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Wavy speedo problem

To: "mgs" <mgs@autox.team.net>
Subject: Wavy speedo problem
From: "james" <jamesnazarian@netzero.net>
Date: Tue, 19 Mar 2002 23:09:00 -0700
List,  I just had Hans send me all of his list replies on his speedo problem,
but they didn't seem to cover mine. So I'll start a similar thread to the one
just finished.

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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