Chevrolet Silverado / GMC Sierra. Service manual — part 998

The control valve assembly consists of 2 components. The main valve body contains the pressure
control valves, the TCC valve, the exhaust backfill valve, and the control main relief valve. The
shift valve body contains the shift valves, the control main pressure valve, and the manual selector
valve. The control valve assembly attaches to the bottom of the main case module and is enclosed
by the oil pan.

Remote Oil Cooler Provision

Ports for remote-mount oil cooler lines are located on the right side of the converter housing near
the converter housing/main housing splitline. Remote oil-to-water coolers require plumbing for
transmission fluid and engine-cooling water. Remote oil-to-air coolers may also be used and only
transmission fluid lines need to be provided. Heat is transferred from the transmission fluid to
either water or air depending upon the cooler type used.

Fill Tube/Dipstick Provision

All 1000 Series models have a fill tube/dipstick provision on both sides of the transmission. A
plug is installed in the unused location.

Park Pawl

All 1000 Series transmissions have a park pawl. The internal parking pawl is engaged by
selection of the PARK position on the shift selector.

Power Take-Off (PTO) Provision

The 1000 Series transmissions have a provision to mount and drive a PTO unit on the left and/or
right side of the transmission housing. The torque converter turbine drives the optional PTO drive
gear. The PTO reflects engine and torque converter characteristics.

TOW/HAUL MODE

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:

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.

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

During acceleration, the TCC is applied in 2nd range and remains applied in 3rd, 4th, 5th

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

and 6th. 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.

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.

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.

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.

Activation

Tow/Haul is selected or de-selected via a switch on the end of the transmission shift lever. A
lamp on the instrument panel will illuminate to indicate that tow/haul has been selected.

Tow/Haul must be selected again, every time the vehicle is started, if desired.

ADAPT FUNCTION

The transmission control module (TCM) produces excellent shift quality by applying closed loop
control that constantly adjusts shift characteristics for changes in operating conditions. These
adjustments are based on vehicle conditions, such as grade, load, and engine power.

The learning process of comparing and adjusting shift parameters is referred to as adaptive
control. Adaptive control establishes initial conditions for shifts and makes during shift
adjustments. The TCM constantly monitors operating conditions, such as battery voltage and
transmission sump temperature, and adjusts shift parameters accordingly. After a shift is
completed, the TCM compares the shift to a target shift profile in the TCM calibration and makes
adjustments before the next shift of the same kind is made.

The Allison 1000 Series transmission consists of 5 clutches. A combination of 2 clutches is
required to be engaged, in order to attain a torque path from the input to the output of the
transmission. The following table indicates the clutch combinations for each gear range.

Gear Range

1-2-3-4

Clutch

4-5-6

Clutch

3rd, 5th

and

Reverse

Clutch

2-6 Clutch

Low and

Reverse

Clutch

TCC

PARK/NEUTRAL

-

-

-

-

X

-

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

The Allison 1000 Series transmission utilizes clutch-to-clutch shift control to achieve range
changes. In every case, except shifts to or from NEUTRAL, 1 clutch is exhausted and another
applied in order to make a range shift. The hand-off between exhausting and applying clutches is
precisely controlled by the use of 2 pressure control solenoids (PCS). These solenoids are labeled
PCS1 and PCS2 in the transmission. For example, to make a 1-2 shift, PCS1 is used to trim
pressure off the low and reverse clutch, and PCS2 is used to trim pressure on the 2-6 clutch. The
TCM modulates the current to both PCS1 and PCS2, which translates to a proportional level of
pressure to the clutch. In order to make a shift, the TCM uses software and calibration settings of
several program parameters in order to determine the level of current sent to the respective PCS.
These parameters are referred to as adaptive values. With a new transmission and TCM
calibration, the adaptive values are set to base calibration level. The transmission uses the base
calibration to perform the first of each type of shift. However, once the transmission has
performed a shift, the TCM evaluates the actual shift and compares it to an ideal shift in the TCM
memory. Based on that comparison, the TCM changes the settings of the adaptive values to a
level that it believes will result in a shift closer to the ideal shift the next time it makes that type
of shift. This is referred to as adaptive shifting. When the transmission/TCM calibration is new,
the TCM is in fast adaptive mode. The TCM is allowed to make large changes in the adaptive
values after each shift. Once the TCM determines that a given shift is close to the ideal level, it
switches to slow adaptive mode. In slow adaptive, the TCM still is evaluating shifts and changing
adaptive values, but is only allowed to do so in smaller increments. When a shift switches from
fast to slow adaptive mode, it is described as converged. The TCM is programmed in order to
switch from fast to slow adaptive mode in as many as 15-20 shifts. It is important to understand
that there are many different distinct shifts recognized by the TCM, and each of these shifts has its
own adaptive values. There are upshifts and downshifts to and from each range, as well as unique
adaptive values for several different throttle regions for each upshift and downshift. It may take a
significant amount of time before most of the shifts converge from fast to slow adaptive, and thus
it is not unusual to experience somewhat harsh or unpleasant shift quality until these shifts are
adapted.

TCC engagement is accomplished by a separate PCS. There are adaptive values for this as well,
and thus it will also require some driving for TCC engagement to converge.

REVERSE

-

-

X

-

X

-

First

X

-

-

-

X

-

Second

X

-

-

X

-

X

Third

X

-

X

-

-

X

Fourth

X

X

-

-

-

X

Fifth

-

X

X

-

-

X

Sixth

-

X

-

X

-

X

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado

If you are experiencing harsh shifts, it is important to verify whether the particular shift is
converged. Use the scan tool in order to determine if the shift is converged.

If the shift is not converged, the TCM is learning how to adapt that shift, and needs to be
driven more, with the intention of performing more of the particular type of shift.

If a particular shift is converged, but still objectionable, it is good troubleshooting practice
to reset the adaptive values for that shift back to base calibration level. This will
automatically reset the TCM to fast adaptive mode. The vehicle should then be driven in
order to allow the TCM to relearn the shift. Many times this will correct the condition. It is
possible to reset individual shifts without affecting the other shifts.

TRANSMISSION INDICATORS AND MESSAGES

NORMAL Inhibits Which Result in a Blinking PRNDL

High engine speed NEUTRAL to Range shifts

When a NEUTRAL to DRIVE or NEUTRAL to REVERSE shift is made, when engine
RPM is high, the shift is inhibited to NEUTRAL. The TCM has torque management
capability and attempts to slow the engine to a point where it makes the requested shift.

High Throttle or Torque Direction Change Shifts

REVERSE to DRIVE, DRIVE to REVERSE, NEUTRAL to DRIVE, and NEUTRAL to
REVERSE shifts, where throttle position is greater than 25 percent, are inhibited to
NEUTRAL. The TCM has torque management capability and attempts to slow the engine to
a point where it makes the requested shift.

Four-Wheel Drive Transfer Case in NEUTRAL

If the four-wheel drive transfer case is shifted into NEUTRAL while the transmission is in

IMPORTANT: WHEN REPLACING A FAILED TRANSMISSION WITH A

REPLACEMENT UNIT, IT IS IMPORTANT TO RESET THE TCM IN
ORDER TO BASE CALIBRATION AND FAST ADAPTIVE FOR ALL
SHIFTS. This can be done in 1 step with FastLearn. If this is not
done, the TCM adaptive values will be at the settings that it
learned for the old transmission, and will be in slow adaptive
mode. Under these conditions, it would take an unacceptably long
time for the adaptive values to converge to levels suitable for the
new transmission.

2008 Chevrolet Silverado 1500

2008 TRANSMISSION Automatic Transmission - Allison - Cab & Chassis Sierra, Cab & Chassis Silverado, Sierra &

Silverado