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speedo needle not moving, but odometer turning


tolerate

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1977 280z.  I will visually inspect the speedo cable this weekend if I have time.  I've got dry rot on the tires and need to get those replaced asap.  

 

just to reiterate my problem, my speedo needle isn't moving, but my trip odometer is turning.

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The needle is attached to a shaft that is attached to a steel cup that rides in close proximity to a magnet that is spun by the cable. (when the magnet spins, it acts on the metal cup causing it to spin even though there is no direct connection)

 

Open the speedo and you'll see.  The needle won't turn if the shaft or cup is buggered up.  The Odometer is run by gears that are driven by the cable,  The cable, Odo gears, and the magnet can all turn without the needle moving.  Or the magnet may not be spinning...

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The trip meter shows that the cable is turning.  The older speedometers use a rotating magnet, and a cup or disc shaped thing, and a spring to show speed.  There's pictures out there is you search "how a speedometer works".   You probably have a spider web or some sort of crud in the mechanism, its kind of sensitive.  You could try to spin it free by driving really fast or use a drill motor on the cable end, or take it out out and see if you can break things loose by shaking it.  Or take it apart and see if it's cleanable.

 

 

There's a nice picture in this link - http://www.uniquecarsandparts.com.au/how_it_works_speedo.htm

Edited by NewZed
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