Tow Haul Mode-downshifting Into Red Line Area Of Tach

lb7dmax · March 27, 2008

I searched this subject and came up empty......

I drag a 10,000 lb 5th wheel with a 04 LB7-generally in tow haul (TH) unless conditions are real good, then I'll turn TH off.

Truck climbs real good, generally downshifts on real steep grades, I don't push it and it goes right up at 50 mph.

However on the down side, in TH, on a steep decline Th will shift down once and hold the speed fine until

I get to a hairpin corner where I have to slow to 20 to 30 mph......then Th shifts down again and I'm into redline territory..3500 to 4000 rpm's...and when I'm out of the hairpin Th will not shift up(to reduce the RPM's).

My tach has a dotted redline from about 3300 to 4800 rpms and then solid red to 5000

rpms.

I've read elsewhere that running into the dotted red line in these circumstances is okay...is that really the case? I read the owners manual today-nothing mentioned that it is ok.

And once I'm over-revving(after the hairpin) how do I get the transmission to shift up?

I love the truck and plan to keep it for life so any help with this question is appreciated.

SKYrado03 · March 27, 2008

I am getting the impression that you are engaging/dis-engaging towhaul-mode while your vehicle is in motion. Yes/no?

I just finished reading my 03 manual on tow-mode, and no-where does it say that it can or can not be done while the vehicle is in motion, but I've only done it while stopped, and haven't had the problems you describe.

mljenkins · March 27, 2008

I just finished reading my 03 manual on tow-mode, and no-where does it say that it can or can not be done while the vehicle is in motion, but I've only done it while stopped, and haven't had the problems you describe.

On my '06 1500 I have enabled/disabled in motion with no adverse affects. But I have only done that on the highway.

On that 3/4 ton of the original post, does it have an Allison? I have a friend who has the Allison and it has a feature to downshift on downgrades, but that's about all I know.

I'm interesting what others say about the RPMs. I have purposely downshifted while going down hill with the trailer and the RPMs get high. The oil pressure goes way up when that happens. I also wondered what the effects on the engine and tranny were under these conditions.

lb7dmax · March 27, 2008

Yes the truck has an Allison 5 speed. TH can be engaged while moving or stationary and I've done both.

When it's in its high rpm condition the trans. will not shift up so I end up disengagin TH. I'm tryin to remember but I think in this condition it takes a while for the TH function to disengage which is disconcerting.

I don't like running in the red zone or not being able to shift out of it but if this is the way the TH system is supposed to work, I'll have to get used to the over revving.

Starmachinist · March 28, 2008

Yes the truck has an Allison 5 speed. TH can be engaged while moving or stationary and I've done both.

When it's in its high rpm condition the trans. will not shift up so I end up disengagin TH. I'm tryin to remember but I think in this condition it takes a while for the TH function to disengage which is disconcerting.

I don't like running in the red zone or not being able to shift out of it but if this is the way the TH system is supposed to work, I'll have to get used to the over revving.

When I drive my dad's '04 Duramax w/ the Allison, I get the same thing where it'll rev right up into the redline but instead of disengaging the tow/haul, I just step on the gas peddle a for a second and then the tranny upshifts again and continues on normally. Hope this helps,

Andy

cammiller · March 28, 2008

Here is a thought. When it gets that high, use the brake a little. THe grade braking is only to assist the overall braking effort. If it still climbs, then it means your going too fast. These systems are not like the engine brakes on a highway tractor.

lb7dmax · March 30, 2008

The link below fully explains the TH high rpm condition, which appears to be normal up to 4000 rpms for the LB7 and LLY engines, though I can't find anything by GM which supports or confirms the Allison theory.......upshift and reduction of high rpms apparently will occur if u run the engine up to 4800 rpms. that's too high for my liking so I guess I'll continue to fumble around and try to disengage TH so I can get a higher gear.

http://www.allisontransmission.com/service...p?CategoryID=11