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 8,000RPM, then dips past 0RPM.
Replacement Analog Calibration IC - top view
Replacement Analog Calibration IC - side view
The replacement IC is made with modern low-mass surface mount components to minimize the effects of vibration and maximize reliability. The factory uses a laser to fine-tune the calibration of each instrument panel. The IC is preconfigured for Corvettes with an 8,000 RPM tachometer, and includes a multi-turn potentiometer to allow for precise adjustment of the tachometer calibration. We use precision tolerance components for maximum accuracy, and Gold plated board and mounting pins for maximum life.
Note: This is Preconfigured for ZR-1 Corvettes with 8,000RPM tachometers, but it is adjustable for non ZR-1s and other engines.
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 Adjustable Analog Calibration IC (1990 - 1996) preset for 8K tach
Supply of solder
Instructions and Troubleshooting Guide are online - Click here