I purchased Gordon Killebrews book on our C4 clusters, and it seems like a few guys are having problems..so here's a quote that may help.

"When the dash lights are inoperative and the high beam and directiona indicators come on dim, look for a bad ground.The most common problem is a poor dash ground on the 153 circuit. This is a black/white striped wire in cavity D3, of the large connector to the cluster.This wire goes to a ambient over-ride sensor, then to the bulkhead connector to cavity B1 to the engine harness wire to the left rear of the engine.The ambient over-ride sensor is located under the dash on the backside of the bulkhead connector on a plastic cover over the wires entering the connector. It is about the size of a dime and 1/2 inch thick with two terminals. One is the 153 circuit. The other returns to the cluster cavity D7. This is a 911 circuit, dark blue wire." I'm skipping the portion about what the sensor does, because it doesn't matter, and if the following works, great, Gordon says if it doesn't just leave the NEW ground attached.
"Ground the 153 circuit at the over-ride sensor." He further states if it doesn't work you may have to pull the cluster.

What I did was splice into the 153 and ran it thru the bulkhead and attached it to the braided ground strap bolt located on the frame rail in front of the battery, or beside it, can't quite remember, but you'll see it.Hope this works for some of you.

 

This is a great post Rick, I have seen quite a few members on here over the last few months that have questions like this. I'm going to copy it...might be able to use the info one day.

 

Well it does matter if you live in Alaska as it kills the cruise circuit below 32 degrees. Note - in '89 this ground was re-routed and no longer goes through the big bulk head connector and you might (well I know somebody who did) spend needless hours trying to find it (unless you look at the Electrical Supplement) Rather it's bundled up with ECM harness on the other side of the firewall. Guess GM realized it was causing a problem.

 

Rick:
Thanks,,,, for all... so please disregard my "where art tho" email... as you can see, i've found it...
thanks
vinnie

 

It does not affect the ambient sensors ability to function, since all you're doing is tapping into an existing ground. Gordon makes no mention of a change in location..but I will say his book only covers 84-89. At least the one I have.

And this fix is usually if the following symtoms occur.

"The dash lights will go dim and the high beam and turn signal lights come on."

If tapping on the cluster seems to help...I'd lean toward a bad or dirty connection at the cluster, in which case you have to pull the cluster..I was nervous about that the first time, but it's realatively easy.

 

What if your cluster lights get randomly dim? Mine never go out, but on occasion they'll randomly get dim and then a few minutes later back to bright. Sometimes they'll stay dim longer than others.

 

Have a similar problem.

 

I'd say one of two or maybe both: Cold solder joint on the dash electronics and/or bad ground, but not totally bad....

 

if randomly dim at nite it could also be hdlite switch dimmer crkt try turning dimmer and see if it goes away

 

 

I had the same problem ...and tried all of the tips, bought the Killebrew book... and still had the random dimming. Got pretty good at taking the cluster out...then apart and learned how to solder in the process. Turned out it was a failing power supply, and once that was professionally replaced (along with the sockets and bulbs) it is like new. Found a local specialty shop..cost around $175 to go through the whole cluster, same day repair...with a 1 year warranty