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But even if I bypass the sensor and ground the wire leading to it to the engine block (which is what happens when the switch turns on), it still doesn't shift.


Well, that doesn't mean that the switch is good, but it probably is. What it means is that there's a problem further down the line. Forget the switch for now, keep the ground wire on the switch and work through the rest of the list I gave you. Keep at it - you'll find it.

Frank

'89 [color:"white"]G-Raider[color:"white"] [color:"black"]Supercharged 3.0L, MegaSquirt 2, lockup A/T, 2.5" exhaust, 172k, Cibie H4s/Oscar SCs, Hella Micro DE fogs, Cobra CB, Superwinch hubs, LSD rear/Aussie Locker front, Bilsteins, Lifeline AGM, Rust-Oleum