afaik, all volvo 'red blocks' use a 3 wire Ox sensor. 2 wires are the heater, the 3rd wire is the signal, the ground is via the exhaust pipes. on our 87 240 (and my 92 740T), they are connected up on the engine side of the firewall, more or less in front of the passenger seat, with two separate connectors. I believe the green wire is the signal and the 2 wire connector is the heater (polarity doesn't matter on the heater, but one side is grounded, the other connected to the in-tank fuel pump circuit)..

 

How to I take off belts besides the alternator on the 1990??

 

each device on a belt has a tensioner and lock screw. loosen the lock bolt(s), and unscrew the tensioner bolt/screw til there's enough slack to get the belt off.

I suggest you get your hands on the Bentley 240 Service Manual, its quite good for these things. especially important is the correct torques on various things when you tighten them back up. novices especially tend to over-tighten small bolts and under-tighten big ones.