1987 dash light problem

gary__June 13, 2011

I have an '87 vette that used to be my daily driver, but has pretty much been parked in the garage for the last 2 and a half years. Take it out a few times to run the previous years tank a gas through and put in fresh before winter.

It has the digital dash. One of the lights burned out a while back making the dash pretty dark. The headlight 'bright dim' switch didn't change it at all. The first time I took it out this year, the entire dash would go out, including the radio. Engine never missed a beat.

Traced the problem to a bad + connection at the battery. I went ahead and replaced the battery (it was old) and cleaned all the connections I could find. There's a bunch of them with fusable links convienently located under and behind the battery. That took care of the complete blackout condition. I replaced the bulbs that backlight the digital dash. It's all nice and bright again. The problem that remains is the non function of the bright dim knob. While I had the instrument panel apart, I cleaned the lens for the auto dim feature. The light switch still feels smooth. Besides, it worked fine last year before it was put away for the winter.

The car looks and runs great, including the dash. It's just not totally the way it's supposed to be. Doesn't bother me at all but I want to sell it if I can. The more of these little things I can take care of, the better to make a sale.

I already checked corvette forums and haven't found the fix yet. A few guys with a similar issue that replaced the light switch, and didn't solve the problem. They usually send the instrument cluster in for rebuild, but never heard if that solved anything.

Hoping there's a brainiac john g type still on this board with an idea.

Sorry to bore you with the long post.

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Hi Gary.

I am back for a bit, got sick of the spammers. Been so long I had to use a new login, the old one wouldn't work.

The dimmer control for your cluster works like this.

Brown wire pin 6 of connector "C" (a 16 pin) brings the parking light signal to the cluster to tell it to turn off full brightness and follow the dimmer control.

Grey wire pin 9, same connector "C" comes from the dimmer through fuse 15. You can quickly check that operation by simply watching some of the other components that controls such as the shifter console, climate console, radio illumination, etc.

If those inputs work correctly, then the cluster itself is at fault. The photocell in the cluster only tweeks the backlight setting. The actual dimming is done with an operational amplifier circuit on the computer (instrument cluster) motherboard.

I worked on a lot of those clusters through the years. You should see what we deal with today in comparison.

    Bookmark   June 14, 2011 at 5:50AM
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Hi John,

I don't read this board much either anymore. There's seldom anything interesting to read on it. Lucky you popped in today.

Thank you so much for sharing. Don't know how you remember so much. You explained it well enough so I can diagnos it now. I only wish the General would have made the wiring harnesses to the dash a couple inches longer.

Thanks again.

    Bookmark   June 14, 2011 at 4:26PM
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Actually I don't recall all of those details, I spent a few minutes and looked the information up. Once I saw the schematic again, the rest was simple.

    Bookmark   June 15, 2011 at 7:50AM
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