Bad ammeter causing cranking problems?

R

Ranham

New member
I'm new here, I'm putting a model a back together for a friend. I have never worked on a model a but I'm pretty mechanically inclined. My problem is it wasn't cranking so I have checked everything out and I have 6.3 volts everywhere except the - side of the coil where it is 5.6. I took a wire and hooked it straight to the coil and it cranks. My question is can a bad ammeter cause this not to crank? Thanks
 
Are you using the red bible as a refernce for this assembly? If you unhook everything from the starter switch but the battery cable it should crank. Then start adding one wire at a time until the problem child is found.
 
with stock wiring only the battery cables,and starter switch on the starter are involved with cranking
if the car is modified you should post pictures to help us sort it out
 
Kurt in NJ said:
with stock wiring only the battery cables,and starter switch on the starter are involved with cranking
if the car is modified you should post pictures to help us sort it out
Click to expand...

It is brand new wiring. My thought is the if wire comes from the starter to the passenger side of the terminal box then to the ammeter then to negative side of the coil and there is a voltage drop there then the ammeter may be causing it.
 
The amp meter only reads the flow across it's own shunt. If it was shorted internally or externally then it would smoke it. There has to be a wire connection to the charging system and electrical loads and that is provided in the terminal box.

I don't know if you have a stock pop out ignition switch or not. In the stock system the ignition switch completes the ground path between the coil and the distributor breaker. The power to the other terminal on the coil should be on the charge side of the terminal box so it's connected all the time.

Make sure you have a good ground connection to the frame. Most folks add a ground strap from the frame to the transmission to insure a beefy ground path in all the right places.

Google the MAFCA wiring diagram pdf file and download that for your reference. It answers a lot of questions for the model As.
 
Ranham said:
I'm new here, I'm putting a model a back together for a friend. I have never worked on a model a but I'm pretty mechanically inclined. My problem is it wasn't cranking so I have checked everything out and I have 6.3 volts everywhere except the - side of the coil where it is 5.6. I took a wire and hooked it straight to the coil and it cranks. My question is can a bad ammeter cause this not to crank? Thanks
Click to expand...

When you say "cranking", do you mean starter motor turning engine over? Or do you mean engine starting and running. Two different things.

The electrical connections on the back of the ammeter or inside the ammeter can be bad (high resistance). This can cause a voltage drop, but only when current is passing through the meter. If in doubt, temporarily connect a jumper between the two brass wing nuts on the terminal box on the firewall. This temporarily takes the ammeter out of the circuit (bypasses it) to see if that fixes the problem.

When measuring voltages at various places in ignition circuit, make sure the points are open or place an insulator between them, like a piece of paper or a thin piece of cardboard.

Original pop-out style ignition switches can have poor electrical connections internally and give funny voltage readings at times.

Hope that helps you chase it down.
 
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If the point are open or a piece of insulating material put between them, then the voltage at the coil should be the same as at the battery. If not then there is another path to ground that is causing the low voltage.

If the points are closed then the voltage will be lower because the points are pulling the voltage down. If there is some resistance in the wiring system then the voltage with the closed point will be very low and the car may not start. Or the battery could be bad. The resistance could be in the ground of the battery or the ammeter as you suspect, or corroded or loose connections. Sometimes you can tell just by feeling around for hot spots.

If you suspect the ammeter, then measure the voltage across it with the points closed.

Do not leave the ignition on with the points closed for very long or you could burn out the coil or points.
 
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