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Here is an old post I made some time ago on repairing the gears in the earlier speedometers. It was the blue/purple gear in Art's image that was cracked in each one:
I posted earlier concerning the problem with the odometer on my 82 GLT. Initially the odometer worked while driven with an electric drill on the bench, but reinstalling it in the car resulted in only .6 miles before ceasing to function once again. The speedometer worked fine. The achives document this as a common failure in this era 240. I actually had a total of three speedometers with broken odometers that I have accumulated. The following is what I found to be a fix for the small gear that proves to be the weak link in the system.
The small gear that is driven by the worm gear from the speedometer head was broken into two pieces in two of the units while one was cracked through on one side and not gripping the shaft it is mounted on. These speedometers were marked on the face as r0,980 and were 85mph types. The gear was purple in these but perhaps they are different colors in different types. All were equipped with what I found out was the Lamda Sond service reminder an auxiliary odometer drive off of the speedometer.
I immediately used an air grinder with an abrasive disk to remove the two gears that were associated with the Lomda Sond reminder. One was mounted on its own shaft (I cut it and its shaft off) while the other was mounted on the end of the shaft that also carried the broken purple gear.
With the second Lomda Sond gear removed the purple gear could be slid off of the end of its shaft. The shaft measures .117 inches. By holding the two pieces together and using number drills to get a hole size on the gear I found the diameter to be about .113 inches (#33). I used 3M Super Glue Gel as an adhesive and a spring clothes pin with additional finger pressure for one minute of curing time. While still holding the gear with the clothes pin I redrilled the hole to .116 (#32).
I used a dental pick to score the shaft parallel to its length. I scribed 5 or 6 lines in the area the gear would finally be pressed to.
Even after drilling to .116 the gear still seemed to be to tight as i tryed to press it onto the shaft, but the next number drill would have made the hole too large- .120 (#31). I drilled it again with the #32 drill and added a little lateral pressure to "hog it out" slightly. It seemed to have a good fit when I pushed it on. The scribed lines in the shaft would provide resistance to slippage and I decided not to use any adhesive on the shaft. I was careful to leave clearance between the gear and the adjacent metal shoulder.
I repaired all three speedometers in about one hour. It is not necessary to remove the face to make the repair, but on the first one I had already done that. If you do remove the face it is necessary to remove the speedometer needle. Before doing that you should carefully lift the needle at the end until it clears its home peg. Release it and allow it to come to rest and make note of where it stops (mine stopped right over the "r"). Before replacing the needle you need to start it into its sleeve with it positioned where it was before removing it. I would advise against removing it as it took quite a bit of force with a crow's foot type tool to remove it- lots of ways to do damage while removing and replacing the needle. It was also pointed out to me that the calibration on the speedometer might be off after removing the needle.
I put the speedometer back in the instrument cluster and reinstalled the cluster. There are only six connections to deal with on my 82 GLT. Disassembling and reassembling the cluster was very straight forward. The only connection you need to take off before pulling the instrument cluster away from the dash is the speedometer cable itself. Failure to do so will result in damage to the plastic piece that clamps the cable to the back of the speedometer. The cluster is held in place by two screws on each side. Access to the two on the left is achieved by pulling the headlight switch knob; the panel dimmer knob, and removing the backing plate. The two on the right are revealed by removing the two square inserts (or the small clock bezel in the Turbo models).
The odometer works fine. Time will tell how permanent a fix this is. I would say the barrier to successful repair of this gear using adhesive would be the size of the hole. Trying to press a gear with a .113" opening onto a shaft of .117" diameter would put too much stress on an already weakened gear. Perhaps these plastic pieces shrink over time and result in the cracked and broken gears found so often in these speedometers. New gears may not be available to the general public, but number drills and quality adhesives are.
Randy
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