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'87 GT--Dash Lights Searching for a Ground?

D

drummingrocks

Member
I've got an '87 GT that I restored as a daily driver, and it's developed an annoying electrical problem that I can't seem to figure out.

When I turn on the parking lights or headlights, the gauges in the instrument cluster immediately begin reading higher--almost as if turning on the lights is somehow sending more voltage through the gauges: the tach, oil pressure, voltage, and temperature gauges steadily rise until they're almost reading off of the gauge face (i.e., what we always called "pegging the needles").

To me, this sounds like something in the lighting circuit is searching for a ground. Keep in mind, this problem ONLY occurs when the lights are turned on, and it happens whether the parking lights and/or headlights are on. Sometimes when driving, I can hit a bump, and the gauges will return to their normal positions for a few minutes. A few minutes later, the next bump will make the gauges start reading high again.

So far, I've:
--Checked and cleaned all the visible grounds on the headlight harness itself
--Verified that all of the ground wires that run inside the car and under the dash are grounded, and that nothing is left loose
--Verified that the ground strap from the engine to the firewall is intact and installed correctly
--Sanded both the ground terminals AND the areas they attach to in the engine bay to ensure a good ground
--Replaced the rheostat switch in the dash
--Pierced the ground wires in the instrument cluster wiring harness and physically grounded the wires with test leads, with no change

Can anybody offer any suggestions as to what to do next? I still feel like the headlight circuit has a poor ground somewhere, and that's causing the circuit to occasionally search for a ground, ultimately making the voltage reroute through the gauges.

Thanks for any help! I've been chasing this particular problem for quite a while now, and to be honest, I'm tired of putting up with it!
 
The problem could be in the instrument cluster. Try pulling the cluster illumination wire from the cluster connector and see if it acts up then.
Sorry I don't have a wiring diagram for something that old to tell you which wire it is.
 
Thanks for the suggestions! I'll give both of those a try today. I just hope there's not a broken wire (or almost broken wire) taped up in a wiring harness somewhere.
 
Update: I checked all of the grounds on the taillight harness, and all showed less than .01 volts on my multimeter.

Next, I tested the ground at the headlight switch and found a .05 volt reading with the key off and the lights off. This surprised me, so I pulled the instrument cluster out and tested the ground wires going into the instrument cluster harness. There are two connectors that plug into the circuit board behind the instruments, and each connector has its own dedicated ground wire. With the key/switch off and the lights off, both ground wires showed .05 volts. With the lights turned on, the voltage jumped up to .15 on both ground wires.

I feel like something is feeding voltage into these ground wires, or something is still searching for a ground. I'd try plugging the dash cluster back in and manually piercing and grounding both ground wires, but you can't access them with the cluster plugged in. None of my wiring diagrams show where the dash harness ground terminates, so until I can find where the ground wires physically hook to the body, I'm stuck. Any ideas?
 
No real ideas to pursue, but you are on the right track.

With your DVM, what are you using for the ground to check for voltage? May I suggest a long starter cable hooked up to the negative side of your battery? Then you can compare voltage from true ground to what you think is ground. If there is significant voltage, I'll bet there's some corrosion somewhere along the ground path back to the battery. Make sure paint is scraped underneath any ring connector to chassis.
 
Well, out of sheer luck, I finally found the problem. Under the starter solenoid, there's a ground bolt that secures several ground wires to the fender apron. This is also the area where the headlight harness plugs into the car's main wiring harness. Out of desperation, I was going to unplug the headlight harness and check for broken ground wires. When I reached to unplug the harness, I noticed that several of the wires in the area weren't secure.

I had left that ground bolt loose under the starter solenoid, back when I pulled all of the wiring out in order to paint the engine bay. I started the bolt back and connected all of the correct ring terminals to it, but I must've completely forgotten to go back and tighten the bolt itself. Several ground wires were just hanging there; my guess is that they occasionally made a good ground, which explains why the dash lights would periodically function correctly. Once I tightened that bolt up on the fender apron, everything started working again!

Gosh, I feel dumb...all of the time spent disassembling the dash, piercing wires, etc., and it ended up being a simple bolt that I had forgotten to tighten! Well, I always said the problem would end up being something simple...

Thanks for all of the help!
 
Here's to you for admitting the problem, after all that I would have told these guys that a rat chewed through the harness and I spent three hours under the car soldering and heat shrinking the wires back together. Glad you figured it out.
 
So....you don't want me to check your shorts anymore? *sigh*...
 
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