Ammeter Shows No Charge

[KenBolton]

Full disclosure, I screwed up. A loose coil wire caused backfiring. After replacement the starter didn’t engage. Installed an 8v battery. Due to poles being on the same side I ran into shorting of the cable clamp to the battery hold down. A roadside insulator didn’t do the trick. Next morning I found I’d installed the battery backwards. Replaced the 8v with six. Everything is good excepted both the ammeter and a new replacement shows discharge but no charge even though my multimeter shows a charge at both the generator and battery. Inspection of wires behind the dash, in the junction box and engine compartment shows no issues. What am I missing?

[Gary WA]

Originally Posted by KenBolton

Full disclosure, I screwed up. A loose coil wire caused backfiring. After replacement the starter didn’t engage. Installed an 8v battery. Due to poles being on the same side I ran into shorting of the cable clamp to the battery hold down. A roadside insulator didn’t do the trick. Next morning I found I’d installed the battery backwards. Replaced the 8v with six. Everything is good excepted both the ammeter and a new replacement shows discharge but no charge even though my multimeter shows a charge at both the generator and battery. Inspection of wires behind the dash, in the junction box and engine compartment shows no issues. What am I missing?

Positive ground?

[Will N]

If you're sure that the generator is putting out a charge by using a multimeter, then just switch the wires on the back of the ammeter right to left and left to right. That will change what's showing as a discharge to a charge.

[Bob C]

With the engine off turn the headlights on ammeter should show discharge.

[KenBolton]

The ammeter does show a discharge with drain on the battery, ie headlights on. What I can’t determine is why two different meters do not register a charge when the generator and battery indicate it is charging.

[Gary WA]

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Cut out defective!? Give it a little tap tap! do you have Les Andrews book-Model a ford mechanic (red) page 4-9) on what your readings should be using ammeter to check!

[ndnchf]

How do you know the generator is charging? What is the voltage at the generator when it is charging?

[rotorwrench]

What are the readings on the multi meter set on voltage and taken at the battery when the car is running? What are the the readings at the output end of the cut out in the same conditions? Is the voltage higher than the normal fully charged battery reading at rest? Normal average reading for a 6-volt in good condition is 6.2-volts. A battery that is charging should be a fair bit higher than that. Likely around 7.1 volts or so give or take a bit. That can depend on the 3rd brush adjustment.

When the generator was run with both higher voltage and reverse polarity, the generator may still have reverse polarity problems. If there was a short that caused a peak current draw through the ammeter, it can damage the ammeter. Does your car have an original ammeter or a reproduction?

I would start by checking the cut out relay or power diode as applicable. It is designed to pass current if the generator is charging. The current should be around 4 or 5 amps minimum. I would definitely re-polarize the field pole shoes as well.

An original type cut out relay is designed to bring the generator on line as soon as the generator output exceeds the battery normal voltage range. Putting an 8-volt battery in there changes the generator output voltage due to the battery being the only thing that regulates system voltage. It's not supposed to put out 9-volts but it will and it may have overheated the armature. I don't recommend using anything but 6-volts around a 3-brush generator with no voltage regulator other than the battery.

Amperage is always read through the meter so any ammeter used to check the system has to be placed in the line. There is no contact to ground as there is when checking voltage. The original ammeters Ford used are far superior to the new reproductions. The originals use a horse shoe magnet mounted inside. The reproductions use a rare earth magnet and I'm not fond of those set ups.

[KenBolton]

I did tap the cutoutwith no change but I think it is a diode model so tapping may not work.??
Genny output and battery charge is 8v according to a multimeter.

[Gary WA]

To eliminate Gen. Do you have another to try out or borrow a good known Gen to try??? 8 volts is high!Have you moved the third brush?

[KenBolton]

End result was a damaged generator end plate and burned windings.

[nkaminar]

To test the generator, remove the cutout to take it out of the system. The remove the fan belt. Then run a hot lead to the generator terminal and it should turn like a motor but slowly.

To see if the generator is generating, run the engine at a fast idle and measure the voltage at the generator terminal, the opposite side of the cutout from where the wires hook up. It should register about 7 volts for a 6 volt car and about 14 volts for a 12 volt car. If it is 6.2 volts or less for a 6 volt car it is not generating. If it is much higher than 7 volts for a 6 volt car then the cutout or the wiring is bad and the generator is not connected to the battery. Do not run the engine for more than a couple minutes with the generator disconnected from the battery or it may burn out.

The voltage at a fast idle should be the same on the wiring side of the cutout. If it is at the battery voltage then the cutout is likely bad.