If your tach does this, the analog calibration IC is the likely cause
Factory Ceramic Analog Calibration IC
The analog calibration IC in your 90-96 instrument panel is a common source of problems. The factory part is a ceramic package which is easily damaged by heat and vibration. The results are the tachometer or other analog gauge reads significantly higher or lower than they should be. Another common symptom is that the tachometer pegs 6,000RPM, then dips past 0RPM.
Replacement Analog Calibration IC
Replacement Analog Calibration IC - Bottom
The replacement IC is made with modern low-mass surface mount components to minimize the effects of vibration and maximize reliability. We use precision resistors for maximum accuracy. Gold plated board and mounting pins for maximum life.
Note: This is for ZR-1s and other Corvettes with 8,000RPM tachometers. We have another replacement IC for non-ZR1 Corvettes with 6,000RPM tachs.
Signs your Analog Calibration IC may be defective
When the ignition is turned on, the tachometer pegs 8,000RPM, then dips below 0RPM.
When the ignition is turned on and the engine is not running, the tachometer should read 0RPM. If not, the Analog IC is likely defective.
The tachometer reads higher or lower than it should by more than a few hundred RPM.
Not all defective Analog ICs look bad, but if yours looks like the one below, it is definitely defective.
Example of defective Analog IC - not all defective ICs look this way
- Other analog gauges exhibit the same behavior
1 - New Analog Calibration IC (1990 - 1996) for 8,000RPM tachs
Supply of solder
Instructions and Troubleshooting Guide are online - Click here