I am using a Mopar square back alternator and I am having a problem with the volt gauge jumping around anything above idle rpm. I can unplug the voltage regulator on the firewall and it fixes the problem. I have taken another volt gauge and hooked it to the output on the alternator and it does the same thing so I think I can rule out the gauge itself. The voltage regulator is adjustable on the back and I turn it all the way down and it still bounces the gauge. It did adjust the voltage output of the alternator to the battery so it works also. I thought maybe it was an alternator problem so I took it and had it tested and everything was ok including the rectifier. Is there something I am missing? Need help!!!
Start it up, with a multi-meter test the voltage at the battery. Is the multi-meter jumping around ? Or is it study? If it's study I'd say you have a gauge or wiring problem to the gauge..... ....
Back in the day we had some Chrysler alternators with the brush slip rings pressed a little too far on the rotor shaft. That put the brushes slightly hanging off the side of the slip ring. There is a soldered connection on the slip ring back side that sticks out a bit. As the brushes wore, they would contact the connection and bounce on the slip rings putting voltage spikes in the output. It would really upset the engine control module.
Have you insured that the alternator has a good ground? You can check with a voltmeter between battery negative and the alternator case. With the engine running and a load on the system such as headlight, you should read less than 0.1 volt between these two points. A higher reading points to a poor ground.
I recently had a similar problem, except mine was fine at higher speeds but fluctuated at idle. Alternator and regulator both had less than one thousand miles on them. Alternator tested good, tried to replace the regulator and got two in a row that were bad out of the box. Name brand, made in USA. Second one fried the alternator. I replaced the alternator with the next version (two field wires) and rewired the circuit which let me use the electronic voltage regulator instead of the mechanical one. Problem solved.
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