Here's a tip for those with 300D's with non-functioning tachs: use good contact spray. I bought a can of Wurth contact spray. This has to be some of the most expensive contact spray made at $16/can. However, it solved two of my longest running problems. The reason I was under the hood was to fix my heater coming on whenever it felt like it. I sprayed the contacts on the valve by the firewall (sorry, forgot the name right now), and it's been working for about a month since then without a problem. While I had the spray out, I figured I'd give a shot on the tach control module. I've tried several tricks posted in the past, like putting something behind the circuit board, resoldering one I got from a junkyard,... The past fixes would normally work for a few days, and then die. I didn't notice it for a few days since I've grown accustomed to it not working, but my tach has been working now for about a month. I guess the $16/can price is worth it.

Dave Sawyer
'83 300D
'91 300TE