[Arne2]

I know this is an ancient thread, but it gave me a clue on my similar issue, and I thought I'd post what I ended up with to help others.

The issue is that the tachs for manual and automatic equipped cars are different. The manual transmission cars have a CASIS upshift light, the auto-equipped cars do not.

The tachs with CASIS have an extra ground spade on the back, sometimes soldered, often just attached with one of the backside screws.

When you get a replacement tach for your manual car, often you end up with one from an automatic. And if you connect it all as normal (adding the ground spade, without which the tach doesn't function), the temp gauge page anytime the ignition is on. Unplug the extra ground and the temp gauge works again (somewhat), but the tach is wonky.

The hint from this thread was that the temp gauge input also goes to the CASIS bit in the tach, via terminal #4.

When I looked at the wiring going to the tach, it was easy to see - terminal #4 has a wire from the sender, and it also forks to the temp gauge.

My suspicion was that if I disconnected #4 from the tach, the temp gauge would work, but I'd probably lose the econo-meter.

This was exactly the case. Once I pulled that female spade out of the 6-gang connector, the econo-meter was dead, but both the tach and temp worked normally. I used some shrink-fit insulation to prevent the terminal from shorting out and killing the temp gauge later, and put it all back together.

All is good now, as I couldn't care less about the econo-meter.