At 09:37 PM 2/15/02 -0500, you wrote:
>I was out in my B .... down in the 40's or 50's, .... needle on the
>speedometer started floating between 50 and 60 mph. .... odometer had quit
>working. ....
You're screwed. (Sorry).
Inside the unit there's a small phenolic gear (two actually) about 3/8"
diameter with lots of little teeth that drives the odometer. This little
gear is driven by a large diameter single pitch worm gear attached to the
input shaft. When the input shaft goes one turn the phenolic gear advances
one tooth. The small gear is on an ecentric shaft carrying a small ratchet
pawl. The pawl advances a ratchet gear one tooth for each revolution of
the small phenolic gear. The ratchet gear has lots of teeth and drives the
first wheel (1/10 mile indicator) on the odometer. The total result is a
high ratio double gear reduction that turns the odometer primary wheel one
revolution per mile of travel.
There is a bit of grease on the odometer mechanism and on the shaft for the
small phenolic gear. With increasing age the grease dries out and gets
stiffer, and the works gets harder to turn. Eventually a couple of teeth
on the small phenolic gear wear out and sheer off, and the odometer stops
turning. Just before the phenolic teeth sheer off there is considerable
drag on the worm gear due to misalignment of these worn down teeth, such
that the speedometer cable takes a slight twist with each revolution, and
then lets loose again, which is what causes the speedo needle to
waver. This often happens when the main odometer is trying to turn over
the 4th or 5th wheel in the line, which has not turned for a long time, is
quite stiff, and loads up the drive mechanism with more stress. Otherwise
it could happen at any time, especially in cold weather when the grease is
even stiffer than normal.
Now the bad news is that this condition is terminal, as the phenolic gear
is damaged. The worse news is that those little phenolic gears are not
available as replacement parts, and are generally only found as donor parts
in other used instruments. Your speedometer will need to be disassembled
for a thorough cleaning (and re-lube) of the odometer mechanism, and the
damaged gear will have to be replaced. For a more detailed description of
how the instruments works, check here:
http://users.ntsource.com/~barneymg/mgtech/speedo/st1.htm
This information is specific to the MGA speedometer, but the basics are the
same, only the number of teeth on the gears is changed to affect the
required reduction ratio for your car.
There are instrument shops (such as MOMA Manufacturing and West Valley
Intruments) who will happily repair your instrument, and will relieve you
of a substantial sum of money in the process. But with some patience and
some small tools you can do the job yourself (assuming you can find the
necessary replacement parts). Start by checking the small numbers on the
face of your speedometer for the required number of turns per mile, and
shop accordingly.
Casual hint: Next week on February 24th the Chicagoland MG Club is hosting
an (indoor) All British Swap Meet and Autojumble at DuPage County
Fairgrounds in Wheaton, Illinois. Expect about 80 vendors and several
hundred visitors. Check here for more information:
http://www.chicagolandmgclub.com/swapmeet.html
Happy hunting, and fun tinkering,
Barney Gaylord
1958 MGA with an attitude
http://www.ntsource.com/~barneymg
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