This 1990 Dodge Ram W150 5.2L had an overcharging alternator. It was putting out 15.5 volts. I discovered that there was a one volt voltage drop in the circuit providing power to the field circuit of the alternator and external voltage regulator. Rather than spend hours tracing the entire circuit through the harness, bulkhead connector, fuse block, ignition switch, etc; I used the existing switched power circuit to signal a simple relay that controlled direct battery voltage, and applied it to the field circuit.
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So it's not the PCM engine controller, it's a problem with the ignition switch feed or fuse link. You may want to investigate this further since you could have a damaged wiring harness. Those models have a really bad wiring harness. Custom repair or junkyard might be only other option. If it works that's good!
The wiring harness on these old Dodges are not easy to repair esp coming off the battery, it's a cluster of a mess on the 88 and even the 92 Dakota I had. They make similar kits for the newer models. As for the underhood harness with fuse links and cables, I doubt you even find a good used one.
What a creative idea, I wouldn't have thought of that. Presumably you searched for and couldn't find the source of voltage drop in the circuit. I was hoping it might be as simple as a crusty ground from battery to engine….