I have a 1990 Ford F-150 4x4 with a manual transmission. About a week ago my speedometer started jumping around from 0mph to about 40mph, no matter how fast i was traveling. Now it reads zero whenever you depress the gas with stress on the engine, but if you let off the gas when driving it starts to jump around again, and it also does this when the clutch is pressed. Im not sure what the problem is.
Sean
This is still a cable driven speedometer with several possibilities. Start at the transfer case end of the cable where the drive/driven gears are. They are just plastic and are always suspect first. Your cable ( 2nd suspect )might connect to a speed sensor ( 3rd suspect ) with a spring clip ( or mount direct ), and bolt to the housing with a bolt in a bracket. When dis-assembled the driven gear stays on the cable/sensor end and the drive gear is inside. Looking in the hole at the drive gear, the teeth should be exactly strait accross their tops…ie; //// not concaved. Same with the teeth on the driven gear. If they’re scooped or concaved they are worn out and need replaced.
You can manualy spin the cable from the bottom to test the opperation of the speedo head ( 4th suspect ). A little cordless drill makes a good test spinner.
With so many variables you’ll really need to check it ( or have it checked ) first to know.