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The last time I started up the wagon I noticed that the (somewhat rare L79) tach I have in the car wasn't working. When I first got it running it wasn't working, but then began working as the car ran for a while.

I was ever hopeful that the problem wasn't the tach itself, but last night I set out to find out for sure. I installed it with a new tach harness and a fresh underdash harness and fuse panel. The wiring all checked out, everything was snug. It was wired correctly. I pulled the tach out of the dash, which is a little bit of a pain, and tested the ground to the instrument cluster from the body, no problems there.

Once I had the tach out I was unsure what tests I could do on it with my multitester. None that are useful as it turns out. Well, at this point I knew the wiring was sound, the signal from the coil was getting there, the coil was working because the engine would run, the ground at the instrument cluster was good, the power wire from the fuse panel was good and the signal was making it to the tach so that left me with the conclusion the tach must be bad. :sad:

Feeling I had nothing to lose, I actually took the back cover off the tach itself. There was nothing obvious in there I could do anything about but nothing looked out of place.

Hoping against hope, I thought I'd wire it up outside the instrument panel and attached a temporary ground wire to the body of the tach. To my happy surprise, it worked!

Still a little confused as to what the problem was, I then noticed that the tiny little screws that hold it to the instrument panel, where the tach gets it's ground, were very rusty! I strongly suspect the screws weren't providing a consistent ground thus the sporadic performance. The body of the tach that touches the cluster housing has some typical light corrosion so no ground was getting through that way either is my guess.

I hit the screws with the wire brush on the bench grinder and I'll reinstall with some star washers to get a good clean bite into clean metal and I'm guessing it will be fine. I also plan to hit the back of the instrument housing and the rim of the tach with some steel wool to clean those surfaces up.

It just goes to prove that age old saying about electrical problems, "make sure the grounds are good". :thumbsup:
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