Matt I share you frustraion. '86 740 no speedo. Unfortunately I have no gas guage at this time also!!!!!! I tested my sensor at the differential. It has a dead short. No resistance. I tested and 86 at the jukyard. It had Infinite resistance.
I passed on that one. I tested one off an 87 740 after taking the diff cover off to get an up close look. It tested 3000-ish Ohms. BINGO. This Agrees with what Dave says. I am going to go to Volvo and ask to see a new one and am going to bring my meter and see what it reads. I think that this figure may be right.
My approach is going to be finding one at the junkyard and swapping the diff cover and sensor as one unit since there seems to be no way in hell to get these things out, and a new one is about $100 (winging the adjustment). I might pay $5 to $10 at a self-serve junkyard.I just wonder if the rear ends are the same i.e what models/years can I get an identical diff cover and sensor off of. This will also allow me to get a look inside my diff to see that all is ok and get a new gasket on it and fresh oil. If this doens't solve my problem I'll kill myself!!!!!!!!!