James, here's a good site to put in your favorites:
<http://users.ntsource.com/~barneymg/mg01.htm>
Specifically, the answer to your question will likely be found here:
<http://users.ntsource.com/~barneymg/mgtech/speedo/st1.htm>
Larry Hoy
> -----Original Message-----
> From: owner-mgs@autox.team.net
> [mailto:owner-mgs@autox.team.net] On Behalf Of james
> Sent: Tuesday, March 19, 2002 11:09 PM
> To: mgs
> Subject: Wavy speedo problem
>
> The car is a '71 roadster, the cable and housing are new,
> trans angle drive is rebuilt. The needle tends to run
> moderately steady about 30mph higher than the actual road
> speed. The odometer is dead on at all speeds when measured
> against mile markers of varying distances from 1 to 10 miles.
> The odometer always ticks correctly and it will roll over
> large (10k) digits. The needle bounces over about 5 to 10
> mph range, but as stated reads very high. Typically the
> needle will read 50mph from just off the line until I start
> aproaching 40 mph where the needle then begins rising in time
> with speed changes. It would appear that on very hot days
> after prolonged drives the needle will start to fall back to
> the correct speed, but I can't call that a repeatable
> behavior since we haven't had a lot of hot days here lately.
>
> IMO it seems that the cable is working well since the odo is
> dead on. I infer from this that the odo is linked to the
> cable and that there is some form of spring restraining the
> needle and that there is a slipping or friction clutch
> between the cable and the needle so that the faster the cable
> turns the more friction and thus the needle is dragged,
> against the spring, farther into the range of the speedo.
> Since the speedo follows road speed over a certain speed it
> would appear that the spring is working correctly, but there
> is too much friction in the clutch and thus the needle
> travels too far into the range at a given speed. This also
> works with the needle dropping to better accuracy as heat is
> applied over long distances.
>
> So how far off am I? and what can I do about it?
>
> TIA,
> james
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