I've got a 19861/2 Nissan pu d21. The other night my son and I were driving home and discussing a voltage problem and doing a little fild experiment trying to figure it out. The home stretch he decides to run the RPMs up probably around 6,000 and waualla we blew both hig beams. Now this wasn't a spike voltage. The voltage (by the brightness of the lights and the rpms of the heater blower motor) seem to a parallelled, relative or proportional to the engine RPMs.
When we pulled into the shop I turned off all accessories and checked the voltage at the battery between post at an idle. Then I ask my Son to slowly run the RPMs up. I was surprised to see the voltage (probably @ 3,000 rpms) go above 16 volts. I suspected the battery had shorted. We left the vehicle running and then I clipped a jumper battery (booster) onto the battery to confirm that I did have some weird battery problem (the truck had never failed to start even in cold weather) and to my surprise when we ran the rpms back up I got pretty much the same results although it seemed the jumper battery did have some effect on the rate of voltage clime initially.
I pulled the alternator off a truck I hauled in from a Buddy's house (same model) and installed it and now everything is back to nominal. Just for future reference does anybody have the answer to this one. I suspect the alternator had issues with diodes and the excessive voltage was AC and eventually the battery would have gone down. But I was checking the voltage in DC mode and my meter shouldn't have been able to read the voltage out put in AC; only the DC output. i.e. If the alternator had bad or leaking diods the DC voltage should have read low? Still confused????
Hollis