Heavy voltage fluctuations while driving

[Sir Goofy]

Good day, long time reader first time poster.

Our 1989 Toyota LE is having trouble keeping a charge. I've removed the wiring harnesses, checked for continuity, cleaned them up, and reinstalled them. Alternator replaced by mechanic and warrantied because it didn't fix the problem.
Installed a new battery and a digital voltage reader in the cigarette lighter to monitor voltage, there is a 0.3 volt difference between the battery voltage and the cigarette lighter voltage. While driving my wife is reporting the voltage fluctuating between 14.4 and 11.9. Thank you for any help given.

[MarkH]

As long as the belt isn't slipping (you'd probably hear it), I'd suspect a break in a wire from the alternator that intermittently looses continuity from vibration or something. That happened to me. In my case, the break was fortunately accessible near the alternator and not in the wrapped part of the wiring harness. I think there are a few hidden breaks in the wiring in my engine compartment. When I wiggle the nest of wires from the injectors, etc, the idle gets rough.

[Sir Goofy]

Was the break in one of the small wires or the larger one? The wire close the make plus was starting to look greater so I sealed it with liquid electrical coating to protect it after I cleaned it.

[MarkH]

I don't remember actually, since it was a couple of years ago and I've done lots of other wiring work since then. I think a break in just about any of them could produce your symptoms. There's also a charge relay in the dash and a couple of fuses. I'd pull them and check for corrosion in the contacts. The sensing wire from the alternator goes to the battery as a separate wire from the main battery cable. I'd check for corrosion there too. There's also a small plastic box in the engine bay with 2 wires bolted together. Make sure there's no corrosion there either.

[Jan-Willem]

I would guess a small wire. The three pin wire towards the alternator, has a pin for sensing (that one isn't it, because if it loses connection, the voltage goes up), but the pin for ignition might be it. if that one has a bad contact, the alternator switches off.
On my van it this wire had a plug on both ends, on on the loom side, one on the alternator side, so I could take it off. I guess this wire is under some stress because of the heat of the manifold.