Speedometer Calibration

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We are not responsible for damage to your speedometer or vehicle resulting from the use of this information.

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First off, if you don't have a source of accurate speed readings such as a GPS or a signal generator or other bench method to calibrate with, or if you get the speedometer out and then decide that it is too risky for you to do yourself, you may contact me. If you ship me your speedometer, I will bench calibrate it to match your particular tire size - provided that you return ALL THE MAGNETS back into the plastic disk in the front axle so that the odometer is set back to the factory setting. Along with the speedometer please include the number of magnets your plastic disk contains (should be 6?), and the tire size you wish to have it calibrated for. Please don't attempt this, screw it up, and then ship it to me. I would far rather that if you don't feel competent to do it, you just send it to me first. If you overnight it in a return shipping paid box (with lots of good padding) I will calibrate and return it same day at no charge. But, if you want to tackle it yourself, here is how you can do it...

I have developed a calibrator for use with this procedure. You enter the number of magnets in your wheel sensor, along with the size of your front tire, and it simulates the vehicle traveling at 30 mph so you can adjust the speedometer. If you are using the calibrator, you can find the instruction page for it here. The number of magnets that the plastic disk in the front axle came with originally was calculated for the odometer, which can't be recalibrated. Therefore, to prevent screwing up the odometer, the number of magnets should NOT be changed from factory. Once you have the correct number of magnets back in, then you can set about calibrating the speedometer the correct way. If the disk that held the magnets is cracked, put some gel type crazy glue in the hole to help hold the magnet from slipping out of the container. If you are putting magnets back in to the disk, try to put them in so that all magnets have the same polarity facing the same direction. Remember the old rule about opposites attracting, and you should be able to use it to end up with all the magnets facing the same polarity to the same side.

If you have a GPS to calibrate against or some other accurate speed reading, then that will work for this procedure if you don't have the calibrator to use (it is free to borrow for anyone that asks). If you don't have something to use as an accurate speed check and you don't have a pulse generator or microcontroller of some sort, then don't take it apart. It isn't something you can guess at. If you do have a signal generator and need information about calibrating on the bench, you may contact me through the Yahoo group. If you give me your tire size and the number of magnets in your disk, I can tell you what frequency you will need to generate and what two wires to feed it to in order to bench calibrate the speedo.

Anyway, to calibrate the speedo, first put the correct number of magnets back in the plastic disk to correct the odometer. Then remove the three bolts that hold the bracket that holds the pack voltage meter and the speedometer in the dash. Then remove the two nuts that hold the speedometer to the bracket. If there is a nylon tie strap holding the wires up then carefully clip it. Unplug the three connectors that connect the speedometer to other wires in the harness for now and remove the speedometer. Remove the two nuts on the back side of the speedometer and pull the back shell off the speedometer. You will see that the chrome front bezel is crimped onto the body of the speedometer all around. With a small screwdriver you need to gently work your way all around the speedometer, lifting the chrome edge around and around to release the front bezel from the speedo body. Once you have it released, pull the front bezel away from the speedometer works. Near the edge of the circuit board is a small trim potentiometer with locking material that looks like nail polish holding it in place. This is put on at the factory after it is "calibrated".

Now that you have the speedometer apart and have access to the trim pot, you can plug the two larger connectors back into the harness. Don't worry about the small one for now, since it is just for the trip reset switch. Gently break loose the material holding the trim adjustment so you can turn it. Now you are ready to have someone drive while you adjust the pot to match a GPS. I recommend adjusting it at about 30 mph. BE VERY CAREFUL not to damage the speedometer or any of its components physically, and DO NOT allow any bare components to touch metal parts in the cab. This is a delicate electronic instrument!!

After you have the pot adjusted so that the speedometer is accurate, take a MEDIUM STRENGTH REMOVABLE LOCTITE or nail polish, and put a single drop along the edge of the trim pot so that it won't vibrate out of place after the Loctite dries.

Now before you begin reassembly, you can take the opportunity to bend the lip of the front chrome bezel so it is even all the way around, and is open enough to be able to reinsert the speedometer into the bezel. Before you insert it, you might want to make sure you wipe any marks or fingerprints from the inside of the glass. You won't get another chance. Now you just have to do all of this in reverse. Insert the speedometer body into the front bezel. Be sure it is fully inserted and once it is in all the way around, then use a pair of pliers to crimp a little spot on each of four sides to hold it in place, and then work your way fully around the speedometer, crimping it all back the way it was to start with. Reassemble the back shell. Mount the speedometer to the bracket. Mount the bracket back in the dash to hold the instruments in place. MOUNTING THE BRACKET BACK IN THE DASH IS THE HARDEST PART OF THE WHOLE PROCESS. Be sure that you reconnect all three of the cable connectors to the harness, and then tie strap any dangling wires so they don't hang down.

There you go - done.