I have a 1997 Chevrolet Suburban. Showing code P0785 when warms up. Have replaced both shift solenoids, checked wiring, checked fuses, replaced ignition switch, tried 3 different computers. Goes in to fail safe. Stays in 3rd gear. Can shift down to second, but then can only shift to third. With my scanner hooked up, I can come to a stop, vehicle stays in third. I can clear the code and vehicle will shift to first, but the light comes right back on. When cold runs and shifts perfect.

 

Likely broken or damaged wiring harness, TCM issue, ECM/PCM issue, might be pin/connector problem (corrosion, melting, broken lock tab etc.)

 

I had the same issue with a '97 Burb. It would probably be the same on any 5.7 liter, 4L60E equipped Silverado, Tahoe, Suburban or the GMC equivalents. Drove me and my mechanic crazy for weeks! Replaced the shift solenoids, harnesses, ignition switch....money money money...

Here's the solution to this issue. The voltage to the shift solenoids should be a steady 12 volts (12V). But somehow the circuit drops to a lower voltage or loses it entirely for an instant. Why? My truck had an old security system installed in the wiring harness. I don't know for sure, but the 12V power to that "black box" may have been tapped off the transmission circuit (fuse #20) and the wire may have been compromised by the Scotchlok tap. Maybe. Anyway, the circuit is #20 in the fusebox. If you remove that fuse and attempt to drive the truck, the transmission will default to "limp-home-mode" (3rd gear) and you'll lose the speedometer, tach and PRNDL lights. And the check engine light comes on with the P0785 code showing. The fuse label calls this circuit "PRNDL". It might be in a different position on other years or models.

In order to stabilize the voltage in that circuit, you need to use a "jumper" wire from the #20 to another fuse. Do not use the illumination fuse (#14) for the source. The parking lights and taillights will glow. Use the radio fuse #17. Take a short (2 inch or so) piece of 16 gauge wire. Strip the ends back about 1/4 inch or a bit more. Position the stripped wire over the fuse position and shove in the fuses making sure to trap some bare wire into the fuse holder with the fuse blade. One side of the blade will do. You don't have to get the entire stripped wire into the box.

This will provide a constant 12V to the #20 circuit and the transmission will shift 1 - 2 - 3 - 4 as normal. This is certainly NOT A REPAIR of the issue. Just a "work around" solution. But it worked for me and solved my problem. Your results may vary! (LOL) Good luck!!