Ammeter showed a discharge

[Will N]

Was driving my Fordor the other day. It started and ran fine the whole trip, but a mile or so from home the ammeter suddenly showed a 15 amp discharge. It normally shows about a 4-6 amp charge when underway. I run a stock generator with a diode cutout. I have a fuse for the whole electric system mounted on the starter. I have signal stat turn signals installed, and no other accessories. When the discharge happened, there was no change in the way the engine ran, so I suspected maybe the brake light switch stuck. When I got home I checked the lights, and none were on. No turn signals were flashing. The fuse did not blow, so I don't think there's a dead-short anywhere. If I revved the engine the discharge would change from -15 amps to about -4 amps, so the generator is putting out a charge. It was a warm day- about 85F. I turned off the ignition and the ammeter went right to 0. I started the car the next day after it had cooled down and the ammeter showed a 4-6 amp charge at a fast idle. I didn't let it run long enough to get hot. Any suggestions on what my have caused the discharge?

[P.S.]

Interesting! A lot to unpack here, and I think possibly a solution.

First, with a 15 amp discharge, the motor will not always run any differently. If it was a hard short, then yes. So, a wire shorting to the car would be a hard short, current limited by the connection(s) between the short and the source. That would fluctuate wildly, and you would have noticed that.

My wild, crazy guess is- The main brushes or 3rd brush on the generator may be worn or the wire(s) contacting the case (ground). Since the drain was 15 amps and the fuse didn't blow, and since the drain was less with higher RPM, I'm going to guess the current was flowing through the windings (limiting the current), so I am going to guess it was internal to the generator.

If it had been light sockets or something behind the dash, the drain would have most likely been enough to blow the fuse. If it has been a wire rubbing through, the drain would fluctuate all over the place from vibration and movement.

[Will N]

I never considered that! I'll take a peek and let you know. This could also explain why it was back to normal after it cooled down. Could be that something expanded in the generator while it was hot. Thanks!

[rotorwrench]

A high amp discharge indicates a complete loss of generator output. If it has a short to ground somewhere then it will blow a fuse if so equipped. Something may be getting hot due to a loose connection. Check your charging circuit and generator components for function and condition. Check the battery as well.

[Will N]

So I took the inspection cover off and there's plenty of meat on the brushes, I went to move the wire on the third brush to see if insulation had chafed off, and wouldn't you know, the wire fell right out of the flag connector holding it to the brush! I can't tell if the strands of the wire broke, or if only a 1/16" of the wire was inserted into the flag connector before it was soldered on. Only about a 1/16" nub of the wire is left stripped beyond the insulation, and that's all that was making (poor) contact with the flag connector. Time to break out the solder gun. Thanks you guys, you hit on the problem right out of the gate!

[ndnchf]

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Better check the other connection too.