I have an 88 740 GLE wagon. I have just finished converting it from AW70 and 4.10 rear to M46 and 3.31 rear. Everything works great except the speedometer. The rear axle came from a 90, and it has 48 teeth on the pulse wheel, whereas the 88 had 96 teeth. I changed the fronts to match, which seemed to satisfy the ABS, but now the speedometer runs at half speed. I can't just swap in the 90 speedometer, as it has a 4 wire connector, not the 3 wire on the 88. I've experimented with the wires but can't get it to work. The + and - wires are the same, but there are two signal wires on the 90, and only one on the 88. I've identified the wires going directly to the speed sensor on the rear axle and coneected them to the terminals on the 90 speedometer, but still I get nothing.
If anyone else has done a conversion like this before and figured out how to make it work, I would appreciate the info. I have all the parts from the 90, and access to a wide variety of other 740s in local salvage yards if any additional parts are needed. I have both the US and UK Haynes manuals, neither of which has a good wiring diagram of the instrument cluster (wires are not all labeled as to color, and some are omitted)