70 Mach 1 with Fluctuating Speedometer below 40mph

7

70 Mach

New Member

Jun 13, 2024

4

0

1

Western PA

Oct 19, 2025

#1

• Oct 19, 2025

• #1

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

Member

Jul 24, 2025

13

16

13

San Diego

Nov 3, 2025

#2

• Nov 3, 2025

• #2

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

5 Year Member

Jan 19, 2021

672

262

83

Laguna Woods CA, fort Mohave AZ

Nov 3, 2025

#3

• Nov 3, 2025

• #3

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.

 

C

CSGT428

Member

Nov 12, 2025

6

9

13

Inland NW

Nov 12, 2025

#4

• Nov 12, 2025

• #4

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

I prefer to be called "Evil Genius"

Founding Member

Oct 10, 2001

2,468

86

99

Western Mass

Jan 19, 2026

#5

• Jan 19, 2026

• #5

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

 

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