91-94 Mechanical Speedometer

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

Reply to
carbide
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Did you pull the cable all the way out, wipe it down good, and then regrease it and reinsert it? If not, try that. If you did and the needle is still shaky it may be that the cable ran dry so long that's it's "sprung" and you need a new cable. What happens to make the needle jerk up and down is that the cable binds for part of a turn, then releases on the other half of the turn which makes it slow down and speed up over and over again which of course makes the needle jerk. It's possible it's the head but every vehicle I've had with a jerky needle has been fixed by fixing the cable. You can probably get a replacement head (used) pretty cheap on Ebay if you want to try that route.

Reply to
Ashton Crusher

Yeah, I did it twice, thinking the same as you. I also carefully inspected the full length of the cable and housing, looking for any damaged spots. It didn't make any difference at all. And removing that instrument cluster is a royal pain.

The key here is it's not the usual rapid ticking of the speedo needle at the rpm of the speedo cable. It works fine until about 55, and then starts swinging erratically at a slow rate- not in time with the cable. AND it reads too HIGH, by 5-10 mph.

Searching this newsgroup I found a lot of cases like this, and other speedo problems. One poster described my symptoms exactly. He tried lubing the cable, and replacing the cable and housing, and finally fixed it by replacing the head.

That's why I went into detail to document it. Hope it helps the next guy, like he helped me.

I'm mulling over buying another speedo on Ebay or from a local junk yard, or using a cycle computer. This sort of thing bugs me, and I'm a tinkerer, so I'll fix it one way or another.

-Paul

Reply to
carbide

OK, I went to a junk yard and pulled a couple of speedometers, one from a Ranger and one from a Bronco. They were both older than '94, and had one less digit in the odometer. I also found another slight difference, the holes on each corner that the Tach and other gauges index off of were smaller, so I had to drill and use nippers to make that fit. The face was also a little different. Unfortunately there were no newer vehicles. I probably should have bought the correct speedo on Ebay, I could have gotten one for about $45 with shipping. However it might have had the same problem...

Quick and dirty speedo removal process: Remove the trim around the instrument cluster. I used a claw hammer. Then remove the screws holding the clear bezel and the black spacer between that and the instrument faces. I used a torx bit on an electric screwdriver with a

12" extension.

Pop out the tach and the gauges starting from the outer ones, just pry gently. The Speedo is held in by them. If it's an automatic, you'll have to deal with the cable that moves the gear shift indicator. I just cut the cable with wire cutters, or you can remove the torx screws holding it.

Then reach down by the gas pedal and pull up on the speedo cable to get some slack, and gently pry out the speedo until you can get a hand behind it. Push on the flat on the white plastic catch to separate the cable from the speedo. You don't want to squeeze on both sides of that, just push on the flat with a finger while gently pulling on the speedo. Squeezing both sides won't allow it to unlatch. That's it.

The Ranger speedo was dusty and had wear marks indicating that the spinning magnets were rubbing on the needle cup, but the Bronco speedo was in pretty good shape. The needle had very little side play. So I bought that for $17.

I reset the odometer to match my old one by driving out a pin that held a series of gears that link the digits of the odo. I slid that pin out, and then the digits were free to turn. I set it to 63999.9, reinstalled the pin and gears, and spun the input with my fingers to verify that it went to 64000.0.

I installed it using my quick and dirty procedure, and it fixed the problem.

Next I took my old one apart- hold the cup while twisting and pulling on the needle, and it comes off. Two screws hold the face on, then two more screws hold the front of the meter to the frame. The needle and odo assy comes off, leaving the frame and input section.

The spinning part has a hole in the end, which is a bushing for the end of the speedo needle assy. That was worn, allowing that end of the needle to flop around, making the rattling noise.

So that's the cause- a worn bushing.

-Paul

Reply to
carbide

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