Recently, I messed up my speedometer. I did a search here on Mud as to how to fix it. Well, my search turned up very little. Oh, I found lots of threads about speedometers and swapping them out and adapters and what year fits what, etc. etc. etc. But, I found nothing about how they worked or how to fix one. I figured a short thread on what makes an FJ40 speedometer work was in order. This covers my 6/71 speedo, and I’m not sure what other years were the same. I’m guessing they are all fairly similar, but I could be wrong.
**Disclaimer** I’m not a speedometer technician. I’m just some dumb@$$ who messed up their own and dug into it to fix it. Use this info at your own risk.
In simple terms, the speedometer works through a gear driven in the transfer case. That gear turns a cable that is attached to the back of the speedometer housing. I had wrongly assumed that the speedo worked by a direct drive via the cable (i.e. however fast the cable was turning was where the needle on the speedo pointed). This is not the case. At least, not quite so simplistic. Read on to see how the speedometer really works.
I started by removing the gauge cluster from the dash. To do this, I removed the speedometer cable, unplugged 2 wire looms for my gauges, and took off the leads to my ammeter (remember to disconnect the neg. battery terminal before hand). Once out of the car, I removed the 4 screws that hold the back cover to the gauge cluster, and then removed the 2 screws that hold the cover to the speedo. At this point, you will have the picture I posted above. Just the bare speedometer assembly. Really, the disassembly is pretty straight forward in all steps.
If you continue to disassemble, you will need to remove the needle. It just presses onto the shaft. Next, remove the 2 small screws that hold the faceplate on. It helps to have a set of small, watch repair type screwdrivers. Lift the faceplate off and you will see what's in the first picture. Continued disassembly will give you what's in the second picture.
To remove the odometer housing, you will need to remove the small brass clip opposite the odometer gear (left side of the speedo housing in the above picture). This will allow you to slide the odometer to the left and out of the housing. There is a bushing that holds the odometer shaft centered. This will slide up the shaft when removing the odometer. When you re-install it, you will have to slide the bushing back into place and insert the clip. It is self explanatory when you actually see it in person.
So, how does it all work? The speedo cable connects in the back to a round magnet. The magnet spins with the cable from the transfer case. This is the “direct drive” part. On top of the magnet sits the housing in the second picture. As the magnet spins, it moves the housing in a clockwise fashion as much as the spring allows. The faster the magnet spins, the more pull on the spring and the needle reads higher.
What spring? Apart, you can see that there is a base section with a spring attached with a bushing to the speedo shaft. The spring goes to the upper section and is attached with a small dowel into a hole in the upper housing. The upper housing is secured with two screws. The lower housing spins to show the speed at which you are traveling. However, it is all dependent on the spring rate. DO NOT MESS UP THIS SPRING. That is what I did by taking apart my speedo and cleaning it. I tweaked this spring. Your speed is directly related to the spring rate. Mess it up and you will have one heck of a time getting it correct again. I’ve had mine apart dozens of times trying to get it right again. I finally have dialed it in, but only with about 20 hrs of messing with it.
The part where the spring attaches to the dowel spins on the upper housing. In this way, you can effectively change the spring rate. Spinning the dowel housing counterclockwise tightens the spring. Small adjustments can compensate for larger tires. If you look in pictures, you can see the “stops” for the speedometer.
In the first picture, look at the 11:00 position. That is the stop on the upper housing. The stop on the lower housing is in the 9:00 position (second picture). It is just a little bump that hits a tab on the upper housing. You want that nub just north of the tab in the upper housing. The spring will pull it tight.
That’s all there is to it. Again, do not mess up the spring. I have taken the bushing off the speedo needle shaft and re-wound the spring a couple of times trying to get it right. It is very tedious and I would not recommend it. However, if you’re like me and have already messed it up, don’t worry. You can get it to work again. It just takes trial and error to get it to the correct spring rate. I’m sure there is a spec somewhere, but I couldn’t find it. Besides, I have no way to measure it even if I did have the spec.
When putting everything back together, make sure that the spring does not bind between the upper and lower housing. Test fit it into the speedometer assembly and check to see that the stops rest against each other lightly. Spin the needle clockwise and ensure that the stops return to each other. If it does, put it all back together in reverse order.
