70 Mach 1 with Fluctuating Speedometer below 40mph

[70 Mach]

My 70 Mach 1 has a 351C engine and FMX transmission with a jumpy speedometer below 40 mph. This model also is equipped with a Distributor Modulator Speed Control Sensor between the transmission and the cluster (Part # DOAF 9E731). Could this be part of the problem? It's strange because over 40 mph it's fine.

 

[9T02R]

Could be as simple as a worn speedo gear. You'll need to pull it out of the trans for a look see. Brian

 

[Mcmahst]

I’ve never heard of that sensor causing a problem like that. The sensor generates a small current when it spins, so usually, when it’s bad, it makes a terrible sound. It’s likely the Speedo gear. They’re easy to change. Good luck.

 

[CSGT428]

Most likely it's a "resistance" or "drag" problem. When the cable has some drag on it from a lack of lubrication or a tight turn in it.... could be the DMSC sensor also. Once it spins fast enough it overcomes the drag problem and stops slowing down and then "releasing" the cable as torque builds up, which is causing the bouncing speedo.

 

[Realmongo]

you can also try removing the cable from the casing and lightly lubricating it with white grease.