I have a 1986 Dodge D150 with a 225 cu in engine, automatic trans, electronic GM ignition (ESC disconnected.) I have replaced the wiring from the alternator to the battery with heavy duty battery cable and a 100A fuse. I am having the alternator output measured at the battery to be very erratic with voltages jumping constantly from 14.8V to 15.8V with the engine idling. The battery is new and fully charged. I have tried four new voltage regulators and nothing changes. I have tried two OEM 75A alternators that are both tested good and no change. I have run a heavy ground wire from the VR case to the ALT to the negative terminal on the battery. The battery negative cable is well grounded to the engine block. I even ran new wires (not using the OEM wiring harness) to the VR to the ALT with no change. When I turn on the A/C the voltage holds steady at about 13.4V and does not jump around. I added a heavy ground wire from the ALT to the engine block with no change. Also, my accessory voltage gauge in the truck reads from 14.4V to 15.4V which is lower than the reading on the multi meter connected to the battery. When I am driving the voltage remains above 14V still jumping around (unless the A/C is on.) I am afraid this high voltage will damage my electronic ignition, battery, gauges, etc. I know this problem points to a grounding problem with the VR but I don't know what else to do. Does anyone have any suggestions beyond what I have already done? Is the heavy battery cable from the alternator causing this problem due to resistance? The problem started when the VR went out and started putting out 18V and the engine started to cut out. I have a new VR with an adjustment on the back but I don't know if I should make any adjustments as the OHM reading of the VR is 1.75 Ohms as it should be. What about using one of the blue VR's that put out a stable 13.4V from Summit Racing? Wound that be too much constant voltage for the battery and not enough for the A/C?

What you haven't done is make sure the reference voltage (switched battery) to the regulator is correct. If that's low the regulator will attempt to compensate. You could have a bad ignition switch, corroded bulkhead connector or any other flaky connection on the +12V line to the regulator. Get out the wiring diagram and track it down.

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Joshua

I did check that. It was a steady 12.46 volts.

And that isn't the battery voltage. The reference voltage at the regulator must equal battery voltage or the regulator will raise the alternator output to raise the voltage.

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Joshua

Okay, that makes sense. I will run a new wire from the ignition switch to the VR and make sure it is the same voltage as the battery. Thanks for the response.

Before doing that, I would check the main power feed going through the bulkhead connector. That terminal has a habit of getting corroded, and having excessive voltage drop. "this"
https://flic.kr/p/2kCSuZD

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

I would suggest, if it’s not on your truck, to do the relay upgrade. This takes all the voltage and puts it directly from battery to alt.

We'll see if the corroded terminal is what causes the voltage drop on buddy's incoming Dodge D150 project.

That linked pic isn't showing the bullhead. The bulkhead is the pass thru, thru the firewall. The linked pic is a plug at the firewall at the back of the valve cover, the fried terminal is the main hot lead from the alt. Which was burnt on my 85 as well. But a direct line from the alternator to the battery would effectively bypass that.the other end of the wire at the fried terminal is on the fender well about a foot back from the "+" battery post and goes thru a fusible link. Another spot I see cobbled and/or corroded.