Have a 95 SC400 and the tachometer is working intermediately.
When I start it up the tach doesn't work until I'm driving for at least 30 minutes, then it works perfectly. I'm pretty sure the motor is ok, but I read in prior posts that there is electrolytic capacitors feeding the F-V encoder chip, and these caps do dry out over time and lose there cap. value, so maybe it's just taking a long time to charge a dead cap. It can't be a lose wire not making connection...
does anyone have the full maintenance manual for the SC400 and see what troubleshooting I can take? What could the problem be? Everything else works on the instrument panel.

 

https://www.clublexus.com/forums/sc-...-and-tach.html

Chips, cap, tracers. Full rebuild. See link for more info on how it all works and pics. Doing just one of the three won't cut it. It all ages equally. Tracers are usually not burned (USUALLY!), but putting fresh caps in without retracing them will fix that for you quick. Half assing anything electronic will find the weakest link for you in a heartbeat.