75 GMC alternator wiring

[hillytim69]

Hello, my pickup no longer has the original bat light in dash and has only an after market voltmeter. Symptoms are it will not charge at idle and will overcharge at high rpm. I think this is because of the missing idiot light which is part of the circuit to make it work. I can't see any wire under the dash for the light. Nor do i know what color wire to look for.Wiring is a bit hacked into by previous owners over the years. What is the easiest way to make this work? would putting in a one wire regulator solve this problem or can I run a resistor or something? I can't drive it due to it charging well over 15 volts at cruise rpm. All ideas welcomed, thanks guys.

[ray_mcavoy]

Yes, if your truck originally had a warning light style instrument cluster, the GEN/BATT light would have been part of the alternator's "exciter" circuit. Not having that light anymore can cause a lack of charging at idle and low rpm. However, that usually won't cause it to overcharge at high rpm.

Assuming your truck originally had a warning light cluster, the original wires that went to the GEN/BATT light would have been orange and brown leading to cavities #6 and #7 respectively of the cluster plug. The orange wire is a +12V feed and the brown wire goes directly to the #1 terminal on the alternator. If everything else is still intact, simply connecting a small bulb (like a #194) between those two wires should restore the low rpm / idle charging. GM used a 10Ω resistance wire to take the place of this bulb in trucks with factory gauge clusters so a 10Ω resistor can be used too as long as the wattage is sufficient. Changing to a 1-wire regulator would eliminate the need for either the bulb or resistor.

The overcharging at higher rpm might be due to a bad connection at the alternator's #2 voltage sensing terminal. Use a voltmeter / multimeter to see if you have full battery voltage present on the alternator's #2 terminal. If not, check the connector and/or wiring. Otherwise it's likely a bad voltage regulator.

[hillytim69]

Wow thanks for the in depth response! I will try the resistor. Still not sure if it was an idiot light or ammeter setup stock. Thanks for the info I will post up whatever I figure out.

[hillytim69]

Found about .5 volt voltage drop at the number 2 terminal with key on engine off. Might be even higher difference running? I dont know if half a volt is significant but I will try a different ign power source for this and see if it helps. Thanks again for helping me understand how this works.