1990 dash lighting problem

[Volleyman]

On my 1990 ZR-1 the backlighting behind the speedo/fuel gauge LCD's works fine (brightness adjusts with control)with the lights on, but it is completely dark with the parking/head lights off. When turning the ignition switch on it briefly lights to full brightness but then goes completely dark. This is in full sunshine outside. The photocell in the dash seems to be good as it changes value with the amount of light hitting it as tested with a VOM. Another post stated there was a power transistor mounted on a bracket under the dash but I have been unable to locate it. Any ideas??

[secondchance]

Now, this is based on my experience w/ 84 dash but worth a shot.
When the gauge goes dark run the car shine a flashlight and see if gauge itself is working or not.
If gauge itself is working what is failing is backlighting.
84 cluster (probably the same for later digital) had halogen backlighting that clips on to fiber board with circuitry embedded. Over time, halogen heat would burn the circuit board and burnt carbon would at times short out the circuit.
Solution for me at the time was grind off emmbeded circuit and hardware all halogen bulbs. Center cluster (digital w/ backlighting) may have similar problem.

[Volleyman]

The MPH and fuel gauge LCD display seem to work fine all the time, just the backlighting is not on when the lights are off. It is very hard to read the speedo in normal light. I think the bulbs lighting the display are fine as when the key is first turned on it lights up bright as if it is doing a bulb check but then goes out with no backlighting of the LCD display.

[secondchance]

Like I said, there probably is carbon track on the circuit board. Therefore, during daylight when higher current is supplied juice is flowing where it was designed to. At night, when less current is supplied, it is shorting out through carbon track.
One way to deal with it is to send it out to outfits like Mid America or Corvette Central for a rebuild. New circuit board free of carbon (resulting from halogen heat) will solve the problem. I had a same experience w/ 84.

[gbrtng]

Looking at 8A-82-1 of the 91 FSM, there's a backlight panel dimming signal from the CCM to the digital display bulbs. It's a .5 gry-red wire from D8 on the CCM to B4 on the instrument cluster. Basically, the CCM grounds the current thru the bulbs.

This panel has nothing in common with the early (84) panel.
Good luck,
Glenn

[secondchance]

I know its different.
However, I suspect digital backlighting technique may be very similar.
Fortunately, I had no problem so far with 91 nor current 94 - knock on wood.

[Volleyman]

The backlight dimming signal from the light switch seems to work fine when the lights are on. My thought is the trouble is in the dash dimming circuit. Either the photo cell on the dash or a possable power transitor mounted somewhere? Anybody know where that might be located under the dash?

[gbrtng]

Those bulbs are controlled by the CCM - but I repeat myself. You will need the FSM to determine which inputs to the CCM to check.

[Volleyman]

The trouble is in the DIC, the photo cell is bad, ordered several from digikey, I hope to find one in the correct range that will work.

Originally Posted by gbrtng

Those bulbs are controlled by the CCM - but I repeat myself. You will need the FSM to determine which inputs to the CCM to check.