My 94 Explorer's mechanical speedometer behaves erratically above about
55 mph. It swings while driving a steady speed, and generally reads 5-10 mph too HIGH.I looked thru past threads here, and found this to be a common problem, with some people saying it was the speedometer cable and others saying it was the head (meter).
I tried greasing the cable with silicone grease, which is the recommended lube. No change, so I removed the head and studied it.
The speedo cable drives a worm gear which drives the odometer and trip odometer, and spins a pair of bar magnets mounted crossways on the end of the worm gear. The speedo needle is attached by a shaft to a metal cup which fits over those spinning magnets. There is some sort of needle bearing in the end of the worm gear that keeps the cup centered over those spinning magnets, and another bushing near the face of the speedo that supports the speedo needle assy. There is also a watch spring that returns the speedo needle to zero.
What I found was a lot of end play and side play in the speedometer needle assy. I suspect this is the cause of the problem, the spinning magnets aren't kept properly centered in the cup.
The operating principle seems to be that the moving magnets induce a current in the metal cup, and the current in turn creates a magnetic field in the cup that interacts with the magnets, causing the cup to rotate in the direction of the spinning magnets, until the magnetic force matches the tension from the watch spring.
I tried greasing the two speedo needle bearing points, and reducing the end play slightly. The speedo is a little more stable, no longer swings as wildly, but it's still not right. Note that this inaccuracy doesn't affect the odometers, since they are gear driven.
So the upshot is the head is the problem. I didn't want to disassemble it completely for fear of messing up the calibration, though if I get a replacement I may. I couldn't see an obvious way to adjust the end play, which would help a lot keeping down on the side play. Pushing the end where the speedo cable inserts removed about .020" of end play, which made a noticeable difference in the side play of the needle assy.
-Paul