Chapter 21 Allison Electronic Transmissions. Objectives (1 of 5) Identify the three generations of Allison automatic transmissions. Describe the operating.

Chapter 21

Allison Electronic

Transmissions

Objectives (1 of 5)

• Identify the three generations of Allison automatic transmissions.

• Describe the operating principles of Allison partial authority transmissions.

• Describe the modular design used in the WT transmission.

• Outline how the WT transmission uses full authority management electronics to effect shifting and communicate with other vehicle electronic systems.

Objectives (2 of 5)

• List some of the service and repair advantages of the modular construction of the WT transmission.

• Group the WT modules into input, gearbox, and output categories.

• Describe how the WT electronic control module masters the operation of the transmission.

Objectives (3 of 5)

• Define the terms pulse width modulation, primary modulation, and secondary modulation.

• Outline how the base WT transmission uses three interconnected planetary gearsets to stage gearing to provide six forward ranges, reverse, and neutral.

• Identify the overdrive ranges. • Describe the WT integral driveline retarder

components and operating principle.

Objectives (4 of 5)

• Outline the function of the dropbox option in one WT model.

• Describe the role of, and the components within, the electrohydraulic control module.

• Outline the essential components in the WT electronic circuit and classify them as input circuit, processing, and output circuit components.

• Describe how SAE J1939-compatible hardware and software allow WT to share componentry and data with other on-board electronic systems to optimize vehicle performance.

Objectives (5 of 5)

• Describe how the WT clutches are controlled.

• Outline the torque routes through the WT transmission in each range selected.

• Describe how diagnostic codes are logged in the Allison WT ECU and the manner in which they are displayed.

• Interpret some of the WT diagnostic codes.

• Perform some basic diagnostic troubleshooting using the Allison recommended tooling.

Shop Talk

• The term full authority is borrowed from the way we categorize engines.

• In the early days of engine management electronics, the term partial authority was used to describe a hydromechanical system that was adapted for electronic management. – This description fits an Allison CEC transmission perfectly. A full

authority engine was one that was designed to be managed by electronics.

• Again, if we use these terms to categorize transmissions, full authority perfectly describes the WT transmission. It is capable of more comprehensively monitoring and controlling all transmission functions.

CEC Transmissions

• The CEC has more in common with a non-computerized Allison transmission than with the newer WT electronic transmission.

• The main difference is an electrohydraulic valve body that controls the hydraulic circuits using solenoids that replace the shift signal valves.

CEC Electronic Control Unit

• The main power and ground inputs should be dedicated. – A minimum of 10V is required to operate the unit. – 16V continuous and 19V intermittent are the maximum voltages

to which the unit should ever be subjected. • The ECU requires continuous power to retain nonvolatile RAM in

which diagnostic codes and TPS calibration data are retained. Nonvolatile RAM is lost if the battery is disconnected from the unit.

• The ECU is a sealed component and is not serviceable in the field.

• PROM removal/replacement is the only field service permitted on Allison ECUs.

PROM Chips

• Allison programmable read-only memory (PROM) chips log data that permit CEC systems to: – Be programmed for a variety of vehicle and

equipment options– Have flexibility of operating characteristics

• The PROM is located inside the ECU and can be accessed through a cover in the ECU housing.

• PROM data is written magnetically to the chip in the same way that audio data is recorded to a cassette tape. This is the only means of logging programmed data to the CEC ECU.

Output Speed Sensor

• The device has a magnetic sensor triggered by a pulse/reluctor wheel on the transmission output shaft.

• Output shaft rotation drives the pulse wheel through a magnetic field, generating an AC voltage signal.

• As the unit rotates at higher speeds, both the frequency and signal voltage increase.

• The pulse wheel uses a 16-tooth gear for on-highway applications. – This 16-tooth wheel has become standard in all

highway truck tailshaft speed sensors.

Throttle Position Sensor (TPS) (1 of 4)

• With a hydromechanical engine– A throttle position sensor (TPS) is a simple

potentiometer device actuated by a pull cable.• With an electronic engine

– Usually only one TPS is required to signal both the Allison ECU and the engine ECM.

• An interface is required.– When an Allison CEC is coupled to electronic

engines, an interface module usually is required. – The function of an interface module is simply to allow

the engine electronics to “talk” to the transmission electronics so that input signals such as that of the TPS can be shared.

Throttle Position Sensor (TPS) (2 of 4)

Throttle Position Sensor (TPS) (3 of 4)

Throttle Position Sensor (TPS) (4 of 4)

Temperature Sensors

• Temperature inputs to the ECU could result in: – Blocking of all shifts when the transmission is 225°F

or below. This protects the transmission from damage that could be caused by heating up too quickly.

– Limiting of transmission shifting to neutral, first range, and reverse (N, 1st, R) when temperature is between 225°F and 125°F.

– When temperature exceeds 270°F, the “hot” light is energized (if equipped), a trouble code is logged (temperature code # 24, sub-code hot #23), and top gear is inhibited. Emergency vehicles such as fire trucks are usually programmed not to inhibit top gear in these circumstances.

Switches

• Forward pressure switch–This switch signals the ECU when the transmission is in forward ranges. One of two types is used and it is plumbed into the clutch apply circuit.

• Reverse pressure switch–This switch signals the ECU when the transmission is in reverse gear. One of two types is used, and it is plumbed into the clutch apply circuit.

• Oil pressure switch–This switch signals the ECU when low fluid pressure or level exists.

Fluidic Oil Pressure Sensor

• A jet of oil is directed at a sensor.

• When the oil level is normal but is cold, the jet is blocked by a bi-metallic strip.

• When the oil level is normal and the temperature is also normal, the jet is dispersed and doesn’t strike the sensor.

• When the oil level is low and the temperature is normal, the jet is allowed to strike the sensor.

Solenoids

• Non-latching solenoids have a plunger that is spring loaded to the “off” position. Non-latching solenoids, therefore, permit hydraulic flow only when electrically energized.

• Latching solenoids must be momentarily energized to be switched, after which the plunger remains in position until energized again with reverse polarity. When a latching circuit solenoid is in the open position, it will permit hydraulic flow through its circuit until it receives an ECU switch command to close it.

Overview of CEC Valve Body

• Latching solenoids replace the shift signal valves.

• The neutral range valve is controlled by one latching and one non-latching solenoid.

• The forward-reverse valve is controlled by one latching solenoid.

• The trimmer regulator valve is controlled by a non-latching solenoid.

Solenoid Designations

• Each solenoid receives a constant flow of main pressure.

• It also is given a letter designation.

• Solenoids A, B, C, and D are latching solenoids that replace the conventional shift signal valves.

• Solenoids are all directly switched by the ECU.

Shop Talk

• An Allison CEC transmission, although electronically controlled, is still a hydromechanical device.

• As such, the importance of correct transmission fluid level cannot be emphasized enough. – If the fluid level is low, the converter and

clutches will not receive enough fluid. If the fluid level is high, the fluid will aerate and the transmission will overheat.

WT Modular Construction

• Input modules – Torque converter module – Converter housing module – Front support/charging pump module

• Gearbox modules – Rotating clutch module – Converter housing module – P1 planetary module – P2 planetary module – Main shaft module

• Output module – Rear cover module– P3 module– Output retarder or transfer gear module

• Electronic control unit

Gearbox Modules

• The gearbox modules combined contain five clutch assemblies and three planetary gearsets.

• All WT transmissions have the hardware for six forward ratios, neutral, and reverse.

• The MD and HD series transmissions are available with either close-ratio or wide-ratio gearing.

• Chassis application will determine which is used. • All B series transmissions are built with close-ratio gearing.

Close- or wide-ratio gearing is determined by the physical characteristics of the planetary gearsets used in the assembly of the transmission.

• The upper two ranges of WT model transmissions have overdrive gear ratios.

Rotating Clutch Module (1 of 3)

• The rotating clutch module is splined to the turbine shaft and, therefore, rotates with the turbine.

• This module contains the turbine shaft assembly, the rotating clutch hub assembly, the C1 and C2 clutch assemblies, the rotating clutch drum, and the sun gear drive hub assembly for the P1 planetary gearset.

• The P1 sun gear supplies constant rotational input to the P1 planetary set.

• The C1 and C2 clutches consist of pistons, return spring assemblies, a drive hub, and clutch reaction and friction plates.

• The reaction plates in both C1 and C2 clutches are splined to rotate with the rotating clutch hub assembly.

Rotating Clutch Module (2 of 3)

• The C1 clutch is applied by hydraulic fluid acting on the C1 piston: This action forces the piston and the clutch reaction plates against the clutch friction plates.

• Because the clutch friction plates are splined to the C1 drive hub, this will rotate providing turning force or torque at turbine speed to the main shaft assembly.

• The means used to apply the C2 clutch is identical to that used to apply the C1 clutch.

• Because the C2 reaction plates are splined to the C2 drive hub, when the C2 piston is hydraulically actuated, the C2 clutch transmits torque to the P2 planetary assembly.

Rotating Clutch Module (3 of 3)

• A balance piston is used to enhance control of off-going and on-coming rotating clutches. The balance piston is the third piston in the rotating clutch assembly and is located between the C1 spring assembly and the C1 pressure plate.

• The balance piston entraps fluid (lubrication pressure) between itself and the C1 piston, thereby balancing piston movement against exhaust backfill pressure behind the C1 piston.

P1 Planetary Gearset Module

• The P1 is the first of the three planetary gearsets in the transmission. The three sub-components of the gearset are arranged as follows: – Sun gear. The sun gear is driven by the rotating

clutch drum. – Planetary carrier. Its pinions mesh internally with the

P1 sun gear and externally with the P1 ring gear. The ring gear of the P2 carrier is splined to the P1 planetary carrier.

– Ring gear. The ring gear is housed in the C3 clutch assembly.

• Rotation of the P1 planetary gears is controlled by the application of the C3 or C4 clutches.

P2 Planetary Gearset Module• The P2 planetary gearset is the second in the WT

transmission. Its three sub-components are arranged as follows: – Sun gear. The sun gear is splined to the transmission

mainshaft. – Planetary carrier. Its pinions are meshed with the sun

gear internally and with the ring gear externally. – Ring gear. The ring gear is spline-coupled to the P1

carrier. • P2 planetary gearset rotation is controlled by the action

of the P1 planetary gearset and by the application of either the C4 or C5 clutches. Input rotation to the P2 planetary carrier can either be provided by the P2 ring gear or by the P2 sun gear, which is splined to the transmission mainshaft.

Output Retarder Module

• The action of the retarder can be compared to that of a torque converter operating in reverse.

• When the retarder is activated, it is charged with transmission fluid stored in the external accumulator.

• When the retarder is not in use, transmission fluid is drained from the retarder housing.

• For the retarder to be applied, the following is required: – The vehicle dash-mounted Retarder Enable switch

must be on. – The vehicle must be moving. – The TPS signal must indicate that throttle travel is

close to zero. – The ABS system must not be activated.

Retarder Controls

• WT retarders are fitted with a variety of apply devices.– Hand lever– Foot pedal– Pressure switch– Auto apply—auto full-on– Auto apply—auto percent-on– Combination

Transfer Gear/Dropbox

• It consists of an output adapter housing, transmission output shaft adapter, transfer case, transfer case charging oil pump, C6 clutch assembly, and a transfer case scavenging pump (auxiliary pump) mounted at the right side PTO provision.

• The addition of the dropbox adds a forward gear to the WT transmission and provides for full-time, all-wheel drive.

• The transfer case is mounted to the adapter housing and is under-slung below the main transmission to provide for forward and rear drive yokes in line with the transmission.

• The transmission output shaft adapter splines to the P3 carrier hub.

WTEC Electronics

• Electronic management of the WT transmission is similar to the CEC, with some refinements.

• There is an increased ability to interact with other on-board electronic systems and better programmability to suit specific vehicle applications.

• The system is networked to the chassis data bus by a J1939 connection.

• The latest generation of WTEC has the ability to broadcast range inhibit, check transmission light data, and send range status to the data bus for display onto the appropriate dash displays.

Input Circuit Components

• Shift selector(s)• Throttle position sensor• Engine speed sensor• Turbine speed sensor• Output speed sensor• C3 pressure switch• Sump temperature sensor• Coolant temperature• Vehicle interface module (VIM)

Input Signals

• The following is a partial list of the input signals the WT ECU may be programmed to process:– Secondary shift schedule– PTO enable– Shift selector transition– Engine brake and pre-select request– Fire truck pump mode

WT Output Circuit Components

• The following are WT ECU switched output circuit components: – A–E clutch solenoids– Lockup clutch solenoid F– Forward latch solenoid G– Vehicle interface module (VIM)– PTO functions

Output Signals

• The following is a partial list of the output signals that WT electronics may be programmed with to supply other chassis electronic management systems. – Engine brake enable– Sump/retarder temperature indicator– Range indicator– Output speed indicator– PTO enable– Engine overspeed indicator– Two-speed axle enable– Lockup indicator– Service indicator– Shift-in-progress indicator– Retarder indicator– Neutral indicator for PTO

Input Circuit

• Input from the operator is sent to the ECU by means of the shift selector.

• Other input signals come from sensors located in the transmission itself and from the vehicle interface module (VIM).

• The VIM handshakes data exchange (both ways) between WT electronics and other vehicle electronic management systems using SAE J1939 communication protocols and hardware.

• Temperature sensors, shaft speed sensors, and pressure switches all monitor conditions within the transmission that are required to compute an outcome.

• Command input signals are usually operator-generated and often require the ECU to make a change in outcome.

Shift Selectors

• A lever and two types of push-button shift selector are available.

• The full function non-strip type push-button selector allows the operator to select any range programmed to WTEC.

• Using these buttons does not override the automatic shifting function of the transmission.

• The operator cannot select a shift schedule that could result in damage to either the transmission or drivetrain.

• When the D range position is selected, WTEC will manage shift schedules using all the ranges programmed to WTEC.

• Selection of any of the numeric ranges will confine shifting within the ranges below the selected range.

Vehicle Interface Module (VIM)

• The VIM contains two 10-amp fuses and between two and six relays.

• WT electronics will operate on either 12- or 24-volt.

• One of the 10-amp fuses protects the main power connection to the ECU. The other protects the feed from the ECU to ignition circuit.

• One relay is used to output a signal to the reverse warning circuit and another is used for the neutral start circuit.

• As many as four other relays may be used for special function outputs.

Throttle Position Sensor (TPS)• A dedicated TPS is only

required when a WT transmission is used with a hydromechanical engine.

• In cases in which an electronically managed engine is used, WT electronics simply shares the TPS signal with the engine by means of the data bus.

• The TPS should be of the potentiometer type.

• The potentiometer is a three-wire variable resistor that converts position to a voltage value by sliding contacts across a resistive strip.

• The TPS used with WT and CEC transmissions is a common component.

Output Circuit

• The output circuit is responsible for effecting the outcomes of the processing cycle of the ECM.

• In the Allison WT, the results of ECM processing are converted into actions by an electro-hydraulic control module.

• This is mounted at the base of the transmission, and the channel plate forms its bottom enclosure.

• Solenoids are used to convert electrical signals into the hydromechanical outcomes required to enable shifting.

Solenoids

• Two types of solenoids are used in the WT electrohydraulic control module.

• Both are somewhat similar in construction but differ in their switching characteristics.

• Both types receive control main pressure from a supply port and may either route that pressure to exit through the solenoid regulator valve or to an exhaust port, depending on the switch status.

Normally Closed (NC) Solenoids

• Closed until energized: An NC solenoid remains closed until energized by an ECM electrical signal.

• Pressure is exhausted: When the solenoid is not energized, main pressure fluid is blocked by the seated check ball, routing it directly to the exhaust passage.

• Pressure is applied: When energized, the check ball unseats, blocking the exhaust port and permitting main pressure fluid to exit to the solenoid regulator valve.

Normally Open (NO) Solenoids

• Open until energized: An NO solenoid remains open until energized by an ECU electrical signal.

• Pressure is applied: When the solenoid is not energized, the check ball is unseated, allowing main pressure fluid to flow into the solenoid through the supply passage and be routed to the solenoid regulator valve.

• Pressure is exhausted: When the normally open solenoid is energized, the check ball is forced onto its seat. This blocks the passage to the regulator valve and routes main pressure to the exhaust.

Pulse-Width Modulation (PWM)

• WT solenoids are controlled by pulse width modulated (PWM) signals.

• Primary modulation controls the amount of on time.

• The primary modulation signal to WT solenoids is delivered at a frequency of 63 Hz.

• The percentage of on time during each 1⁄63 second is referred to as the solenoid duty cycle.

• A 100% duty cycle would represent the maximum signal.

• A 0% duty cycle would indicate no signal.

The WTEC Diagnostic Tools

• The Allison electronic service tools (EST) required to read and reprogram WTEC electronics– MPSI ProLink digital diagnostic reader (DDR) – The Allison digital optimized connection (DOC)

software used with a personal digital assistant (PDA)

The WT Hydraulic Circuit

• The primary components of the hydraulic system– Transmission fluid – Charging pump – Three integral filters – Electrohydraulic control

module – Breather – C3 pressure switch

• There are seven sub-circuits that make up the hydraulic circuit. – Main pressure circuit – Control main circuit – Torque converter circuit – Cooler/lubrication circuit – Clutch apply circuit – Exhaust circuit – Exhaust backfill circuit

Hydraulic Circuit OperationDuring an Electrical Failure

• An interruption of electrical power will result in the solenoid regulator valves locking in their NO or NC states.

• To minimize the impact of an electrical failure, the WT transmission defaults to totally hydraulic operation.

• The C1 and C2 latch valves are used to accomplish this default mode of operation.

• When an electrical failure occurs, the latch valves engage specific clutches based on the range the transmission was in when the failure occurred.

• In case of an electrical failure, the latch valves and the two NO solenoids, A and B, default to “limp home” mode by reverting to total hydraulic operation.

Torque Paths and Range Management

• In each range, the gearing is staged through the transmission.

• The stages are sequenced numerically, but the stages of gearing do not necessarily correlate with the numbering of the planetary gearsets.

• For instance, first range gearing is single stage and takes place in the P3 planetary gearset.

• As many as three stages of gearing may be used, depending on the range selected.

Shop Talk

• Learning hydraulic circuit and torque power flows helps visualize transmission operation so they are a key to achieving a proper understanding of Allison transmission operation.

• For the Allison specialist tech, knowing the hydraulic circuit factors and torque power flows are essential to diagnose performance problems.

Clutch Application Chart

• See Table 21-1 on page 622 of the textbook.

Throttle Position Sensor Initial Adjustment

Neutral Operation

• Both the CI and C2 clutches are released. • The C5 clutch is applied, but no output occurs because a driving

clutch is not applied to move the planetary gears. The result is that no output torque occurs.

• If the output shaft is rotating while in neutral, the stationary clutch applied will depend on the shaft rotational speed.

• This varying application corresponds to the neutral ranges identified as N1, N2, N3, and N4.

• These ranges are dependent on the application of the clutches C5, C4, C3, and C2 respectively.

• The application of a specific clutch during neutral operation means that one of the two clutches required to ensure a range selection when the transmission is shifted to D is already engaged.

Neutral Power Flow

• Only the C5 clutch is applied

• Torque from the turbine shaft is not transmitted beyond the rotating clutch module and the P1 sun gear.

• If the vehicle is rolling when in neutral, different clutches may be applied. This controls the speed of rotating components.

First Range Operation

• Clutches C1 and C5 are applied.• C1 applies torque to the P3 sun gear. Because the C1 is applied,

the rotating clutch assembly acts to lock the turbine shaft and mainshaft, causing them to rotate as a unit.

• P3 sun gear is the input. Torque is transmitted through the mainshaft to the P3 sun gear, which is splined to the mainshaft.

• C5 holds the P3 ring gear. The C5 clutch being engaged prevents the P3 ring gear from being rotated.

• P3 carrier is the output. The P3 sun gear acts to rotate the P3 pinion carrier, which is splined to the output shaft.

• Torque route is as follows: converter turbine shaft, mainshaft, P3 sun gear, P3 pinion carrier, output shaft.

First Range Power Flow

• In first range, the C1 and C5 clutches are applied.

• C1 locks the turbine shaft and main shaft together. The P3 sun gear is part of the main shaft module, so it becomes input for the P3 planetary gearset.

• C5 holds the P3 ring gear. The P3 carrier is splined directly to the output shaft. The P3 sun gear is input and the P3 ring gear is held, so the P3 carrier becomes the output.

First Range Operation WT with Dropbox

• Clutches C3 and C6 are applied.• The C6 clutch holds the mainshaft stationary. A WT

transmission with a transfer case has a mainshaft that extends from the main housing through the P3 planetary gear assembly and output adapter to the C6 clutch.

• When the C3 clutch is applied, the P1 ring gear is held.

• Torque route is as follows: – turbine shaft, P1 sun gear, P1 planetary carrier, P2

ring gear, P2 pinion carrier, P3 ring gear, P3 pinion carrier, output shaft adapter, transfer drive gear, P4 pinion carrier, P4 sun gear, output shafts

Second Range Operation

• Clutches C1 and C4 are applied.

• The applied C1 clutch locks the turbine shaft and mainshaft so they rotate as a unit.

• The applied C4 clutch holds the C2 ring gear.

• Torque route is as follows: – turbine shaft, mainshaft, P2 sun gear, P2

pinion carrier, P3 ring gear/mainshaft,P3 sun gear,P3 pinion carrier, output shaft.

Second Range Power Flow

• In second range, the C1 and C4 clutches are applied, making the P2 and P3 planetary gearsets work together to provide the output.

• The C1 clutch locks the turbine shaft and mainshaft together and this drives the P2 sun gear.

• The C4 clutch holds the P2 ring gear. The P2 sun gear is input and the P2 ring gear is held, making the P2 carrier the output.

• The P3 sun gear is splined to the mainshaft so it rotates.• The P3 ring gear is splined to the P2 carrier, but it rotates at a

slower speed than the P3 sun gear. • The P3 ring gear acts like a held member and the P3 sun gear

becomes input. This makes the P3 carrier the output, and it is splined to the output shaft. The C1 and C3 clutches are applied, and all three planetary gearsets work together to provide the appropriate output.

Third Range Operation

• Clutches C1 and C3 are applied.

• The C1 clutch locks the turbine shaft and mainshaft.

• The C3 clutch holds the P1 ring gear.

• Torque route is as follows: – Converter turbine shaft, mainshaft, P1 ring

gear/ P1 sun gear, P1 pinion carrier, P2 sun gear and P2 pinion carrier, P3 ring gear and P3 sun gear, P3 pinion carrier, output shaft.

Third Range Power Flow• The C1 and C3 clutches are applied, and all three planetary

gearsets work together to provide the appropriate output. • C1 clutch locks the mainshaft to the turbine shaft and the rotating

clutch module rotates the P1 sun gear. • The C3 clutch holds the P1 ring gear. Because the P1 sun gear

is input and the P1 ring gear is held, the P1 carrier becomes the output.

• The P2 sun gear rotates with the mainshaft. • The P2 ring gear is splined to the P1 carrier, so it rotates.

Because the P2 ring gear rotates more slowly than the P2 sun gear, it acts like a held member. This makes the P2 sun gear the input, the P2 ring gear “held,” and the P2 carrier the output.

• The P3 sun gear rotates with the mainshaft. • The P3 ring gear is splined to the P2 carrier, so it rotates with the

P2 carrier at a speed slower than the P3 sun gear. This makes the P3 sun gear input, the P3 ring gear held, and the P3 carrier the output. The P3 carrier is splined to the output shaft.

Fourth Range Operation

• Clutches C1 and C2 are applied.• The C1 and C2 clutches rotate with the turbine shaft.• The C1 drive hub is splined to the mainshaft so that

when it is actuated, the mainshaft will rotate at turbine shaft (input) speed.

• The C2 drive hub is splined to the P3 ring gear (through the P2 pinion carrier), which results in the P3 ring gear rotating at turbine speed.

• Torque route is as follows:– Converter turbine shaft, mainshaft, P3 sun gear and

P3 ring gear, P3 pinion carrier, output shaft.

Fourth Range Power Flow

• In fourth range, both rotating clutches C1 and C2 are applied

• The C1 clutch locks the turbine shaft to the mainshaft. • The C2 clutch locks the turbine shaft to the P2 carrier. • Because no stationary clutches are applied, all three

planetary sun gears, carriers, and ring gear rotate at the same speed and in the same direction as the input from the turbine shaft.

• This results in a direct drive or 1:1 ratio.

Fifth Range Operation

• Clutches C2 and C5 are applied. • The C5 clutch holds the P3 ring gear.• The application of the C2 clutch locks the

turbine shaft and the P2 pinion carrier so they rotate together.

• Torque route is as follows: – Converter turbine shaft, P1 sun gear, P1

pinion carrier, P2 ring gear and P2 planetary carrier, P2 sun gear, mainshaft, P3 ring gear, P3 sun gear, P3 pinion carrier, output shaft.

Fifth Range Power Flow

• In fifth range, the C2 and C3 clutches are applied and all three planetary gearsets work together to provide the appropriate output. The P1 sun gear is rotating with the rotating clutch module.

• The C3 clutch is holding the P1 ring gear stationary, making the P1 carrier the output.

• The P2 carrier is rotating at turbine speed because the C2 clutch locks the turbine to the P2 carrier.

• The P2 ring gear is splined to and rotating with the P1 carrier. The P2 carrier is rotating faster than the P2 ring gear, so it is the input. The ring gear acts like a held member. This makes the P2 sun gear the output.

• The P2 sun gear rotates the mainshaft and the P3 sun gear. • The P3 ring gear is splined to and rotating with the P2 carrier. However, the

P3 sun gear rotates faster than the P3 ring gear, so the sun gear is input and the ring gear acts as a held member. This makes the P3 carrier the output, and it is splined to the output shaft.

• This gear range produces an overdrive.

Sixth Range Operation

• Clutches C2 and C4 are applied.

• The C2 clutch locks the turbine shaft and the P2 pinion carrier, causing them to rotate as a unit.

• The C4 clutch holds the P2 ring gear.

• Torque route is as follows: – Turbine shaft, P2 pinion carrier, P2 sun gear,

mainshaft, P3 ring gear/mainshaft, C3 sun gear, C3 pinion carrier, output shaft.

Sixth Range Power Flow

• The C2 and C4 clutches are applied, and the P2 and P3 planetary gearsets work together to produce the appropriate output.

• The C2 clutch locks the P2 carrier to the turbine shaft. • The C4 clutch holds the P2 ring gear. The P2 carrier is the input

(from the turbine shaft) and the P2 ring gear is held so the P2 sun gear becomes the output.

• The P2 sun gear rotates the mainshaft and the P3 sun gear. The P3 sun gear is input and the P3 ring gear acts like a held member. This makes the P3 carrier the output, and it is splined to the output shaft.

• This gearing produces an overdrive.

Reverse Operation

• Clutches C3 and C5 are applied.

• The C3 clutch holds the P1 ring gear.

• The C5 clutch holds the P3 ring gear.

• Torque route is as follows: – Turbine shaft, P1 sun gear, P1 pinion carrier,

P2 ring gear, P2 pinions (carrier held, reversing direction), P2 sun gear/mainshaft, P3 sun gear, P3 pinion carrier, output shaft.

Reverse Range Power Flow• The C3 and C5 clutches are applied, and all three planetary gearsets work

together to produce the appropriate output. • The P1 sun gear rotates with the rotating clutch module. • The P1 ring gear is held by the C3 clutch, making the P1 carrier the output. • The P1 carrier is splined to the P2 ring gear. The P2 ring gear becomes the

input for the P2 planetary set. • The C5 clutch holds the P3 ring gear, which is splined to the P2 carrier.

Because the P2 ring gear is the input and the P2 carrier is held, the P2 sun gear becomes the output. Because the carrier is the held member, the P2 sun gear rotates in an opposite direction (counterclockwise) to the input direction of rotation.

• The P2 sun gear rotates the mainshaft in an opposite direction. Because the P3 sun gear rotates with the mainshaft, it also turns in an opposite direction.

• The P3 sun gear becomes reverse input for the P3 planetary gearset. • The P3 ring gear is held by the C5 clutch. The P3 carrier becomes reverse

output, and it is splined to the output shaft.

Lubrication

• Allison recommends the use of TranSynd synthetic oil in all of their transmissions.

• TranSynd is formulated jointly by Allison and Castrol.

• If this oil is used, the oil drain intervals can be extended by three times depending on the specific application.

• Dexron is also approved. • Make a practice of referencing either Allison or

chassis OEM service literature when servicing automatic transmissions.

Range Verification

• WT verifies the transmission range continually whether or not a shift is in progress.

• The test involves checking the current gear ratio by comparing the turbine and output shaft speeds.

• This ratio is then compared with the ratio logged in the ECU memory for the commanded range.

• If the two ratios do not match, a diagnostic code is logged.

Off-Going Ratio Test

• The off-going ratio test is performed when a shift is in progress. • Within a set time after the ECU has commanded a range shift,

the ECU calculates the ratio between input and output speed, comparing this ratio with that of the previous range. For instance, if the speed ratio of the previous range is still present after the range shift has been commanded, the ECU will deduce that the off-going clutch failed to release.

• The shift commanded will be repeated twice to verify the fail to shift condition. If this does not result in the shift being effected, a diagnostic code is logged and the ECU commands the transmission in the previous range.

• The off-going ratio test is applied during the interval between the turbine speed shift initiation point and the pull-down detected point.

Oncoming Ratio Test

• The oncoming ratio test is performed by WT electronics near the end of a shift in progress sequence.

• The oncoming ratio test checks turbine speed and output shaft speed to determine whether the transmission is in the new range commanded by the ECU.

• When the ratios fail to match, the ECU assumes the oncoming clutch did not actuate and will log a code.

Shop Talk

• The off-going and oncoming ratio test can detect clutch slippage almost immediately. This provides a considerable advantage to the technician when troubleshooting WT transmissions.

• WT electronics can command a hold in the previous range to protect the transmission when severe clutch slippage warrants this failure strategy. By the time the driver complained of a slippage problem, significant transmission damage had already occurred, which was a major problem in Allison CEC transmissions.

C3 Pressure Switch Tests

• The C3 pressure switch is an important diagnostic indicator.

• The C3 pressure switch signals the ECU as to whether pressure is present or not in the C3 clutch. – For instance, when C3 pressure is not present and

should be, a diagnostic code is logged and a DNS condition is immediately put into effect, with the result that the transmission remains in the current range.

– If the opposite occurs, that is, C3 pressure is signaled in a range when it should not be, the result is the same. A diagnostic code is logged and a DNS condition results.

DNS Light

• Whenever WT electronics sense a condition that could damage the transmission, transmission electronics, or the vehicle drivetrain, shifting is restricted and the DNS light is illuminated.

• The ECU should be checked for fault codes whenever there is a transmission-related problem.

• If the DNS light fails to illuminate at startup, the system should be checked immediately. Continuous illumination of the DNS light indicates the ECU has logged a diagnostic code.

• When the DNS light is first illuminated, eight seconds of short beeps are emitted from the shift selector.

• The beeps are an audible alert to the operator that shifts are being restricted.

• Additionally, the SHIFT digit on the shift selector display will remain blank and the ECU may not respond to shift selector requests.

Shop Talk

• The DNS light is not necessarily illuminated every time a diagnostic code is logged.

• The DNS light should illuminate every time the engine is started. It will remain lit for a few seconds only.

Limp Home Mode

• When the DNS light is illuminated, the intent is to inform the driver that the transmission is not operating properly and requires repair.

• However, the transmission can usually be operated for a short time in the selected range in “limp home” mode, provided the ignition key is not switched off.

• When the DNS light illuminates when the ignition key is cycled from off to on, the transmission will remain in neutral until the fault code responsible for the DNS condition is cleared.

• The converter lockup clutch is always disengaged whenever transmission shifting is restricted or during any critical transmission malfunction.

Accessing Diagnostic Codes

• The diagnostic data reader or DOC is used to access the diagnostic codes logged in the WT ECU.

• WT diagnostic codes are numeric and consist of a two-digit main code and a two-digit sub-code.

• The WT ECU logs these codes and produces them for readout by sequencing either the most severe code or the most recent code first, followed by the order in which they were logged, beginning with the most recent and working backward.

• A maximum of five codes may be logged. As codes are added, the oldest inactive code is dropped from the list first. Should all the logged codes be active, the codes are listed in order of severity. When the number exceeds five, the first logged code in the sequence is dropped.

Summary (1 of 8)

• Allison CEC Transmission is a partial authority electronic management system that adapts a hydromechanical Allison transmission to computerized monitoring and management.

• Primary input signals to CEC electronics are the shift signal, TPS signal, and road speed data.

• Three types of ECU are used to manage the CEC electronics: Splashproof, Sealed Standard, and Sealed Plus II.

• An electrohydraulic valve body is used to convert the results of logic processing into hydromechanical outcomes.

Summary (2 of 8)

• The WT transmission uses a modular design and full authority management electronics to effect shifting and communicate with other vehicle electronic systems.

• The WT modules may be removed from the transmission and serviced as separate units.

• WT modules may be grouped into input, gearbox, and output categories.

• The WT input modules are the torque converter module, the converter housing module, and the front support/charging pump module.

Summary (3 of 8)

• The WT gearbox modules are the rotating clutch module, the main housing module, the P1 planetary module, the P2 planetary module, and the mainshaft module.

• The WT output modules are the rear cover module, output retarder module, or the transfer gear/drop box module (depending on the model).

• The electronic control module is a computer that masters the operation of the transmission.

• The WT transmission uses interconnected planetary gearsets to provide multiple stages of gearing, depending on the range selected.

Summary (4 of 8)

• The WT transmission may have an integral driveline retarder that can enhance the vehicle service brake applications, especially those that require 10% or less of the peak service application pressure.

• The driveline retarder consists of a stator, rotor, housing, and control valve assembly and functions similarly to a torque converter driven in reverse.

• One WT model is available with a dropbox or transfer gear for four-wheel-drive vehicles.

Summary (5 of 8)

• The electrohydraulic control module houses the solenoids, sensors, valves, and regulators required to effect the results of ECU processing into outcomes.

• SAE J1939-compatible hardware and software give WT transmissions full multiplexing capability.

• WT has an EEPROM chip that permits programming to tailor transmission performance to operating requirements.

Summary (6 of 8)

• Codes are usually displayed in the order in which they are logged with the most recent displayed first. In cases in which one code can produce more severe consequences, it is displayed first.

• A maximum of five codes can be logged into the WT ECU at one time. As codes are added, the oldest inactive code is dropped from the list. If all codes are active, that with the lowest priority is dropped first.

Summary (7 of 8)

• WT transmissions have similar integral hardware and potential for ratio ranges.

• Programming determines the actual number of ranges available in a chassis.

• Clutches are controlled by solenoids switched by the ECM.

• WT solenoids are switched by pulse width modulated (PWM) signals.– PWM is the controlling of output signals by

varying the percentage of on and off time.

Summary (8 of 8)

• The term primary modulation is used to describe the amount of time a solenoid is energized. Secondary modulation describes the current flow through a solenoid.

• Understanding the power-flows through the WT transmission can be a useful diagnostic tool.

• Codes logged into the WT ECU can be classified as active or historic (inactive).