I have a working '94 940 speedometer which I need to put into an '87 740. I know they are not plug compatible. I still need to make it work. I need your advice.

Problem: the '87 turbo sedan's speedo connects to the speedo sensor on the differential through the ABS system, but the '94 turbo wagon's speedo connects directly. Another problem: the '87 speedo sensor shares a ground with the fuel level sensor, but the '94 has separate grounds.

So far I have encountered three separate results.
1. If speedo and fuel level sensor grounds are electrically tied together, then the speedo reads 0 MPH and fuel guage is below empty.
2. If speedo and fuel level sensor grounds are separated per '94 wiring diagram and fuel sensor wired directly to combined instrument's fuel sensor inputs but speedo sensor is routed through '87's ABS system, then the speedometer reads about 0.2 of road speed. (When the car is going 40 MPH the speedo reads about 8 MPH.)
3. Same situation as 2 but speedo sensor is wired directly to speedo inputs (+ and -), the speedometer reads about 2x road speed. (When the car is going 40 MPH the speedo reads about 90. Whee.)

Does anybody have any sage advice for this? I have Volvo's wiring diagrams for both cars but neither one has a detailed schematic for the speedo's inner workings.

Thanks in advance for the help.