Tow Haul

[john857]

I have a question about towing? I have the HD 2500 automatic with the 6 L. I always tow in overdrive with the tow haul on. I never tow anything very heavy about 5000 lbs. I see on some posts that some only tow in 3 gear and not in overdrive. Just wondering why?

[496 CUBIC INCHES]

With the light duty trannys (4L60E) its common to tow in 3rd instead of O/D,because the tranny may "hunt" for the right gear when in O/D. When the tranny goes back and forth between gears,the tranny will heat up and if it gets too hot it will ruin the fluid as well as the tranny. Heat is an automatic trannys worst nightmare.

[496 CUBIC INCHES]

I have the Allison,so I put it in "D" and go. I don't use the T/H mode unless I'm going down a long steep hill,and then I use it for the grade braking feature. I don't use the T/H mode because I don't like the way it shifts on anything other than downhills.

[johnnyo4]

I use the Tow-Haul mode all the time when towing a 10,000 lb 5th wheel. It works flawlessly. The Allison people did a wonderful job with this transmission.

 

A real big plus in the tow-haul mode is that the transmission is locked in all gears. This generates much less heat.

 

This GM Technical Release should answer many questions:

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The Allison LCT 1000 automatic transmission is offered in 3/4- and 1-ton Silverado and Sierra trucks with the 8.1L gasoline and 6.6 Duramax diesel engines (fig. 1). This robust transmission offers five forward speeds and one reverse. The forward and reverse gear ratios are accomplished by five clutches.

 

TIP: The Allison is unique in having no bands, no sprags, and no roller clutches.

 

Transmission performance is controlled by six solenoids operated by the Transmission Control Module (TCM), which is linked by a Class 2 data line to the Powertrain Control Module (PCM).

 

Operating Characteristics

As these unique features suggest, the Allison transmission has some unique operating characteristics. A customer who is familiar with only a lighter-duty drivetrain may interpret perfectly normal operation as a cause for concern.

 

You can avoid unnecessary diagnosis if you can understand the reasons behind these characteristics. Here are some highlights.

 

Grade Braking – The primary purpose of the grade-braking feature 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.

 

TIP: The engine is designed to operate in the dashed red zone (fig. 2) on the tachometer but not in the solid red zone. The engine can operate in the dashed red range but cannot produce any power in this range. Do not operate the engine in the solid red zone.

 

When driving downhill on grades with heavy loads, the vehicle’s inertia can be great enough to increase vehicle speed and consequently engine speed without applying the throttle. To increase the availability of grade braking and provide for additional engine braking, the engine has been designed to allow operation in the dashed red zone region. Operation in this region is normal and will not damage the engine.

 

Refer to Service Bulletin 02-07-30-004B.

 

Tow/Haul and Overdrive Disable – If the vehicle has both Tow/Haul and Overdrive disable features (standard on 2003-04), tap the button once to enable or disable Tow/Haul; press and hold the button to enable or disable overdrive.

 

TIP: This feature may be added to 2001-02 vehicles by installing a switch and wiring, and reprogramming the transmission control module (TCM).

 

Refer to Service Bulletin 02-07-30-051A.

 

Tow/Haul Mode – Tow/Haul (T/H) can be used at any time, even when not towing. Some drivers may not like the T/H shift characteristics when the vehicle is lightly loaded, so normal mode would be selected.

 

When the vehicle is not heavily loaded, some drivers switch from Normal to T/H mode during closed throttle downshifts from highway speeds to help slow the vehicle and then switch back to Normal mode after they’ve stopped.

 

If T/H is not used when towing or when heavily loaded, the driver can expect higher transmission sump temperatures, more wear and tear on the brake system, and increased shift cycling.

 

T/H shift mode significantly changes the transmission’s shift pattern to reduce cycling and to deliver better performance, control, and cooling when towing/hauling heavy loads.

 

For instance:

- Upshift points are raised at light to mid throttle position to use more of the engine’s available power for accelerating. Downshift points are raised to enhance engine braking to help slow the vehicle.

 

- During deceleration, the torque converter clutch (TCC) remains applied at closed throttle to much lower vehicle speeds to significantly improve the effect of engine braking.

 

- 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. The combination of applying the TCC in 2nd range and the raised upshifts and closed throttle downshifts results in more positive shift feel when operating in this pattern. This is especially true when operating the vehicle while lightly loaded. In Normal mode, TCC generally applies only in higher ranges and is dependent on throttle position.

 

Refer to SI Document 1480544.

 

Overdrive Disable – The selector button is located on the end of the column shift lever. The Overdrive Disable is turned on and off by pressing and holding the button at the end of the column shift lever until the IP indicator light illuminates. The Overdrive Disable feature prevents the transmission from shifting into 5th gear. Overdrive disable can be activated in either Normal or Tow/Haul mode.

 

TIP: Be careful not to inadvertently select the Overdrive Disable function when only the Tow/Haul function is intended. If so, the driver could sense that grade braking is not working due to the fact that when in normal mode, grade braking will not command downshifts below 4th range.

 

Park Pawl – The Allison transmission is equipped with a park pawl designed to hold 26,000 lbs GCVW. The transmission incorporates much larger parts than other automatic transmissions, and the lever effort to shift the vehicle out of park may be highter than lighter duty vehicles. An occasional clunk may be heard when the selector lever is moved from Drive or Reverse to Park. This is normal and there is no damage occurring during these shifts.

 

Clutch Operation – The Allison is a clutch-to-clutch transmission. When shifting from one range to another, the transmission drops one clutch and brings another clutch on to complete the transition to the new range. In order to accomplish this smoothly, the handoff from the off-going clutch to the on-coming clutch has to be timed precisely.

 

If the on-coming clutch gets capacity before the off-going clutch is released, the shift has a tie-up feel. Under this scenario, three clutches momentarily have capacity at the same time. When this occurs, the operator of the vehicle may experience a head bob (a sensation where the vehicle momentarily slows down). As soon as a tie-up shift is sensed by the transmission, the off-going clutch is immediately dropped to minimize the shift feel.

 

If the off-going clutch drops capacity before the on-coming clutch gains capacity, the shift will flare. Under this scenario, only one clutch momentarily has capacity, resulting in the transmission being in a neutral gear state. This will result in an increase in engine speed during the shift and a shift bump caused by a loss of acceleration. As soon as a flare shift is detected, the transmission will utilize torque management to control engine speed until the on-coming clutch gains capacity.

 

TIP: Both tie-up and flare shifts can be experienced during the adaptive process (See Adaptive Shift Controls below). This is normal as the transmission controls learn the environment under which shifts are being performed. Shift quality will improve as more shifts of each type are made.

 

Cold Operation – When the transmission is cold, transmission fluid becomes more viscous and flows more slowly. This change in transmission fluid viscosity results in the on-coming clutch filling more slowly and the off-going clutch exhausting more slowly. Due to different flow paths for the oil in the transmission, this difference in timing is not the same for both the on-coming and off-going clutches. To compensate for these differences, the electronic controls will modify on-coming clutch fill times and off-going clutch exhaust times as sump temperatures change, using a process known as characterization.

 

Characterization uses the individual transmission adaptive shift parameters and adds a differential factor for cold transmission fluid. Additionally, the adaptive process is turned off during cold temperature operation, so shift quality is optimized during warmed up operation where the majority of drive time will be spent.

 

When temperatures are very cold, the transmission will inhibit certain ranges. The chart below shows range availability as a function of transmission sump temperature.

 

Sump Temperature Range(s) available

Above 5° F (-15° C) All ranges

-13° to 5° F (-25 to -15° C) 1st thru 4th ranges only

-31° to -13° F (-35 to -25° C) 2nd and 3rd ranges only

-31° F (-35° C) or lower 2nd range only

 

Torque converter clutch operation will be inhibited when transmission sump temperature is below certain levels. The gas-powered trucks will inhibit torque converter clutch operation below 68° F (20° C) transmission sump temperature, and the diesel-powered trucks will inhibit torque converter clutch operation below 86° F (30° C) transmission sump temperature.

 

Shifts may be more noticeable with a cold transmission. This slight difference in shift quality is normal.

 

Heater Performance Mode – On diesel-powered trucks, a feature to shorten engine/cab warm-up times is provided. This feature raises part throttle upshift points (2-3, 3-4, 4-5) to increase engine speed when cold weather conditions are detected. Under these conditions it is normal when the upshifts seem to hang or be delayed.

 

Adaptive Shift Controls – The Allison transmission is equipped with adaptive shift controls, which continually compares key shift parameters to pre-programmed ideal shift conditions. The transmission makes adjustments in order to approach the ideal shift for vehicle speed, load, and road conditions. During this process, some shifts may feel different as the transmission determines optimum settings for a particular shift.

 

Shift Stabilization – The shift stabilization feature minimizes shift busyness, or hunting between ranges. The transmission is designed to determine before making an upshift whether the engine will be able to maintain vehicle speed in the next higher range. If it senses that it will not, it will prevent the upshift from occurring.

 

4 Wheel Drive Operation – When operating in 4LO, there is a very deep gear ratio reduction and the resulting shift feel will be exaggerated. Sometimes a grinding noise may be noticed when going to Park from Drive in 4LO. The deep gear ratio causes the internal parts to be still rotating for a short period of time after the vehicle comes to a stop. The operator should wait 1 to 2 seconds after coming to a stop before engaging Park.

[Shaners]

When towing less than 5K lbs, I'll tow in OD with the tow/haul engaged. If the tranny hunts in and out of OD alot in hill terrain, I'll pull it down to "3". This is with my 6.0L, 4.10's and 4L80E tranny.