. When turning on the ig switch, starting the car, or when the car stalls on the go, my eyes are on the ammeter.

There are some good old days articles from the restorer that will help you learn the many diagnostic uses of the ammeter's behavior. Make sure your car has the November, 1929 wire change on page 390 of the Service Bulletins applied. It takes just 1 minute to apply the change. This allows the functioning of the ig points to be seen on the ammeter. Saves time when chasing ig problems.
 
Katy, I don't know the max ouput. I switched the car to LED head and tail lights. The sum of the loads, if all were on, is well below the 20 amps. The horn will be the biggest draw and its momentary. The alternator has worked well and looks good too. The car came with it. I hope to replace it with a restored generator from NDNChief if it fails.
 
I have installed many 6v Power Master PowerGens. I have never needed to jump the ammeter in any of the cars and I never had a problem.
 
Two thoughts, .....First, why bother making a jumper when just move both wires to one terminal stud.

Second, rge Ammeter is the fuse that Ford designed into the electrical system. When the Ammeter blows due to a short, then the engine dies. Connecting the two wires is the bandaid to get the vehicle back home where proper repairs can be made. Are you certain this Ammeter is still functionable??
 
I use the ammeter with my original style 6v generator as a yes/no instrument. It is good to see it twitch while starting to be sure that the key is in the right spot, is showing a charge while driving, and to make sure the third brush is low enough for enough power when I rarely drive with the stock headlights on. I don't think it was ever meant as a precision instrument. A suitable fuse in the lead from the starter terminal is better protection than relying on the ammeter, IMHO.
 
I think you are missing the point of the jumper wire in this (and my) situation. The jumper wire allows only some (let's say half ) of the current to still go through the ammeter to avoid overloading the ammeter. Now the ammeter will read half of what is actually going to the battery. You just need to double the reading in your head to get the correct number.

Charlie Stephens
 
Hello, I recall awhile back vendors and customers were stuck with some poorly made amps, probably some still out there. I understand what you are says with the higher output alternators , the meter still works but only shows half of actual output to avoid damage of amp meter .Cars of that era was common to run negative charge when running lights,
Not like modern cars that compensate for loads .The cutout only makes connections if generator is putting out current. disconnect if not , no actual amperage regulation.
 
Brent, I put an ohm meter on it and it looked ok, but I replaced the gauge because it looked iffy.
 
A wire with the purpose of bypassing some of the current is know as a shunt wire. How much it lowers the ammeter reading depends on the amount of resistance in the wire. It can be calculated if a person want to use it as a calibration adjustment for the ammeter reading. It can be used as sort of a safety device to protect the ammeter if the current rate is too high. I've had to use them on helicopters when converting from a 70-amp alternator to a 100-amp unit on machines that had search lights or other high amp loads.
 
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