Tachometer drops out and battery light comes on

[DeadheadNM]

Brainstorming an electrical issue w my 99 K2500 Suburban 6.5L diesel.

Starts great, runs great. Once I'm on the road for a while the tach needle seems to abruptly lose signal and drops and the battery idiot light comes on with an ammeter volt drop. Engine runs fine all the while and the issue is intermittent but reproducible. It was a fire department rig meaning a lot of additional electrical devices were spliced in and removed before I purchased it most of which I've previously gone through and cleaned up.

I'll check the volts at the batteries while running and off, make sure there's no volt loss between the neg terminals and ground, and run through and clean all the grounds. Batteries and cables are basically new AC Delco items. The alternator isn't OE as there's a "remanufactured" sticker on it.

Does the tach needle dropping while the battery light illuminates point to a common etiology that explains both, like a faulty voltage regulator or bad ground? I'm lost as to how these are related and no idea where the tachometer lead for a diesel originates.