No Speedo but odometer works

[Croc1990]

1983 k20 350/sm465

So I picked up this truck couple months ago and just slowly picking away at it. Issue I have is Speedo doesn't move but odometer does. I first suspected the cable so disconnected it on both ends oiled it up hooked back up to speedo crawled underneath with drill spun cable in reverse and Speedo moves.
Hooked bottom end up took do a spin and same thing no Speedo but odometer works. Am I missing something?
I know can just get a gps or use phone for speed but it's just nice to see the speedometer move

 

[chengny]

1983 k20 350/sm465

So I picked up this truck couple months ago and just slowly picking away at it. Issue I have is Speedo doesn't move but odometer does. I first suspected the cable so disconnected it on both ends oiled it up hooked back up to speedo crawled underneath with drill spun cable in reverse and Speedo moves.
Hooked bottom end up took do a spin and same thing no Speedo but odometer works. Am I missing something?
I know can just get a gps or use phone for speed but it's just nice to see the speedometer move

The odometer rack is hard coupled to the cable via a gear and rarely fails.

But the speedometer is driven by a little magnetic coupling. IOW there is no mechanical connection between the spinning cable and the needle that indicates your speed.

The theory of operation of a cable driven speedometer:


1. The speedometer cable, driven by the gear that rides on the transmission output shaft, spins at a rate that varies with road speed.

2. The other end of the cable is attached to a magnet which spins around (at the same speed as the cable) inside the speed cup.

3. The magnet rotates continually in the same direction (in this case, counter-clockwise) - unless the vehicle is operated in reverse.

4. The spinning magnet creates eddy currents in the speed cup.

5. The eddy currents make the speed cup rotate counter-clockwise as well - in an attempt to catch up with the magnet. Remember that the magnet and the speed cup are not joined together in any way—there's an air gap between them.

6. There is a hair spring attached to the speed cup's output shaft. The hair spring limits the amount that the speed cup/output shaft can move for a given road speed.

7. The pointer - that actually indicates the vehicle's velocity in MPH/KPH - is rigidly fastened to the end of the output shaft.

8. So as the speed cup/shaft rotates, so does the pointer - up the dial, indicating the car's speed.

A sucky drawing of a speedometer:

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Can you move the needle with your finger? Maybe it's just stuck - poke it and maybe it will pop free.

 

[gpmorgan]

Great post chengny. I learned something today.

 

[Croc1990]

I'll take the bezel off and try