Speedometer

[jadney@vwtype3.org (Melissa & Jim)]

>Date: Sun, 08 Dec 1996 15:43:53 -0800
>From: Eko Indarto <echo@indosat.net.id>
>To: type-3@umich.edu
>Subject: Speedometer
>
>Hi... thank you for all your help 
>
>Now have already remove the speedo from the dash. Let's start with initial 
problem..
>If you all recall my earlier mail, I heard strange, intermittent noises, 
came from the 
>speedometer. The noises was becoming worse and worse, and before I could 
open and check 
>the real causes, the speedo was not working anymore (There was no strange 
sounds 
>anymore). I managed to disassemble the speedo. Now I have separated the 
assembly that 
>holds the needle and the scale, and the assembly that holds the speedometer 
cable. I 
>found nothing broken or damaged there. Can anybody explain to me how this 
mechanism 
>works, so I can have something to start. Or.. you can give me some advice 
to repair this 
>gauge.

If both the speedometer and odometer have stopped working, then I would bet 
that your problem is with the cable, not the speedometer head.  The cable is 
anchored to the wheel, so it should not be possible to turn it from the 
front seat.  Likewise, if you drive around the block, the cable should be 
turning.  It should also not be possible to pull the cable out of the sheath 
from the front seat.  If yours fails any of these tests replace the cable.

If you look at the chain of gears, the operation of the odometer will be 
obvious, but the spedometer is much less so.  It works on a principle of 
magnetics that produces drag from a rotating magnet.  The magnet is the cone 
shaped piece on the inner end of the input shaft.  It spins inside a metal 
cup which attaches to the speedometer needle.

The rotation of the magnet produces eddy currents in the cup which then 
produce their own magnetic field which then reacts with the first field in 
such a way that it tries to slow down the rotating magnet.  Since this is 
not possible, the only effect is the reaction torque on the cup which turns 
the cup and the needle.  The cup/needle are spring loaded, so their 
equlibrium position is determined by the balance between the spring return 
torque and the magnetic drag torque.

Since the magnetic drag torque is approximately linear with the rotational 
speed of the magnet (= rotational speed of your left front wheel), and the 
spring return torque is linear with the angular displacement of the needle, 
the equlibrium position of the needle is proportional to the speed of your car.

Be careful with your parts, it is easy to break the inner needle pivot 
and/or the tabs that hold your two parts together.

Jim
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       Melissa Kepner                                    Jim Adney
       jadney@vwtype3.org              jadney@vwtype3.org
                             Laura Kepner-Adney
                             Madison, Wisconsin
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