2000 Silverado 4x4 tranny problem
I wonder why you say "changed the wiring harness" - did you just replace an old one or install one for a different year that what the trans originally had.
I'm grasping for straws here but since no obvious problem/solution jumps out for me, nor evidently anyone else, I want to start with basics on the assumption the trans was rebuilt correctly and you only have an electrical or compatibility problem.
If only 2nd gear is unavailable, this is most likely a servo problem. First check the servo end play by pressing on the servo cover with e.g. a crowbar and estimating how much it moves in. Spec is 1/16 to 1/8 inch. Around 1/4" you lose 2nd and/or 4th gear. A newly rebuilt trans should have a new band and therefore be in spec.
A broken servo piston, broken seal or worn body could cause loss of 2nd gear. Sonnax makes a set of o-ring seals that work better with a worn body.
Servo/band problems affect both 2nd and 4th gear, but symptoms often start with either gear and not both.
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This should force 3rd gear. Does this give you 3rd gear?
Confirm that you had 3rd gear with the 1st servo?
It is very strange that you have 4th gear but not 3rd; not sure I have ever heard of that.
Perhaps if the band is not releasing.
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