I was looking under the hood of my 1988 Vanden Plas for the reason my tach isn't working when I found a broken yellow wire near the battery, I traced it as best as I could through the loom and it seems to go to a barrel key switch mounted on my relay cover on the left side of the radiator. Now my curiosity has the best of me, what is this switch for, and could it be responsible for my tach doing absolutely nothing?

 

 

I don't believe it is original. Could it be for an aftermarket alarm system?

On your 1988, the tachometer signal comes from the ignition amplifier. The insulation on the wiring at the coil and ignition amplifier tends to harden and crack, so it's possible you could have a short in that area.

Another possibility would be corrosion on or looseness of one of the electrical connectors on the back of the instrument cluster. If you remove the knee bolster, you can reach up behind the instrument cluster to check that the electrical connectors are fully seated. To clean the terminals it's easiest to disconnect the instrument cluster, which is just secured by four screws, two at each lower side. I have photo albums showing how the knee bolsters and instrument cluster are removed - see the link in my signature.

Cheers,

Don