Looking for suggestions on a possible root cause (e.g. in the instrument cluster, or is the switch at fault...)

1985 California-spec 300TD.

When I move the headlight switch from the 'Off' position to the Parking lights (one detent clockwise), the temperature gauge reading will climb from approximately 85 degrees to something closer to 100. Similarly, the gas tank needle will register more than it was doing with the light switch in the off position.

There is no additional deflection when turning the headlight switch to the headlights on position, the gauges remain 'artifically' high.

General consensus, is the issue in my headlight switch (worn ground contact?), or do I need to dig deeper into the instrument cluster/grounds?

Thanks in advance,
Dave

Almost definitely ground. Had the same problem on mine where any load would cause the gauges to bounce - turn signals, lights, hazard etc. My first attempt to ground the panel did not work and I went through replacing oil pressure gauge and an bunch of other stuff. Redid the ground to a better location and the problem went away. I put a quick disconnect in the line to facilitate easy removal of the cluster.

This is so common, it should be almost routine for our old cars to add a ground to the cluster.

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Definitly a bad ground. Behind the instrument cluster is a big cluster of ground wires. Take the 10mm bolt out and remove all the ground wires and clean each ring connector, the bolt, and where it's all mounted. See if that helps. Also check that you have good ground connection between each gauge in the cluster. They are grounded through the bodies of the gauges, so clean the contact points where each gauge overlaps the next. Might also check that all the wires on the headlight switch are securely attached.

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