Through a long sad embarassing story I had to get a replacement used K10042 speedometer for my 1989 240 automatic. The place I got it from has a warranty and return policy. The replacement speedometer indicates too fast by about 20%.
I put it into my original cluster and it still reads too fast. Before I send it back and search for another (hard to find) I wanted to see if there was a fix. My previous speedometer seemed to work fine except for it hung at about 55 MPH. Thanks to my wonderful fix it skills it is now toast. Another lesson learned. My wife can only remember the ones I mess up and not all the ones I fixed successfully. I am sure you can relate. I don't understand all the theory of this thing which might help.

1. How do I check the transmitter on the rear axle? Could it be the problem?

2. Is there a place the wire harness has a problem?

3. Is there any hope for the "new" speedometer? Should I try changing out the
circuit board off my old speedometer with the "new"one?

4. Any places to find another K10042? The junk yards around here are out.

5. What question didn't I ask that I should have?

I have the same problem with my '82; except that is gear driven! Well it is kinda the opposite problem. Not sure how much it is off but I was doing "80" and my buddy was doing 90 and I was pulling away from him!!

I digress. To a solution:
There is a gear that is driven by the worm that 1 breaks and 2 is what we used to swap out to recalibrate speedometer units. If memory serves the gears were often blue. I will see if I can't take a picture to clarify.

Good luck!

Andrew

Due to an even longer and more complicated story (changed the car from auto to stick, and swapped the rear end) I have a speedometer with the opposite problem. Unfortunately, this is on a 740, so we can't just swap them and see if it is solved.

Does the replacement unit read consistently high, or does the percentage of error increase along with the speed of the car?

To answer your questions:

The sending unit on the rear axle is operating correctly if the speedo works when the car is moving, even if it reads incorrectly. It may be that the sending unit is the wrong one for the speedo you now have. If you don't have ABS, you may be able to fix the problem by swapping the sender. If you have ABS things get more complicated.

The wiring to the sensor sometimes frays or breaks. When this happens, the speedo doesn't work (at all)

The best hope for the "new" speedo might be to send it back and get another one under the warranty.

If both the original and the replacement are K10042, there is some other designation you need to be aware of. The difference between the two may be due to year, a different number of teeth on the tone ring (the thing that the speed sensor reads) or it may be between manual and automatic transmission cars (which have different axle ratios)

Try to make sure the replacement comes from an 89 automatic, with or without ABS, whichever yours is. There may well be other years that will work, but without a parts book, I am unable to tell you.