Help: Weird trans/tcase issue. 4lo popping out with po872

[THEREDDEVIL]

Ok sorry if this gets long and boring. But I'm stumped and my builder is puzzled. Short history is I broke my driveshaft last year sled pulling, new shaft and replaced the rear bushing in the tcase. All was good, added a tcase brace at the same time. 2wd and 4hi I have no issues. Last Thursday I was at a sled pull and was gonna make 2 hooks. First hook went great, second hook I put tension on the sled and released the brake and the truck felt like it popped out of 4lo, then grabbed real hard and threw a po872 and po700. I freaked out and pulled off real slow. Tried to put truck in 2wd and it grinded real bad. Shut it down, cleared codes, engaged 2wd and drive home with no issues. Changed spin on and no debris. From what I hear a po872 means e shift solenoid is stuck, and the valve body needs pulled to free it up. I never touched it, just cleared code and been driving fine like normal. Is my tcase the culprit? How the heck can 2wd and 4hi work great? But not 4lo. I've made 7 passes at the strip in 4hi with no issues.

 

[Mike L.]

Ok sorry if this gets long and boring. But I'm stumped and my builder is puzzled. Short history is I broke my driveshaft last year sled pulling, new shaft and replaced the rear bushing in the tcase. All was good, added a tcase brace at the same time. 2wd and 4hi I have no issues. Last Thursday I was at a sled pull and was gonna make 2 hooks. First hook went great, second hook I put tension on the sled and released the brake and the truck felt like it popped out of 4lo, then grabbed real hard and threw a po872 and po700. I freaked out and pulled off real slow. Tried to put truck in 2wd and it grinded real bad. Shut it down, cleared codes, engaged 2wd and drive home with no issues. Changed spin on and no debris. From what I hear a po872 means e shift solenoid is stuck, and the valve body needs pulled to free it up. I never touched it, just cleared code and been driving fine like normal. Is my tcase the culprit? How the heck can 2wd and 4hi work great? But not 4lo. I've made 7 passes at the strip in 4hi with no issues.

Sounds like a transfer case problem although the 872 code is kinda weird and I haven't seen that associated with T/C yet.

 

[THEREDDEVIL]

Thanks for the response mike. I figured I would throw it at u and see what u thought. Is there an actuator or a solenoid that keeps the tcase in gear? I'm wondering if I jacked something up when I split the case to press the bushing in.

 

[Mike L.]

The forks in the TC have plastic inserts on them and they can wear out.

 

[Tony Burkhard]

And you may find the slide coupler in the planitary has wear and forcing it out of 4-lo. If you tear it apart you can call me to help walk you through it. Im guessing that when it popped out of 4-lo the tcm thought it sliped the clutches, it saw stator rpm's but no output rpm's.

 

[THEREDDEVIL]

And you may find the slide coupler in the planitary has wear and forcing it out of 4-lo. If you tear it apart you can call me to help walk you through it.

Thanks tony I appreciate it. I guess I will b pulling it out again no doubt. Lol

 

[THEREDDEVIL]

i wanna bump this up. i was mesing around toady testing to see exactly what my issue is before i pull the tcase. i was in 4lo and couldnt build boost without it popping out gear. i then tried to take off with about 1/2 throttle from a dead stop in 4lo and it acted fine. i then shifted into 2wd and then to 4hi, and back and forth to make sure i could hear the actuator changing the modes. so i went to take off in 2wd and the truck barely moves, same in 4hi. but now i can build boost in 4lo and it works fine. its like my issue went from being 4lo related to being 2wd/4hi related. i will be pulling the case tonight now that i finally have time.

 

[curt]

fork inserts

The forks in the TC have plastic inserts on them and they can wear out.

i had to change these fork bushings out as well(they look likewhite plastic) i was going down a hill in 2 hi let off the accelerator and heard a instant "zing sound" so put my foot back into it then used the brake to slow down with torque applied from accelerator pedal! talked with gm tech said if i caught it right away it mighta saved the planetary and pinion gears ($1300 bucks) i split case and gears were ok just needed new fork bushings! like $20 bucks!!!!!! so if u ever have this happen when u unload the torque(downhill,etc) step on it and brake same time to slowdown to re-engage gears under load! not sure about the 4 lo issue i would think it would be like the 2 hi the gears would pull itself in like a "undercut" gear!