JimLentz

I have a '92 with a ZF6 and the LT1. I think this is a new problem I noticed after flushing the cooling system, although I didn't get coolant on anything electrical. Only think I did electrical related was put in new knock sensors as part of the coolant flush. My tach appears to read correctly except when I shut the car off it drops below zero. I am pretty sure it never did this before, but I have seen photos of other C4s with the tach sitting below zero when off. Is this a common problem and any pointers on fixing it?

WVZR-1

KEY-ON (not running) and TACH parks at 0 you have no problems. If it parks at other than 0 with KEY-ON (not running) you have issues. Confirm with a scanner or with C68 a calculation from the AC Control.

JimLentz

I have not hooked up a scanner yet, but with key-on and and not running the tach goes to 0. When I then turn the key off it drops below 0.

WVZR-1

If KEY-ON (not running) parked at 0 is the easiest way to determine that your '90+ C4 has a TACH calibration issue. It can vary widely to below or well above but if KEY-ON doesn't return it to 0 then there's issues for sure.

Incorrect other than that needs to be confirmed with a scanner - there is no other way. Many try to use MPH - axle ratio etc to confirm and that just doesn't work.