On 6 Jul 93 Adrian Jeffries wrote:
>My speedo reads incorrectly since the overdrive was fitted. I understand
>it is possible to recalibrate the things but I cant figure out how.
>I've looked at one that was in bits & all there seems to be is a spinning
>metal bar (driven by the cable) within a metal cap (the needle).
>I've got the TR4 early domed instruments - cant remember if they are
>Smiths or Jaegar though.
>
>Could some kind soul enlighten me?
>
Glad to. The metal bar is a permanent magnet, which generates eddy
currents in the cap proportional to the bar's rotational speed, which
produces a proportional torque, which deforms the hairspring, etc, ultimately
causing the pointer to point to the wrong number on the face :-(
Certainly the easiest way to correct this is *if* there are different
speedo cable drive gears available to account for various transmissions
and tire sizes. I have no idea whether these are available for TR4s,
but I'd bet a wad others in this group know.
If that fails, speedometers can be calibrated by adjusting the bar's
magnetic field strength. For my MG TF-1500 I set up a drill motor on
the bench with a speed controller, optical shaft pickup and frequency
counter to drive the speedo at known speeds. I put a large DC current
thru a Radio Shack tape degausser to magnetize the speedo's bar to a
high field strength, then carefully used AC on the degausser to decrease
the field strength till the speedo read correctly. It worked!
-Dave Plumpe
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