Another A farmall charging problem

I am having trouble getting it to charge correctly. At half trottle and above it puts out 5 amps.

With the digital volt meter, the reading jumps all over the place between 7 and 11. With the analog meter, it shows around 5 volts.

When I ground the field terminal on the generator the voltage appears to stay the same but the amperage increases from the usual 5 to 10 amps.

I took the generator apart. The brushes looked like new but the communtator had grooves in it. I sanded it and cleaned the space between the bars. I put it back together with the old brushes. It still acts the same way. Would new brushes correct the problem? Go ahead and chew me out for not putting in new brushes, I got it coming. Just didn't want to make the trip to town.

If the above doesn't work, try temporarily grounding the generator field terminal at the light switch. If the ammeter now shows a charge it says the problem is with the light switch (bad switch or field resistor).

If THAT doesn't work, problem's gotta be in the wiring (broken wire) between the light switch and the generator.

Also note: When using your voltmeter to test system operation, stick the probes directly on the battery terminals. If the generator is charging the voltmeter should show 7.0 volts or a bit higher at the battery. Also as you observed, some DVM's give erroneous readings on generator systems on account of electrical system "noise" caused by the generator. Your analog meter will give you the most accurate readings for this type of work - presuming it's in calibration of course…

Hope this helps!