Tach whine repaired

[DAVIS2X1]

The tach on my R90/6 worked but had a whine and jumpy needle above 3,000 RPM. I thought it might be the the cable but it still prevailed when the tach was driven via my variable speed drill.

I took the tach apart and ran each part separately. The input ran smooth. the speed disk ran smooth. Upon further inspection I could see that the speed cup was backing up on the disk, and the disk was rubbing on the speed cup. The repair is as follows.

1. Remove instrument cluster.

2. Remove the two back pieces, instrument light assembly and finally the tach.

3. Remove the two screws to remove the tach input.

4. Loosen the three screws that hole the driving disk to the tach body.

5. Remove one of the screws and insert a 2.5 mm flat washer between the two assemblies. Repeat for the other two screws.

6. Reassemble.

There is a pivot bearing on the disk that the speed cup fits into. I beleive that over the years the shaft wears the pivot deeper and allows the interference.

 

[tvrla]

That could be, but it's more likely the shaft wore. Off the top of my head, I think the shaft needs to extend .140" out of the cup. It takes a special tool to adjust the distance - that's a mighty tiny shaft and you don't want to bend it!

Like old mechanical clocks (you know, how people told time before digital?) these things should be lubed every now and then.

A jumpy needle can be caused by worn bushings besides a sticky cable. The cable is the first thing to check and the easiest fixed.