I have an "82 300D with 284000 miles. Lately, when I step on the brake pedal or signal a turn, or push a window button, the glow plug light comes on and the gas gauge and coolant gauge go to zero. It stays that way until I stop, shut off the ignition, and start it up again. Someone told me the ignition switch needs to be replaced, but I'm wondering if it's not a short somewhere else. Any thoughts?

 

This is a classic description of a poor ground probably behind the dash gauges. Brown wires are generally grounds. As each switch is activated it is getting a ground thru another circuit. Pull the speedo cluster and ck the brown ground wires behind it.

 

Pulled the instrument panel, found the brown wires, cleaned all surfaces with emery cloth and reinstalled. No change. Well, not exactly true - now car won't shut off. I see a small, black, rigid vacuum hose coming through the firewall from the engine compartment in a group of hoses and wires just above the steering column. I must have knocked it off when I was contorting around under the dash, but I don't know where it goes. Thoughts?

 

There is a vacuum hose at the rear of the ignition switch which goes to the top of the vacuum valve mounted to the back of the injection pump. trace that hose from under hood to the ignit. switch. I still say you have a poor ground somewhere. Possibly the multiwire plug on the left side of the speedo cluster or the printed circuit itself.