Never heard of a sensor on the suspension of a 900c. I guess I would check the FSM (Factory Service Manual) for it, to verify it exists. If it does, the FSM may have a work-around for it: tie that wire either to the battery's positive terminal, or Ground it, to fool the circuit.
Why not go with sealed, airless shocks and liberate yourself from the hassles of needing compressed air? I replaced the Fork Springs, and rear, air-assisted shocks on my Voyager, with Progressive Fork Springs, front -- no air assist needed with them (OEM had 7.5 lbs. air in front forks, not needed/not recommended with the Progressive Fork Springs), and the back Shocks are sealed, airless Progressive models. The bike handles fantastic -- much better than the tired, OEM air-assisted suspension system. I would never return to air-assisted suspension due to the hassles of messing with low pressure forks (hand pump, and special gauge required), and putz'ing with gas station air for the rear suspension.
I replaced my Fork Springs and rear shocks with Progressives on my 1979 Honda 750K, first. The OEM suspension was tired... Afterwards, the bike rode/handled like a modern bike. The difference was amazing. After that, upgrading my heavy touring bike, was a no-brainer.
Just be careful of lowering the rear end too far. By going with shorter shocks, you have lessened your suspension's range of travel, which can cause issues beyond just comfort going over bumps. Cheers!
1979 CB750K (sold, 2012, but not forgotten)
1983 Kawasaki 440 LTD Belt Drive (sold, 2011)
1993 Kawasaki Voyager XII
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