the blue truck has speedo issues

[1low4x4]

Hey......i broke the original speedometer cable in this truck by not seating it fully into the cluster and the tip broke off. So i put in the speedo cable from the red truck that was still good and it now works, but its way too fast. Like its showing pegged out when im probably dping 70 mph.

Both ends are in...but i did fool with the speedo gear in the trans a bit. Could that have altered it any, me turning it? Or what else could be the case? Thanks...

85 c10

 

[1low4x4]

Bump

 

[Boone83K10]

Did the red truck have a transducer on the speedo? Yes messing with the speedo gear could alter the output.

 

[chengny]

The problem is most likely within the speedometer head as opposed to the cable/drive gear. Specifically, the hair spring was probably stretched and weakened when the original cable was breaking up. See 6 below:

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 vehicles 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.