How Tow\Haul Mode Works, Trans not shifting

[rmiller110]

Ok, so after towing my trailer with tow\haul on the truck will not shift into 3rd or 4th gear. Putting it back into tow\haul does not change the situation. It's an '05 with 48k on the clock.

Is tow\haul servo driven? Computer driven?

Transmission shop is going to rebuild the trans for $1900, but I want to make sure they're fixing the actual problem. They say the clutches are bad and therefore not shifting to 3rd or 4th.

 

[D'Hag]

I'd say your transmission was on the verge of problems, and towing a trailer just finished the job. The ultimate cause is hard to say.

Tow/Haul mode is computer controlled, and does nothing more than alter the shift points and shift firmness. Sort of like installing an old-fashioned towing shift kit, but with the push of a button. When I run mine in Tow/Haul mode, pulling a 2000-lb boat/trailer combo, here is what I notice--

• Disengages torque converter lockup at lower throttle positions.
• Shifts down to 3rd gear sooner when pulling a hill.
• Less likely to shift all the way down to 2nd gear when pulling a hill, mainly because 3rd gear was engaged sooner.
• Firmer shifts.
• Stays in lower gears longer, or all shift points are higher in the RPM band.
• Wide-open-throttle shift points are higher in the RPM band.

 

[disem]

It turns out that 3-4 gear clutch pack is a weak point on these trannys. I had the exact problem with my Tahoe. 3-4 gears - gone. Replace the cluches with heavy duty. That will include more discs. Also change the sungear shell. That's a weak spot too. The improved sungear shell is called "the beast". It's 3 times stronger than the original.

 

[oomindwarpoo]

You'd think chevy would catch on by now even in the new Tahoes/burbans.. 4 speed tranny, others have 5 some with 6 speeds now. WTF?

 

[05Yukon]

You'd think chevy would catch on by now even in the new Tahoes/burbans.. 4 speed tranny, others have 5 some with 6 speeds now. WTF?

2009 comes with a stronger 6 speed tranny.

 

[D'Hag]

You'd think chevy would catch on by now even in the new Tahoes/burbans.. 4 speed tranny, others have 5 some with 6 speeds now.

There is a problem with 5 and 6 speed automatics-- More gears = more parts. Not just more gear sets. More servos and valves and solenoids and a whole bunch more stuff to go wrong. Not to mention vastly increased production costs that get passed on to the buyer.

If the engine has a strong, wide torque band (like mine seems to have), then 5 or 6 gears aren't necessary. And just because a transmission has more gears does NOT mean it still won't break in those upper gears.

Where GM (and others) need to "catch on" is that the upper gears and clutch packs need to be beefier from the factory. It's an old, simple, well-known principle that the manufacturers certainly know-- Transmissions break when running in the upper gears. Axles break when running in the lower gears.

But then, they're also concerned with their bottom line. An extra hundred bucks in a few thousand transmissions adds up fast!

 

[Polecat]

I just had mine rebuilt, 03 with 77,000 miles...get the Corvette servo...mine shifts good and firm now.


another hauling question....towing a car, do you have to tow in 'two/haul' mode, or what happens if you use regualr OD?