The way a speedometer works (omitting the odometer for a moment) is that
the speedometer cable spins a disc with a magnet attached to it, very
close to a small steel "pie pan". The pie pan is attached to the
needle, with a spring pulling the needle towards zero. As the magnet
spins, it imparts some rotational force to the pie pan, which pulls
against the spring, bringing the needle off of zero. The faster the
magnet spins, the harder the pie pan pulls against the spring, the
higher up the needle goes.
Two things can cause the needle to "bounce" the way you describe:
1) Abrupt changes in the rotation speed of the plate with the magnet,
which are often caused by a drag in the speedometer cable. The inner
cable winds up until it has enough force to get past the drag, then
"sproing", it whips the disc extra fast for a moment. Then it drags and
starts winding up again...
2) Some kind of intermittent and more direct coupling between the plate
and the pie pan. Either:
a) Things aren't lined up, or are worn out causing the two to "tick"
together
intermittently, which means you're looking at having it rebuilt
or replacing it).
or
b) Some grease, oil, or other fluid has gotten in there to create an
intermittent "viscous
coupling between the plate and the pie pan. In that case you
just need to clean it out.
Spray Carb Cleaner into the space between the plate and the pie
pan, let it dry, then
carefully relubricate the odometer works and axles with a light
machine oil (a few drops
of sewing machine oil is plenty). The gap between the plate and
the pie pan should be
dry.
On my wife's Magnette, the culprit was an over-enthusiastic greasing of
the speedometer cable, which proceeded to pump the excess into the
speedometer.
Chris K.
> Date: Tue, 24 Jul 2001 20:40:49 -0500
> From: Tyson Sherman <mts3@ra.msstate.edu>
> Subject: Re: My B doesn't like me
>
> That did it. Thanks. I didn't realize removing
> the tach would be so easy. It's a lot easier than
> the speedometer because of that dreaded blade
> sticking down, also known as the bracket for the
> trip reset knob.
>
> Now I have a full set of working gages.
>
> What would make the speedometer needle wiggle at
> speed. It gets bad at about 50 mph but I didn't
> get up any faster than that. I seem to remember
> it would get worse where at 70 it looks like
> you're going anywhere between 50 and 90.
>
> Thanks,
>
> Tyson
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