I HAVE AN 2OO2 BUILT 2/19/02, IT SEEMS THAT WHEN IM IN TOW HAUL MODE THE TRANNY IS IN A LOWER GEAR THAN REG MODE, WHEN IM DRIVING AND PUSH TOWHAUL OFF THE TRANNY SHIFTS, I THOUGHT YOU HAD 5TH GEAR WITH TOWHAUL ??????? IF IM IN NORMAL MODE AND PUSH TOWHAUL IT WILL DOWN SHIFT.PLEASE TELL WHAT IM MISSING?

Shift characteristics do change when in Tow Haul,definately more aggresive as there is less slip .... What you are feeling is the torque converter locking not a down shift I believe when you engage tow haul button...


Mac

Mac is right. I remember when I first got my truck and the converter locked up, I though the Alli had six gears. It appears the Alli in T/H mode will lock the converter around 20mph (2nd gear). When in regular (Not T/H) the converter locks at approx. 50 mph. Edited by: CMDMAX

He said "downshift" which would be going from a higher gear to a lower gear. If that's the case he may have been far enough below T/H fifth gear lockup speed for it to downshift to fourth.

TC

To get 5th gear w/ T/H mode u need to get over 50mph, close to 60mph, to get it to shift.


I don't normally use it when towing because of this.

By John kennedy posted 10-01-2002 10:03 PM

Everything you wanted to know about grade braking and more:

The grade-braking algorithm's primary purpose is to utilize engine braking to slow a heavy vehicle
on steep grades in order to reduce wear on the traditional braking system. The method used to slow
the vehicle is by overriding the PRNDL position, effectively pre-selecting the next lower gear range
automatically. Because the transmission is electronically controlled and there is no mechanical
linkage that needs to be moved for a pre-select downshift, implementation of this algorithm can be
done completely in software with no hardware modifications.

This algorithm takes into consideration several factors before commanding a pre-select downshift.
These are the primary inputs:

Throttle position
Brake state (The brake must be applied to activate grade braking)
Vehicle acceleration/deceleration
Grade/Load
Vehicle speed
The determination of the PRNDL position is made by a logic subsystem. Several factors are
calculated and weighted and then a sum is calculated. When this calculated sum surpasses a
threshold, a PRNDL override is commanded.


Important
The shift speeds for grade-braking are controlled/determined as a result of "Calculated Sum" logic.
However, the grade-braking shift point will never be greater than the Pre-Select Shift Point.

The following charts provide the maximum Pre-Select (manual) shift points:

8.1L Gas Engine (RPO L18) with both 4.10 and 3.73 axle ratio Shift
Transmission OSS RPM

5-4
4624

4-3
3266

3-2
2543

2-1
1474

For the L18 gas engine, maximum engine speed immediately following a preselect or grade braking
downshift can be as high as 4,650 RPM. After a grade braking downshift, if vehicle speed continues
to increase, an upshift will occur at 5,000 RPM engine speed.

6.6L Diesel Engine (RPO LB7) with 3.73 axle ratio Shift
Transmission OSS RPM

5-4
4017

4-3
2832

3-2
2196

2-1
1214

For the LB7 diesel engine, maximum engine speed immediately following a preselect or grade
braking downshift can be as high as 4,000 RPM. After a grade braking downshift, if vehicle speed
continues to increase, an upshift will occur at 4,800 RPM engine speed.


Important:

Grade Braking is not intended to reduce the need for great care by the driver when driving a heavily
loaded vehicle down a grade. Drivers should continue to take all normal and appropriate actions to
keep the vehicle under control at all times.


1)Does grade-braking feature work on level roadways while trailing or hauling heavy loads and
applying brakes? The feature is actively calculating whether or not to command a downshift while on
level roadways, but it is much less likely that it will command a downshift under these conditions.
The reason is that the Transmission Control Module (TCM) is using vehicle acceleration/deceleration
as a determining factor. Since the vehicle is more likely to respond to vehicle brakes on level
surfaces vs on a grade, grade braking is less likely to command a downshift.

2)Does grade-braking work in Tow/Haul and normal mode? Yes, grade-braking is active in either
mode.


3)Can I turn grade-braking off? No, automatic grade-braking is always an active part of the
transmission control system.
Does Tow/Haul have to be selected? No. In normal mode, the grade-braking feature will pre-select
only the 5-4 downshift, whereas in Tow/Haul mod

<H1>Tow/Haul Mode</H1>


Tow/Haul mode significantly changes the transmission shift pattern to reduce shift cycling and to deliver better performance, control, and cooling when towing or hauling heavy loads. For instance:
<UL =BULLET>
<LI>Upshift points are raised at light to mid throttle position to use more of the available engine power for acceleration. Downshift points are raised to enhance engine braking to help slow the vehicle.
<LI>During deceleration, the torque converter clutch (TCC) remains applied at closed throttle at lower speeds to significantly improve the effect of engine braking.
<LI>During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th, and 5th. This improves the drivetrain efficiency and significantly lowers transmission sump temperature when towing heavy loads. In Normal mode, the TCC generally applies only in higher ranges and is dependent on throttle position.
<LI>Tow/haul is designed to be most effective when the vehicle and trailer combined weight is at least 75 percent of the gross combined weight rating (GCWR) of the vehicle.
<LI>Operation of tow/haul in a lightly loaded or non-loaded vehicle will not cause damage. However, there is no benefit to the selection of tow/haul when the vehicle is unloaded. This situation will cause a firm shift. The tow/haul switch is not a performance switch.
<LI>Selection of tow/haul when unloaded may result in unpleasant engine and transmission driving characteristics and reduced fuel economy. Tow/haul is recommended only when pulling a heavy trailer or a large or heavy load. </LI>[/list]<A name=ss1-1480544><A href="http://service.gm.com/servlets/BlobShtml?ShtmlFile=1480544&amp;psdid=271&amp;evc=sm#ss1-1480544" target="_blank">
<H5>Activation</A></H5>
<UL =BULLET>
<LI>Tow/Haul is selected or de-selected via a switch on the end of the transmission shift lever. A light on the instrument panel will illuminate to indicate that tow/haul has been selected.
<LI>Tow/Haul must be selected again, every time the vehicle is started, if desired. </LI>[/list]

Thanks for the information guys!http://www.dieselplace.com/forum/smileys/Clap.gif

Last night was the first time I tow my trailer (10,000 lbs) on the hwy. I used the tow/haul mode and I liked how it works. Trans downshifts when slowing down and makes stopping a pleasure. When I use to tow my trailer with my old truck I use to s**t myself everytime I had to stop. Always thought I would never stop in time LOL. So far this truck is a great tow truck. It feels like there is no trailer on the truck when pulling or stopping. I only feel a differents in the ride. It's also nice that the truck and trailer are the same with. It's just great !!!!!!

I have a buddy with a 2002 D/A chev pulling a 25 ft fver exactly the same as mine. His problem is his allison downshifts to 4th and stays there at the slightest sign of work while mine will pull in 5th gear all day with very few downshifts and upshifts easily as well. On a trip down to Montana last weekend his ran in 4th most of the time and mine ran in 5th. He was directly behind me for the whole trip, going and returning. Anyone know if there is a reflash for the 2002's to correct this problem, or what this difference could be? By the way we both outpulled our other buddy with his Dodge easily going against a 30mph wind on the trip back. That was after pulling him and his trailer out of the muddy campgound we were in. Gotta love it!http://www.dieselplace.com/forum/smileys/Cool.gif


on edit -I thought I'd mention our trucks both have about 17000 miles on them and he said it acted the same in and out of tow haul, any thoughts appreciated.Edited by: army

Army, have your friend get his truck in to check for latest TCM updates. Tell the service writer about the shifting situation as there is a TSB out on this. After I had my 02' updated,I liked the shifts much better, but had the 5th holding thing already taken care of with the Juice.

Thanks motovet- yea he's going to give that a try. We'll see how it works.

So tow/hual mode is just the trailer and not the overdrive lockout? I always use the tow/hual but, I use the lockout when going down grades, merging into traffic and stop and go traffic. This is all new to me and I learn for this site off of other people.

There are several things to think about when towing with our dmaxs. When you are pulling a load with a standard transmission, you normally take your engine to a higher rpm before shifting, to keep the engine in its so called power band. When you have an overdrive last gear, going up a grade you normally downshift when the rpm go down below say 1800 rpm. The tow-haul switch basically changes shift points up and down, keeping engine in optimum rpm for power, locks the torque converter in the mid gears to eliminate slippage and heat buildup. All this to make your tranny last longer. If you have increased hp and pull alot of weight use the towhaul switch. If you are pulling a grade use the od lockout, high hp and overdrive will shorten tranny life significantly

I have an 05 with the two way switch. If I put it in tow haul mode to lock the torque converter earlier will I still get overdrive to save on fuel? Or does T/H mode also eliminate overdrive? Is overdrive lockout mode like normal drive but without overdrive?

OD lock out is seperate from TH mode, you can lock out OD though in 03's and above by holding the TH button till the OD light comes on. TH will hurt fuel milage unloaded since it will run higher r's even when you arnt on it.

but will it still shift into OD in T/H mode even though it holds in gear longer?

yes

I just got my 2005 Chevy D/A last week, but I did read the owners manual carefully. Tow/haul is for the gas engined, 4 speed hydramatic transmissions. The owner's manual for my truck calls the switch on the gear lever of my truck an "overdrive lockout".

Bearsfolks,
I have an 04' with the tow/haul and OD lockout. I don't think that has changed on your 05'. To check this, just tap the button on the gear selector lever and it should go into T/H. To activate OD lockout you would have to hold down the button for a couple of seconds.

Dan

I tried your suggestion. Thanks. It is tow/haul and overdrive lockout, depending how long you hold in the button. I could have done this wrong for a long time without your help.