I think I can add something to the thread on speedo
repair. The following was on the Brit Cars list:
>>Oblig BC stuff: Anyone ever do any work on a Jaeger TR speedo? The
>>needle on mine will not return to zero at rest and moves sluggishly
>>when.it does move. If I disconnect the cable, the needle falls back
>>to zero fine. I have removed the unit from the dash, opened it and
>Barry and all,
>
>I have exactly the same problem, I have 2 speedos for my 4A, and both
>exhibit this behaviour where as soon as you touch it everything is fine.
>In fact one of mine is an alround Virtual Reality dial as it reads high
>all the time (they have the same number). Have dismantled and cleaned
>both and they seem perfect, put them back in the car---same old story.
>I worked with a guy who used to work for lucas, and he told me how they
>calibrated speedos. The magnet behing the alluminium disc is always too
>strong (i.e. they read very high). The finished speedo was placed in a
>jig with a known speed drive, the operator had to set a target MPH on the
>speedo, the jig was surrounded by a large "degaussing" coil and the operator
>was able to pulse this coil to demagnetise the spinning magnet in the speedo
>head, therefore lowering the needle position to the target MPH.
>I guess Jaeger did the same...
>
>This doesnt help us much other than to suggest that there is no mechanical
>way of adjusting them (needle repositioning and return spring fiddling are
>trial and error methods I guess..).
>
>All I can say is, if you do get some info, let me know if you solve the
>problem.
>
?Cheers
>
>
>Dave Lacey '67 TR4A CTC 77965O (probably doing 25 mph in the car park as
>we speak!).
My experience:
I also dismantled the speedo from my Jaguar to find out why it
behaved erratically and sometimes got stuck at an incriminating
high speed value. I had already replaced the cable which helped
somewhat. The speedo has similar construction to that described
above, with the needle attached to an alumin(i)um disk that has
magnets glued to it. The disk turns from the magneto-viscous
coupling (a word I made up if it doesn't exist) to the spinning
steel wheel driven from the cable. The restoring force is
provided by a spiral spring. The needle and disk assembly are
supposed to pivot freely like a clock balance wheel does because
the axle of this widget is sharpened to a point and sits in a
conical depression on a hardened steel surface. This pivot can
get gummed up, especially if oil creeps out of the end of the
speedo cable. So I cleaned mine but it didn't improve. Then I
noticed that the pivot had too much sideways freeplay. When the
needle turned it could slip sideways and get stuck, not returning
until it got bumped or something (undoing the speedo cable,
for example, would also let it slip back). I tried reducing
the freeplay by tightening the preload adjusting screw on
the pivot to no avail. Looking through a magnifying glass
I discovered that the hardened surface where the needle should
pivot has worn away.
So the fix is to replace this bearing surface. Trouble is its
all integral with the bit where the cable attaches.
I don't know if such a spare part is available, but if it is
I sure would like to hear about it.
Cheers, Patrick.
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