odd speedometer issues (NA/AT)

[KDS_cars]

Im having some issues with my speedometer. when the car is started (engine running) the speedo will bounce between 45-50mph (while the car isn't moving) consistently. Once the car is shut off it will fall to 20mph and stay there until the cluster is tappen. When I first bought the car it didnt read at all. Pulled the cluster and found a loose screw, tampered with by previous owner most likely. I tightened it to receive these issues. Diagnostic wise I have tested continuity at the ecu and at the connector into the cluster. all checked out. (pin 53 to 116 on ecu, yellow with black tracer to solid blue on cluster) all checked out fine. When I tested voltage the ecu read 0-9v consistently fluctuating and at the cluster connector I couldn't get a reading at all but the ecu and 45-50mph jumps matched up. (all with car running for voltage checks. I back probed everything I tested). The speed sensor at the transmission has been replaced, I had to do away with the connector since the previous owner cut the connector off the old one. I did this a while back thinking it was the issue and I found an oem one cheap. I tested the old and new one and both worked fine (tested with a drill out of car) both read the same as each other but I put the new one in just incase. iI has the proper ecu i checked the model code and based on the chart I found online the model is for a 1990 NA AT which I have. However it is a dinin performance ecu. Possibly a cluster issue but if so why would it be moving at all? everything else works (tach,trip,odo,fuel etc). I have not been able to find much else online at least for 300zx's with Automatics. The things I have found were all speed sensor issues which I have already replaced. If it is the cluster is their any test i'm missing I could try? I would rather not just fire the parts cannon at it and find out its not the issue.

[KDS_cars]

for anyone who cares I have tracked down the issue to be between the alternator harness and main engine harness. somewhere in the bay or dash the wire is disconnected / broke. Green death maybe. It has voltage at the alt harness but not the main engine harness connector. I plan to run a new wire as neatly as a can. to tear apart the harness is a lot of work mostly because i plan to build the motor down the line. at that point id replace the harness anyways.

[DCaff300ZX]

Thanks for sharing this, will probably help someone later via search.
Just a thought but with your future harness plans and the common fragility and age issues of the aged Z32 harness, personally I'd replace the harness now and avoid jury-rigging and other work that you'd have to un-do later. Might fix/help a few unknown or un-noticed issues along the way, plus the Z32 is a wiring nightmare and extra wiring isn't a great idea anyway.