OK... I need ideas on how most efficiently to test this. Let me know your thoughts on how to figure this out, or if you have an EWD that might provide info for me or others to figure this out, let me know.
I removed the instrument cluster and cover and found no obvious problems - no physical obstructions and while I didn't completely disassemble the cluster, could not see any evidence of leaking capacitors, corrosion, etc that might strongly suggest the gauge itself is bad... though it still might be as with a slight jiggle and tap the tach needle dropped from permanently reading about 1200 rpm to now reading 0. Since I didn't see obvious problems, I put the instrument cluster back in and started the engine to see if the fact the tach now read zero plus any small jiggling etc might have fixed anything. It still doesn't work... reads zero at all times.
I had the dash apart while also installing a new stereo and found from some prior owner a wire tagged on to what I think is the ECU (rectangular aluminum box behind and above the glove box, 2 connectors on the back) that was labeled "speed sensor". The label was not a factory label but appears to have been added by somebody like a stereo installer. It was tagged onto a green wire in one of the connectors on the ECU. That tagged extension from the prior owner is not being used for anything at the moment, but I've left it in place and coiled up for now until I confirm it can be removed (since I'm not quite sure what it was for... probably a nav system input, but not positive since I didn't think the KZJ78 had an electronic output for vehicle speed). There is a green wire of approximately the same color as the one on the ECU connector that got the "speed sensor" splice that is on the instrument cluster connector on the same side as the tachometer, but green wires aren't uncommon so it may not be the same circuit.
Two options come to mind for the source problem: 1) Tachometer is bad... signal it's receiving is good, but the tach itself needs to be replaced. or 2) Tach is good, but it's not getting a signal. Technically, they aren't exclusive of each other.... I could have both a bad tachometer that is simultaneously not getting the right signal. I'm still inclined to think it's a bad tach since the unit was stuck at a fixed reading even when the car was off. That said, especially given I know there's a goofy tag on an ECU wire labeled something to do with speed, I'd like to try and confirm the tach is getting an appropriate signal.
Any suggestions on how and where to check this signal? I'd like to avoid cutting up the wires around the ECU if at all possible. Here are a couple photos that might help with ideas. I don't have an EWD matching the KZJ78 (1995) to help me identify wires:
Connector on back of instrument panel that is closest to the tachometer:
ECU connector with the prior owner's "speed sensor" wire spliced in:
