[MRRICH]

Rewired the engine compartment of my 68 RS after a serious electrical fire under the hood. New engine and light harnesses installed.
All lights, engine, gauges working properly, however when you turn on the lights (head or park) the temp gauge pegs to the right and both turn signal indicators light in the cluster. Have checked all bulbs and tried the usual ground tests to no avail. Have some ideas but before I dig into the dash harness thought I'd see if anyone has run into this.
Thanks
Rich

[dnult]

It definately sounds like a ground issue. My first suspicion is that the ground tab on the instrument panel is not installed or installed incorrectly. It is located at the top of the instrument cluster between the cluster and the dash near the high beam indicator. It's a very common problem that will cause the sympoms you describe.

As for other grounds, to confirm if they are good or not, get a Digital volt meter and power up the electrical system (lights on etc.). Then measure the voltage between the battery (-) and the various ground points. If the grounds are good, the meter reading will be very low (less than 0.02 volts or so). If you see more than 0.5V you've found a bad ground somewhere between your test points.

[MRRICH]

Thanks Dave
Will try your suggestion with the digital voltmeter - never thought of that - my way was to run jumpers from battery to whatever I thought could be the problem. My next attempt will be to remove the cluster and check that, although there must be a good ground because the clock and tach work,
or am I just hoping?
Rich

[dnult]

Grounds (actually bad connections in general) are funny things. Small loads like the clock and tach don't demand enough current to cause a voltage drop accross a resistance. But when you up the current (like turning on headlamps) suddenly things change. Also, small loads can find alternate grounds through light bulbs and other paralell circuits that'll defy logic.

[JimM]

I'm with Dave.

What generally happens is the signals will "find a ground"
In the case of the tach and clock, that is thru the filaments on the parking lights / turn siganls. But this only works until you put p[ower to the paking lights and turn signals, then ground is no more and everything goes nuts.

The temp gauge problem is a good clue, maybe this isn't a common dash ground problem.
What about engine/frame grounds?
When I first put mine together, I attached the small ground wire from the battery to the inner fender per the book, had all sorts of grounding issues. Turned out the whole rad support (and everything attached to it) was floating at 12 volts!
Moved the battery ground to the rad support and added lots of star washers to the sheetmetal bolts and it all behaved way better.

[MRRICH]

I'm pretty good with electrical but over the years have gotten away from a lot.
You have given me some valuable reminders of what to look for and this weekend I plan to attack this issue and hopefully will be able to let you know the outcome.
Rich

[blk/gld 68]

Originally Posted by dnult

It definately sounds like a ground issue. My first suspicion is that the ground tab on the instrument panel is not installed or installed incorrectly. It is located at the top of the instrument cluster between the cluster and the dash near the high beam indicator. It's a very common problem that will cause the sympoms you describe.

As for other grounds, to confirm if they are good or not, get a Digital volt meter and power up the electrical system (lights on etc.). Then measure the voltage between the battery (-) and the various ground points. If the grounds are good, the meter reading will be very low (less than 0.02 volts or so). If you see more than 0.5V you've found a bad ground somewhere between your test points.

Dave i have a black wire behind my cluster attached to center dash support, but not attached to anything. Is this wire supposed to be attached to the cluster?
mark

[dnult]

My cluster is packed away in a box and I forget specifically where that wire attaches. Can someone else tell mark where the black ground wire attaches? I think there is a screw on the back of the tach or speedo, but I'm not sure. Definately though on the 68, you'll have that goofy bracket around the high beam indicator that is ground for the instruments. A lot of guys have tossed that bracket in the trash and used their own ground wire though. It also seems that the order of assembly on the ground bracket is important. All the pieces can be there but if they are stacked in the wrong order the ground will be intermittant.