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?