NEED HELP 2002 Chevy Suburban Transmission Slipping in 3rd Gear

[BlueDazing]

My 2002 Chevy Suburban started slipping in 3rd gear. I put in a rebuild kit which included changing out the 3/4 piston, the 3/4 clutch pack, the EPC solenoid, and the 3/2 solenoid. I put it back in and it's still slipping in 3rd gear. What could it be?

 

[mrrsm]

In the same "Cut Of The Cloth" ...the Vehicle losing 3rd Gear is THIS Thread provides more relevant assistance and useful information during this discussion:

SOLVED! - Sudden loss of 3rd/4th gear

2003 Suburban LT 5.3L 4x4 210xxx miles No check engine codes Transmission is a 3KCD No previous issues with transmission. Never noticed any slipping gears previously. Complete fluid change out done personally ~70k miles ago with GM fluid. Fluid still looks and smells good. Gauge cluster has...

gmtnation.com

 

[BlueDazing]

In the same "Cut Of The Cloth" ...the Vehicle losing 3rd Gear is THIS Thread provides more relevant assistance and useful information during this discussion:

SOLVED! - Sudden loss of 3rd/4th gear

2003 Suburban LT 5.3L 4x4 210xxx miles No check engine codes Transmission is a 3KCD No previous issues with transmission. Never noticed any slipping gears previously. Complete fluid change out done personally ~70k miles ago with GM fluid. Fluid still looks and smells good. Gauge cluster has...

gmtnation.com

I'm going to swap the transmission with the 4L60E from my Chevy Trailblazer. Is there any chance the issue could be caused by not the transmission, but the computer system in the truck?

 

[mrrsm]

Doubtful. Ordinarily... the workings of the *innards* of all Hydro-Matic Transmissions depend upon Four Things, monitored by ...and added to the PCM's Calculus of what the required Shifts Up and Down the Power Spectrum are necessary to maintain the Balance of Torque Input to Output Demands:

(1) Transmission Fluid (Quantity AND Pressure via the Fluid Pump driven by the Drive Train).
(2) Temperature. High Temps THIN Down the Fluid and weaken its properties-lubricity.
(3) Timing...or "The WHEN" that certain Solenoids activate to Direct & Re-Direct Fluid Flow.
(4) Inner Component Quality and Condition. Worn Friction Disks and Failing Solenoids, etc.

THIS is "The ATSG Trouble Code Diagnostic Bible" that is the *Bees-Knees* of information anyone working with GM 4L60E to 4L80E Transmissions will benefit from having and referring to from the shelves of their Mechanic's Library:

 

[59840Surfer]

Failure of 3rd/Direct gear is typically a primary in this transmission for numerous reasons.

It's just not usually something simple like a solenoid or old fluid. It's along the lines of a blown seal from a non-vulcanized piston, a cracked/cocked piston or it can even be ratcheted steels .... all are a tear-down and rebuild.

Swapping with another transmission is kinda risky as they both have to be of the same calibration --- and that's not usually the way things go.