Did I Just Get Rear Ended?

[2010LTZ4x4]

I'm at a light and accelerate to move foward and then need to stop quickly and I could of swore I just got hit from behind,

the whole truck seem to move a head and I had my foot full on the brake and of course there was no car behind me.

Has anyone else had this happen, is there something going on in the drive train??

[canadiantowman]

I'm at a light and accelerate to move foward and then need to stop quickly and I could of swore I just got hit from behind,

the whole truck seem to move a head and I had my foot full on the brake and of course there was no car behind me.

Has anyone else had this happen, is there something going on in the drive train??

 

nope it's just the normal, bugs the hell out of me.

[Major_Kong]

Do a search on greasing the slip yoke.

[Pender1]

yea, mine does it every once in a while. almost feels like it's having a hard time downshifting fast enough.

[SouthCo]

Haven't had that happened but don't think I've ever accelerated and then had to get back on the brakes quickly.

[07YukonDenali]

This is "normal" according to GM.

[tanked_darren]

Slip yoke, it isnt hurting anything, you could grease it, or have the dealer do it, but I don't even bother.

[kdixer]

It is just a "gremlin" in the 6spd transmission. That is the problem with cars getting "smarter". There are so many calculations being performed for the TCM to command shifts that sometimes the electronics cannot calculate them fast enough for certain situations. When you accelerated from the light the control modules were going thru the calculations for the upshift sequences. But then the sudden stop required a different shifting sequence than what was being processed. I think it has something to do with the transmission releasing and applying it's various clutches as it downshifts thru the gears. It is almost as if the truck is trying to propel itself forward even though you are pushing the brake.

[Sir Dan]

It is just a "gremlin" in the 6spd transmission. That is the problem with cars getting "smarter". There are so many calculations being performed for the TCM to command shifts that sometimes the electronics cannot calculate them fast enough for certain situations. When you accelerated from the light the control modules were going thru the calculations for the upshift sequences. But then the sudden stop required a different shifting sequence than what was being processed. I think it has something to do with the transmission releasing and applying it's various clutches as it downshifts thru the gears. It is almost as if the truck is trying to propel itself forward even though you are pushing the brake.

It's not just in the 6 speed tranny. The good ole' 4L60E does the "bump stop" thing too. Greasing the slip yolk helps for awhile, but eventually it's back to the bump and grind.

[ChevyTech77]

I'll be willing to bet that this is not a bump stop slip yoke issue as I have the same feeling in my '11 and know what the slip yoke issues fell like as I've repaired literally thousands of trucks with that issue sense about '96ish not to mention that I'm pretty sure of what the OP is talking about as it is a weird feeling that I myself have only felt in the newer trucks with TM type ecm/tcm calibrations.

 

This is the TAC motor hanging the throttle open just a bit causing the idle to hang. Add on the fact of the tight 6l80 clutch pack apply status and this is what you feel. Simiply just a calibration issue in the ecm and tcm that I wish that I knew why GM did it so that it could be taken back out. Maybe over time it will be we'll have to wait and see.

[kdixer]

I'll be willing to bet that this is not a bump stop slip yoke issue as I have the same feeling in my '11 and know what the slip yoke issues fell like as I've repaired literally thousands of trucks with that issue sense about '96ish not to mention that I'm pretty sure of what the OP is talking about as it is a weird feeling that I myself have only felt in the newer trucks with TM type ecm/tcm calibrations.

 

This is the TAC motor hanging the throttle open just a bit causing the idle to hang. Add on the fact of the tight 6l80 clutch pack apply status and this is what you feel. Simiply just a calibration issue in the ecm and tcm that I wish that I knew why GM did it so that it could be taken back out. Maybe over time it will be we'll have to wait and see.

 

So would you happen to know what parameters I could possibly get Black Bear to tweak in my custom tune to eleminate this? My truck has done this since new. Like the OP stated it is mainly during sudden or aggressive stops.

[RyanbabZ71]

I'll be willing to bet that this is not a bump stop slip yoke issue as I have the same feeling in my '11 and know what the slip yoke issues fell like as I've repaired literally thousands of trucks with that issue sense about '96ish not to mention that I'm pretty sure of what the OP is talking about as it is a weird feeling that I myself have only felt in the newer trucks with TM type ecm/tcm calibrations.

 

This is the TAC motor hanging the throttle open just a bit causing the idle to hang. Add on the fact of the tight 6l80 clutch pack apply status and this is what you feel. Simiply just a calibration issue in the ecm and tcm that I wish that I knew why GM did it so that it could be taken back out. Maybe over time it will be we'll have to wait and see.

 

So would you happen to know what parameters I could possibly get Black Bear to tweak in my custom tune to eleminate this? My truck has done this since new. Like the OP stated it is mainly during sudden or aggressive stops.

 

 

Justin has been unable to tweak mine to get it to stop this. Doesnt happen all the time occasionally here and there. I did take a log of a ride and send it to him and nothing really stood out.

[ChevyTech77]

Just spitballing here but I'm not sure if doing something to quickly pull down the idle a bit or downshift the transmission quicker would be possible. If people are saying that Justin can't do it at BB, that's probably why GM hasn't done anything with it either. You'd have to tell the ecm and tcm that the vehicle is braking just that one certain way, everytime it happens, so that they could make just the necessary adjustments. Sounds simple enough but again, there's a reason that it hasn't already been done, I am sure.

 

Maybe I'm way off as well but that is what it feels like to me and the first time it happened to me I thought that I was going thru the vehicle ahead of me there for a split second. As annoying as it can be I now know how to not make it happen so I have so far just adapted my driving so that it don't.

[kdixer]

Just spitballing here but I'm not sure if doing something to quickly pull down the idle a bit or downshift the transmission quicker would be possible. If people are saying that Justin can't do it at BB, that's probably why GM hasn't done anything with it either. You'd have to tell the ecm and tcm that the vehicle is braking just that one certain way, everytime it happens, so that they could make just the necessary adjustments. Sounds simple enough but again, there's a reason that it hasn't already been done, I am sure.

 

Maybe I'm way off as well but that is what it feels like to me and the first time it happened to me I thought that I was going thru the vehicle ahead of me there for a split second. As annoying as it can be I now know how to not make it happen so I have so far just adapted my driving so that it don't.

 

Justin has sped my shift times up and it still does it. It acts to me like maybe the TC is locked up for a split second. I always assumed it was the calculations the TCM goes thru to "hand off" the gears with it's clutch to clutch design. As one clutch unlocks for a gear, the next gears clutch locks. These pressure rates are different based on throttle input and speed, and when you suddenly go from accelerating to stopping, it sort of confuses the seguence.

 

I have also had that feeling from time to time cruising around 25-35mph. You can feel it lug the engine similar to lugging a vehicle equiped with a manual transmission under 1k - 1.5k rpm's. Then it will unlock the converter and downshift.

[2010LTZ4x4]

I guess theres no reason to grease the slip yoke after reading the explanations as to why this

"bump stop" happens.

I think I can live with it, and if this is the only thing that has happened to the truck after 13 months of owning it, I'll take it.

Like "Chevytech77" stated i will just adjust my driving so that it doesn't happen to often.