2017 GMC 6.0 Vortec - Manual Shifting?

Always keep in mind that when an automatic trans up shifts or down shifts, thats is when the box makes the most heat.

It makes heat through hydraulic actuation and it makes heat as the clutch packs engage under heavy load and gear ratio selections are programmed to feel smooth. It's the smooth feel that makes the heat (and wear) because the clutch pack hydraulic pressure is modulated for smoothness. That slipping makes the heat.

While I don't have a GM, I manually downshift out of OD into direct prior to ascending a hill when the motor isn't producing a gob of power (Torque) and the clutch packs can engage with less residual heat. I run a shift kit as well and my shift kit increases the hydraulic pressure and how fast it rises, thereby clamping the packs quicker.... less heat

SidecarFlip wrote:
Always keep in mind that when an automatic trans up shifts or down shifts, thats is when the box makes the most heat.

It makes heat through hydraulic actuation and it makes heat as the clutch packs engage under heavy load and gear ratio selections are programmed to feel smooth. It's the smooth feel that makes the heat (and wear) because the clutch pack hydraulic pressure is modulated for smoothness. That slipping makes the heat.

While I don't have a GM, I manually downshift out of OD into direct prior to ascending a hill when the motor isn't producing a gob of power (Torque) and the clutch packs can engage with less residual heat. I run a shift kit as well and my shift kit increases the hydraulic pressure and how fast it rises, thereby clamping the packs quicker.... less heat

I might be wrong but you don't ever want your clutch packs to slip. You want your torque converter to do the slipping. That's what it's for.

Ok, I'm not sure what you mean by "before it should." Maybe you misspoke?

If indeed you meant before, lugging the engine is bad for efficiency. You will use far more gas letting the engine lug down before shifting, than letting the transmission shift and the engine rev up. The engine has a range of RPMs where power and efficiency are optimum, and modern automatic transmissions are designed to keep the engines in that range.

Lots of discussion lately in towing threads about downshifting and letting the engine rev, and many people claim they get BETTER fuel economy by doing that than forcing the engine to lug down.

OK..so when should the +/- manual shift switch for on the shifter be used?

My 2001 3500 8.1 gas would downshift when towing quicker than I wanted it to do but that did keep the speed up. With the 05 duramax It wants to stay in the gears longer than what I feel is ok so I hit the button to go out of overdrive and let the engine run free. I think your truck is doing the same.

chevman

I don't know what truck you have, but my 2011 GMC Sierra 2500HD with 6 speed auto has the M position and +/- on the gear shift. My understanding and the way my truck works is that is the top gear the trans will use. For example if I am on the freeway and have some gentle hills and my transmission is shifting to 6th and then to 5th and then back to 6th, I can go to M mode and set it to 5 and it won't shift to 6th then. My manual states that it is not to used to "manual shift" transmission. It is nice to keep it from "hunting" and it helps keep the trans cooler since shifting up and down constantly generates lots of heat.

You can use the toggle switch to change gears when it's in the M mode. I drop the gear manually on mine prior any large hill.

Thanks KW/00 and haweye-08.

Sounds like you guys actually own one and knew what I was referring to. ;)

transamz9 wrote:

SidecarFlip wrote:
Always keep in mind that when an automatic trans up shifts or down shifts, thats is when the box makes the most heat.

It makes heat through hydraulic actuation and it makes heat as the clutch packs engage under heavy load and gear ratio selections are programmed to feel smooth. It's the smooth feel that makes the heat (and wear) because the clutch pack hydraulic pressure is modulated for smoothness. That slipping makes the heat.

While I don't have a GM, I manually downshift out of OD into direct prior to ascending a hill when the motor isn't producing a gob of power (Torque) and the clutch packs can engage with less residual heat. I run a shift kit as well and my shift kit increases the hydraulic pressure and how fast it rises, thereby clamping the packs quicker.... less heat

I might be wrong but you don't ever want your clutch packs to slip. You want your torque converter to do the slipping. That's what it's for.

Your clutch packs always slip to some degree as the internal ratio's change. They aren't an on-off switch. If they were, you'd rip your universals out or twist the drive shaft. The more they slip, the more heat produced and conversely, the smoother the shift is. How an automatic transmission works.

Manual mode for GM is more like top gear range limit. You cannot force the trans to hold a taller gear than computer wants. If you don't want it to downshift, let off the accel pedal.