Repaired the combination gauge (compass/accel) in my Z

To add to the sum total of Z knowledge I offer the following other tidbits about the digital combination gauge:

During a bench test having no display at all is a VFD power supply problem. Check voltages at the TDK daughterboard. You should have ~2.5-3VAC between the outer pins in the group of three, and -26V to ground for all three in that group.

The 9-pin SIP, Toshiba TA7900S, is a voltage regulator controller + Power On Reset + Watchdog timer. Pin four is the output, should be +5V to ground.

If all the segments appear at powerup, the microcontroller is probably held in reset. Pin 9 of the TA7900 is the watchdog reset, and should have a 32Hz square wave present. My "good" combi gauge micro wasn't outputting the watchdog reset signal, which I fixed by jumping pin 9 to pin 4, disabling the watchdog. No reason for it, this is not a safety critical system.

On the whole, this assembly was not built by the A-Team at Niles. Seems more like an intern's summer project electrically. I would hazard a guess that Nissan went back to analog gauges for 86-89 because these were such shit.

Probably but mine have been replaced with re-worked onenes over 15 years ago and now totally reliable. But the digital stuff for sure was a problem.