6-5 hard downshift

[LB72004]

so i have searched and searched and the only thing i can find is info on the LML hard 6 to 5 down shift and a GMs update to fix it.

from all of the descriptions of the problem it sure seems like i am suffering from the same issue except it is on my LMM. the update appears to only apply to the LML.

AND... my issue only started shortly after getting stuck in some mud and rocking the truck back and forth to get it out. i shifted between 1st and R until i got enough grip and momentum to get out. then a few days later i noticed the 6th to 5th down shift starting to bang.

i have tried resetting the TAPs and it smooths out the shift and it seems fine but then a few days later it starts to bang into gear. a little at first then more harshly as time goes on.

From reading into the issue and what the LML update is supposed to do to fix, it is lowering the on coming clutch pressure on the 6-5 down shift. so tried playing around with the {D5188 to D5191} cells, dropping them a little at a time and resetting the TAPs. it seems to help but in the end it always seems to come back to the hard down shift.

as a last resort / test i tried disabling the 6th to 5th Kickdown Adaptive Learn Enable {D5056} and resetting the TAPs again. i thought that since it always seems to shift nice and smooth after resetting the TAPs and it seems to adapt itself to the hard shift then preventing it from adapting may keep it nice smooth. i know this is not the best thing to do but i was getting desperate and tired of whiplash.

anyways, it still adapted back to the hard downshift. so now i am looking for answers on the forums.

do any of you think i may have burned a clutch pack or two when rocking the truck back and forth using 1st and reverse? maybe the C3 as it seems to be used in R and 5th?

i didn't limp the transmission or at least it didn't throw any CELs or codes at me. i also recently towed 14K-ish lbs from Sacramento to Reno on the I80 (up hill most of the way) and stayed in 5th most of the time and it seems to not skip a beat.


what do you guys think. getting close to needing my built transmission now or is there other programming changes i should try?

thanks, Jason

[LB72004]

Opps, I just realized the {D5188 to D5191} cells are for full throttle only. that would explain why it would keep adapting back to the harsh down shift. looks like we don't have this table available. :doh2:

Hmm, what now???:nixweiss:

[HamptonMtn]

My 2007.5 LMM GMC does the same thing. I've been researching this for a couple of yrs now and haven't learned anything. About 1 yr ago a transmission guy suggested I install a bullydog, so I did. It didn't fix the problem, however, it did give me some more info on the bullydog display. I've noticed that when engine is under a bit of load (such as a hill) and low rpm's, the tranny is shifting from D6 TC locked, down to D5 TC locked. I always thought I had D1 to D6 inclusive and didn't know anything about the TC locked thing (which acts as another gear). It seems to not shift to D6 from D6 TC locked so I'm wondering if this could be the issue. Perhaps a solenoid issue? I can overcome the hard shift by stepping on the gas a little more when on the incline, but it will still shift from D6 TC locked to D5 TC locked without the loud bang. I can also put it in manual. Any suggestions anyone?

I should also add that I had the transmission fluid changed and both the internal & exterior filters changed but the hard 6 to 5 hard shift continued.

[LB72004]

the way i go around it was to change the shift speed. or the MPH it down shifts at so that it is operating at a different "adapt" number then before. i had to try a lot of different numbers and wait for it to finish adapting then see how it ran. slowly it got better and i left it as it worked fine most of the time.

then i put 4.56 gears in it and that threw the shift MPH way out so i readjusted it again and it didn't return. now i have a build trans from Mike L and it has no such issue.

[HamptonMtn]

I think that if my truck would shift from 6 TC Locked to 6 to 5 TC Locked a little sooner at a bit higher rpm's, the tranny wouldn't hard shift. How do you get the thing to downshift at a higher rpm though?

[LB72004]

in your trans tune, the table is D5009