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speedo repair

1K views 9 replies 4 participants last post by  John long 
#1 ·
i have a 58 chevy truk, long story short my speedo went bad. it reads pegged at anything over 55mph
i ordered a new speedo and it was so jumpy i returned it along with 2 others that arrived mangled

i had to remove the new speedo to return it so on a whim i decided to disassemble the old one and check it out to see if i could repair it
not much in there: a spinning magnet, a disc that holds the needle and a clock spring
the odometer is no more complicated than the speedo, just more parts and it works fine and is accurate
i cleaned a few pieces of crud off the magnet, reassembled it and it worked great up to 55 mph

i'll tear it apart again but what should i look for?
anyone here do speedo repair?

speedo (actually running 100mph)



guts and back





not much to it; it IS fixable :D


:thumbup:
 
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#2 ·
Often thses problems are caused by the drive cable, before going crazy with fixing the speedo's innards I put a new cable between it and the tranny.

Yes they are pretty simple the cable end turns a small magnet inside a non-magnetic metallic cup of brass, copper or aluminum. The spinning magnet creates an eddy current in the cup making it magnetic. This couples the cup to the drive cable, the faster the cable mounted magnet turns the stronger the eddy current field this pushs against a spring the difference in the forces is read as speed by the needle position. Short of problems with the odometer gearing about all that goes wrong inside the speedo is the spring craps out or the bearings all this rides on wear out.

The cable portion also wears which can make for a jerky rotation or may thrust the magnet into the cup so the cup becomes friction driven, this should not happen there should be no sign of mechanical coupling between the magnet and the cup.

Most decent size towns have a shop that specializes in fixing and calibrating these things.

Bogie
 
#3 ·
I am guessing the spring is weak. 50 is the center point and past 50 the needle swings downhill. From 0 to 50 it is an uphill journey for the needle.
The weight of the needle itself may be covering up for the weakness of the spring, until it "tops the hill" sort of to speak.
Maybe to prove that theory you could hook a drill to a cable and run the speedo, and tip it on its side so the needle cant swing under its own weight , and the magnetic field would be all that is affecting it.
Just a though.
For it to work up to 50 and be accurate tells me the magnet and drum is OK. Something else must be affecting it, like gravity.
Good luck Ogre.
 
#9 ·
after installing many of these i figured out how easy it is to modify the odometer mileage
my mileage since the rebuild should read 24700, i had reset the last one to 25000 and had to re-zero it to ship it back
this would probably be illegal in a newer vehicle :D



i also did the crimp on plugs to facilitate the instrument panel removal
which i have used many times since i put truk back on the road

 
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