4L80E, 1st gear and reverse only

Truck is a 1995 Silverado K1500 6,5TD, 4L80E.

After I parked the truck for a couple of hours after a 40 minute drive, the transmission will not shift out of 1st gear. It seems like it will try to shift but the instant it shifts to 2nd it will downshift into 1st, almost locking the wheels. There were no prior issues or warning signs with the transmission.

Fluid level is fine and there is no burned smell. Transmission temperature on the Autometer gauge is normal (160-180F).

There's no codes set when connecting the GMTDScan Tech software.

I have checked fuses and grounds. Also checked for any apparent connection issues with the wiring on the transmission. Shifter linkage only allows for engine start in P or N, as it should. Actual shifter positions (PRND321) corresponds with the shown position in the GMTDScan software.

When I manually select the 1/2 solenoid ON with the GMTDScan to shift into 2nd gear, the truck brakes to a standstill like using the footbrake. No movement in 2nd, 3rd or D. Reverse works fine.

I have tried to swap the transmission input speed sensor with another one. No change. Both the input and output speed sensors show RPM's in the GMTDScan.

I don't know what to try next, and I would hate to replace or rebuild the transmission just to find out that it was an electrical issue.

Any ideas will be greatly appreciated!

Thanks
Anders

Try reading for codes the old fashoned way with a paper clip on the service port under the dash. or you can take it to an auto parts store. many of them check it for free. The cable on my laptop was causing problems and not reading codes. The 1995 had both OBD1 and OBD2 if my memory is correct. Which is yours?

Sounds like a shift solenoid issue.

As in one is stuck

Try manually selecting (with gmtd scan) the 3-4 solenoid several times as well as the 2-3 and the 1-2