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Why wont my speedometer work, cable looks fine


jnfritz

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So I’ve been going through my 71 521 here and there and I’m finally getting to the speedometer issue; it doesn’t work.

 

Ive seen on the form that there is a black cable that goes from the transmission and screws into the back of the speedometer.  I have that cable, and the cable

seems to be in working condition. 

 

When I screw this cable into the transmission, and I drive, I don’t see the end of the cable spinning like it’s suppose to.  When I screw it into the speedometer,

it obviously doesn’t work either.

 

What’s going on here?

 

Since the interior cable isn’t spinning when I drive, is there something wrong with the transmission where I plug it into?   I'm guessing the speedometer is purely mechanical, there isnt a ground or power wire that needs to be wire up, right?

 

 

 

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Pure mechanical! Lack of engagement of the [usually] square end of the cable with the worm gear in the transmission gear housing. Most likely? Your transmission worm gear has stripped. Get a new gear. Lots of discussion on this site about various transmissions and the color coding and gear ratios of the speedometer drives. Google is your friend!

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Disconnect the transmission end. Pull the inner cable out. Be ready with a towel or something to collect it. It's very long and slippery and if it's OK you don't want to get it dirty. The transmission end has a ferule crimped on the end that engages the plastic pinion gear. The inner cable itself is just a small long coil that is very flexible. The speedometer end is squared to engage the dash speedometer. I imagine the inner cable is broken. 

 

Speedocable006Large.jpg

 

Speedocable005Large.jpg

 

The most likely causes are damage to the outer sheath that allows water and dirt to enter. It can either get so rusty it jams and the transmission just twists it off or it freezes in the winter and the transmission twists it off.

 

If this is the case just replace it. The 620 cable should fit, later years are slightly longer but too long will work just fine. They used to be about $18 at Rock Auto or NAPA???

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I'll take the cable out of the truck tonight and inspect it further. I still don't think the inner cable is broken.

 

 

what is that small blue gear? is that the worm gear that could become worn? can that be easily taken out and replaced?

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Yes that's the pinion gear. It's matched to the differential ratio and tire size. The number of teeth varies to compensate. There's no good reaason for the pinion gear to wear out. When speedometers stop working, it's the cable that is the weak link. It's suspended outside the vehicle and is exposed to rocks, dirt and salt water damage.  

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When I screw this cable into the transmission, and I drive, I don’t see the end of the cable spinning like it’s suppose to.

 

Cable broke in middle. turn one end and see if other turns.

 

pinion teeth break or mine the shaft ran out of oil the the teeth sheared.

 

got one out of a 510 or 521. best to get 521 as its geared calibrated for it but a 510 one will work for now if you nned a basic speedo.

 

pull the 10mm bolt out and pull the whole unit out.

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When I screw this cable into the transmission, and I drive, I don’t see the end of the cable spinning like it’s suppose to.

 

Cable broke in middle. turn one end and see if other turns.

 

pinion teeth break or mine the shaft ran out of oil the the teeth sheared.

 

got one out of a 510 or 521. best to get 521 as its geared calibrated for it but a 510 one will work for now if you nned a basic speedo.

 

pull the 10mm bolt out and pull the whole unit out.

 

 

This

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