Loose Ground

Q: Loose Ground

I have a 1969 coupe, that has a pecuiliar problem. Whenever the headlights are on, the turn indicators & hi beam indicator are dimly lit. When the turn signals are engaged,(either direction) they all blink. Furthermore, the gas guage indicator drops as soon as the lights are engaged. If you turn the rheostat on the headlight switch all the way down (where you can’t see the lights), the gas reads normally, and the the turn signal indicators work properly. I have replaced the headlight switch and the dimmer switch. I have not been able to locate a wiring diagram of the car, particularly the instrument panel, short of searching for an original service manual, (and shelling out the big bucks for one.) If anyone has dealt with this type of problem, or happens to have the wiring diagrams, I would greatly appreciate it.

A: I had the very same problem on my 69. It turned out to be bad dash ground. The dash gauges/lights are grounded through a thin metal strip that is sandwiched between the radio and the dash. If the radio has been removed or changed or if the connection is not tight, your dash lights/gauges will go nuts. I corrected mine by adding a ground jumper from the back of the gauge cluster to the metal dash at a convenient location.

A: The turn signal indicators and gas gauge are likely source by the same circuit that sources the cluster lamps. When the dash lamps are at the brightest intensity, most of the current is diverted toward the shorted lamp and starving the turn indicator (flasher which requires significant current to operate) and gas gauge. This is why the turn signals stop working. Moving the rheostat to the dim the cluster lamps removes the short and current is available to turn indicators/gas gauge causing them to work correctly.

A guess is that there is a low resistance shorted lamp on the cluster assembly. The short should identify itself by excessive heat. Feel around the back of the cluster for a spot that appears warmer then the rest. If you don’t find it, remove cluster lamps (instrument illumination only) one at time until all have been checked. If all the lamps are removed and the problem seems to remedy as each lamp is removed, there is a low resistance short in the main feed to the rheostat. If all the lamps are removed and the problem persists, there is short between the rheostat and the circuit card on the back of the cluster.

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