How to Troubleshoot the 90-96 Corvette Dimmer Circuitry...

The LCD display in the instrument panel is dimmed by the photocell and electronics in the Central Control Module (CCM) first.  If the photocell is dark, then the CCM looks at the input from the dimmer rheostat on the headlight switch.  The photocell brightness always overrides the dimmer rheostat on the headlight switch.  Warning bulbs in the trop monitor are never dimmed.  The other displays and panel bulbs and courtesy light are dimmed solely by the dimmer rheostat on the headlight switch.

Note:  Test with the car out in the bright sunlight.  The light in the garage isn’t enough to test properly. 

Note:  There is a smoothing algorithm that causes the brightness to change slowly (up to four minutes after a change in the brightness and/or the dimmer rheostat).

1. Test with the headlights off first.  The system should run at full brightness. 
2. If that doesn't work, try shining a light into the photocell port in the top left of the trip monitor to the right of the cluster.  If the light works, the photocell may have shifted vales and likely needs to be replaced - we have these for sale!
3. Put black tape over the photocell.  It should dim completely in about four minutes. 
4. Turn on the headlights.  The brightness of the LCD should respond to rotating the headlight switch knob when the photocell is dark.

 Warning!  Remove the black tape from the photocell before driving!

The 90 Corvette Factory Service Manual describes this process, complete with wiring diagrams.