Paul,
There's another possible cause for optimistic reading and then jerking
around.
The cable, mechanical tach or mechanical speedo, turns a cup inside
of which is a magnet that's attached to the needle. The magnetic flux (or
something) of the spinning cup pulls the magnet against a return spring.
If the bearing that supports the spinning cup dries out, or if it wears
so that the cup can contact the magnet, the pieces touch and the magnet
gets snatched around rather dramatically.
Margaret Lucas explained this to me when I called after noting a few
peggings of the tach needle of my TD. It was either 9,000 rpm or tach
trouble. I could eliminate one of those.
Bob
> .. But when I went for a drive this afternoon,
> it's suddenly bouncing around wildly and reading about
> 40-50 mph high. But it literally bounces between, say
> 70 and 120 mph. Wild bouncing.
|