08 Avalanche 5.3 4x4, 175k on engine. P.O. had trans rebuild about 20k miles ago. Today on highway out of blue at 70 mph started revving in 4th. 1st &2nd fine, 3rd slips a lot but finally shifts in to 4th, 4th also slips a lot. 2500 rpm at 45 mph. I had my HPTuners so ran a scan.



I am well aware the 3-4 clutch pack is a frequent failure point. Seems to slip less in 4th than 3rd. Wondering where to start diagnosing. Will do a pressure test once I get home. Is there a way to test the 3-4 apply piston for leakage from the VB? Could the spool in the VB be stuck or leaking? Anything else to look at before dropping the trans?

 

Is there enough fluid in it? what does it look like?

 

Slightly overfull, clean and pink. Fluid was burnt when I got the truck so I changed it. Installed an external cooler. I watched the temps the whole time I was limping it home and did not let them get over 220. Put a pressure gauge on it and it runs 140 psi on the 1-2 shift but never gets over 80 in 3rd or 4th. Just logged this datalog with HPTuners but not sure what it means. Trans tune is stock so a little baffled why the PCS would be near 1 amp in 3rd approaching shift. ECM tuner is very close to stock also, only turned off AFM and adjusted PE ratio up to 12.6 from the stock 11.1 Nothing else touched. I don't think that LP desired value can be correct, I am no expert at trans tcm data.