What typically happens when the needle bounces is the clearance between the
magnetic disc and the aluminum cup occasionally gets to zero (they touch).
The magnetic disc is connected to, and spins with, the cable. The aluminum
cup (non-ferrous) is connected to the needle. The very slight
magnetic attraction of the aluminum cup is what is indicated by the
needle. The cup is kind of being dragged by the spinning magnet. When the
two touch, the needle bounces. Why would they touch? Bad bearings
supporting one, or the other, or both. Or, if the cable is too long it can
push the magnetic disc into the aluminum cup.
or so it has been explained to me.
Peter C
_
/
| |
needle __ | |___ > cable
spindle | | magnetic
| | disc
\_
Aluminum cup^
----------------------------------------------
--At 07:12 PM 1/27/2001, David Massey wrote:
>Message text written by "Bob Rochlin"
> >Is it true that when a speedometer starts to bounce that it is a magnetic
>problem and the it can be re-magnetized? Honest, I was told that by a
>responsible source. Does any one know how to do it?
><
>
>I'm no expert but the magnet theory doesn't make sense. If the magnets
>loose their magnetism the unit would just read low.
>
>If the needle bobbles about it is either the cable (as someone suggested
>already) or the speedo needs rebuilding (probably bearing problems). I
>have rectified this problem with a rebuild a couple of times when the new
>cables didn't.
>
>Dave
>57 TR3 (rebuilt Speedo)
>71 TR6 (rebuilt Speedo)
>80 TR8 (nope, still good)
>
>P.S. New cables all.
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