Beyond The Basics - Vintage Tech Advice

We help answer some of our readers tech questions in regards to dash lights on a 1973 Mach 1 and a collapsible steering column for a 1965 hardtop!Dash Lights
In 2010 I purchased a 1973 Ford Mustang Mach 1. Upon getting it home, I discovered that the dash lights do not work with the exception of the turn signals and the high beam indicator. To try to resolve the issue, I have replaced the fuses in the fuse box, bulbs, headlight switch, printed circuit board on the back of the speedometer, and the voltage regulator that snaps onto the printed circuit board. All the lights on the outside of the car work when the headlight switch is on. Turning the headlight switch will also turn on the dome light inside the car. Recently, someone told me that the dash ground could be the problem. Can you confirm this, and tell me where this would be located or other possible solutions to the problem?
Dave Frank
Via the Internet
The fact that you have working lights on the outside indicates that the headlight switch is powered correctly, therefore your problem has to be between the headlight switch and the dash cluster. The power runs through the headlight switch rheostat (which adjusts brightness) to the fuse box where there is a 4-amp fuse dedicated to the lighting circuit, and up to the dash cluster and other illuminated items, such as the radio and clock. The wires in question are light blue/red striped and can be checked using an ordinary 12-volt test light. The headlight switch must be pulled on so the illumination circuit is live during the tests.

Rotate the rheostat counter-clockwise. If the radio and other items illuminate normally, then the circuitry from the switch and fuse box is okay and your problem is isolated to the cluster itself. Check for power on the light blue and red wire at the snap-in connector on the back of the cluster. If there is power at the connector, then the issue is with the cluster/printed circuit. No power means there is a break in the wire between the headlight switch and the cluster.

The circuit is simple and wiring problems are rare; usually the connector is the problem. A redundant ground using a jump wire can be attached to the cluster to double check the ground circuit.
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