TRANSMISSI ELECTRONIC CONTROL SYSTEM OF 797B OFF-HIGHWAY TRUCK

TRANSMISSI ELECTRONIC CONTROL SYSTEM797B OFF-HIGHWAY TRUCK

Transmission Electronic Control System

The Transmission Electronic Control Module (ECM) (1), and the Chassis ECM (2) are located in the compartment at the rear of the cab.  These controls are ABL2C ECMs and have two 70-pin connectors, capable of receiving and sending many signals.  Diagnostic and programming functions must be performed with a laptop computer with the Electronic Technician (ET) software installed.  The ECMs can be reprogrammed with a "flash" file using the WinFlash application of ET.


The Transmission ECM performs most of the transmission control and torque converter lockup functions.  The Chassis ECM receives input signals from the body position sensor and the shift lever switch.  These input signals are sent to the Transmission ECM and are also used to control the transmission shifting and the torque converter lockup functions.

The Transmission ECM also controls other functions such as overspeed protection, Control Throttle Shifting (CTS), Directional Shift Management, Top Gear Limit, Neutral Coast Inhibiting, Torque Limiting, Speed Limiting, and Fail In Gear Protection.

The Chassis ECM controls the hoist system, starter protection, reverse neutralizer, body up gear limit, load counter, back-up alarm, accumulator bleed down, and neutral start functions.

Transmission ECM shifts transmission

The main purpose of the Transmission ECM is to determine the desired transmission gear and energize solenoids to upshift or downshift the transmission, based on information from the operator and/or the machine.

The Transmission ECM receives information (electrical signals) from various input components such as the shift lever switch (from the Chassis ECM), the latching pressure sensor, the Transmission Output
Speed (TOS) sensors, and the Converter Output Speed (COS) sensor.

Based on the input information, the Transmission ECM determines whether the transmission should upshift, downshift, engage the lockup clutch, or limit the transmission gear. These actions are accomplished by sending signals to various output components.

Output components include the clutch supply valve solenoids, the lockup clutch solenoid, and the latching solenoid.

Manual Shifting: In the Manual Shifting mode, the Transmission ECM will shift the transmission based on the operator's shift lever selection. Manual operation encompasses REVERSE, NEUTRAL, and FIRST. The maximum forward manual gear is also the minimum automatic gear.

Automatic Shifting: The Transmission ECM provides fully automatic shifting from the minimum automatic gear to the top gear limit.

In the Automatic Shifting mode, the operator sets the shift lever to the maximum gear range desired.  All automatic shifts up and down are speed dependent and are pre-programmed for each gear into the Transmission ECM.  The shift points are raised slightly by the use of the retarder or service brakes.  When the transmission reaches the gear range selected by the operator, no further upshifts will NORMALLY occur. During normal shifting, the Transmission ECM does not allow a turnaround shift until approximately 4.5 seconds after a shift occurs.  The purpose of the delay is to allow mechanical and hydraulic conditions to stabilize before an opposite shift.  This delayed turnaround shift feature is commonly referred to as the "anti-hunt" feature.  The service/retarder and parking/secondary brake inputs eliminate this feature.

Customer Transmission Lockout: The purpose of this lockout feature is to provide the customer with a secure method of locking out the transmission during service work.

The user interface is a single switch to ground that goes directly to the Transmission ECM on Pin J1-31.  The Transmission ECM reads this input and determines if the transmission should be locked out.

Below are the conditions that the Transmission ECM looks for before allowing the transmission lockout status to change state.

Transmission Lockout Status OFF
The only way that the lockout status can go ON is if all of the following conditions are met:
* Shift lever is in neutral.
* Ground speed is 0.0 km/h (0.0 mph).
* Transmission Lockout Switch ON (switch open).

Otherwise, the Transmission Lockout Status will stay OFF.  As long as the lockout status is OFF, the transmission will shift as normal
Transmission Lockout Status ON
The only way that the lockout status can go OFF is if all of the following conditions are met:
- Shift lever is in neutral.
- Transmission Lockout Switch OFF (switch grounded).
Otherwise, the Transmission Lockout Status will stay ON.  As long as the lockout status is ON, the transmission will be held in NEUTRAL.

Note that the transmission lockout feature will never cause a shift into NEUTRAL.  The actual gear of the transmission will always be in NEUTRAL before the status can go from OFF to ON.

Likewise, the transmission lockout feature will never cause a shift out of NEUTRAL.  The requested gear (after the lockout goes away) will always be NEUTRAL after the status goes from ON to OFF.

TOS sensors

Two Transmission Output Speed (TOS) sensors are located at the rear of the differential housing. The two sensors provide redundancy and improve the reliability of the TOS signal.  The Transmission ECM compares the two signals and diagnoses if one of the two signals is invalid due to an out of range signal or noise on the signal.  If the sensor signal is determined to be invalid, the last valid speed value is used.  The sensors are two wire "passive" type speed sensors.  The sensors generate an AC frequency signal. The Transmission ECM recalculates the frequency signal to correspond to transmission output shaft rpm.

The Transmission ECM uses the TOS sensors to determine when to shift, but the shifts always occur at a precise engine rpm. The engine rpm is known because the Transmission ECM knows the gear ratios of each gear range of the transmission, the differential, and the final drives.  The Transmission ECM also estimates the circumference of the tires.  The Transmission ECM uses the gear ratios and tire circumference to calculate the engine rpm for any ground speed. The Transmission ECM also uses the Converter Output Speed (COS) sensor (see Visual No. 9 STMG-2) signal for completing the shift strategy.  The Transmission ECM uses a cross-check strategy and will lock in a gear if it cannot validate two of the three sensors (two TOS sensors and one COS sensor).  The cross-check strategy must have two good sensors before allowing a shift.

NOTE:  After a repair, the truck must be moved to send a valid speed signal to the ECM before an active fault can be removed

Transmission ECM controlled functions

Besides controlling the Transmission Shifting and Torque Converter Lockup, the Transmission ECM also controls other functions such as Engine Overspeed Protection, Control Throttle Shifting (CTS), Directional Shift Management, Top Gear Limit, Neutral Coast Inhibiting, Torque Limiting, Speed Limiting, and Fail In Gear Protection.

Engine Overspeed Protection: If the engine speed (based on machine travel speed and gear) increases to a pre-determined level, the Transmission ECM will upshift the transmission ONE gear position past the operator's selection to protect the engine from overspeed. If the transmission is already in the top gear, the Transmission ECM will shift the torque converter into CONVERTER DRIVE.
Control Throttle Shifting (CTS): Controlled throttle shifting is used to smooth shifting and reduce driveline stress during all automatic transmission shifts.  The Transmission ECM sends a signal to the Engine ECM through the CAT Data Link during each transmission shift to reduce or increase the fuel flow, which reduces the torque during a shift.  During automatic upshifts, if the throttle position is at 100%, the Transmission ECM sends a signal to the engine control to momentarily reduce the "Desired Engine Speed." During automatic downshifts, if the throttle position is at 0%, The Transmission ECM sends a signal to the Engine ECM to momentarily increase the "Desired Engine Speed."

Directional Shift Management:  Directional shift management is used to reduce driveline stress during directional shifts.   The Transmission ECM sends a signal to the Engine ECM during directional shifts to reduce the fuel flow, which reduces the torque during a shift.

If the operator shifts from NEUTRAL to FIRST or REVERSE with the engine speed above 1500 rpm, a signal is sent to the Engine ECM through the CAT Data Link and "Desired Engine Speed" is reduced to low idle.

Top Gear Limit: The top gear limit is FIELD programmable by use of the ET service tool. The Transmission ECM comes from the factory set to the maximum gear available (SEVENTH GEAR).  The transmission will NEVER shift to a gear above the programmed top gear.

Neutral Coast Inhibiting: When the transmission is in gear and the shift lever is placed in NEUTRAL, the machine will remain in gear until the machine travel speed has been reduced from the lL to lC shift point (Lockup to Converter drive) [(approximately 8 km/h (5 mph)]. At approximately 8 km/h (5 mph), the Transmission ECM will shift the transmission to NEUTRAL.  Keeping the transmission in gear above
8 km/h (5 mph) will discourage high-speed coasting in NEUTRAL. High-speed coasting in NEUTRAL can reduce transmission life.  This function does not prevent coasting in NEUTRAL, but makes it more difficult.  The operator CAN COAST in NEUTRAL if he starts down a hill in NEUTRAL and travel speed is below 8 km/h (5 mph).  If the
operator does coast in NEUTRAL, at speeds above 12 mph, engine speed will increase to 1300 rpm and an event will be logged by the Transmission ECM as "Coasting In Neutral."  This information can be reviewed using the ET Service Tool.

Shifts to REVERSE from a Forward Gear are inhibited until travel speed is below 3.2 km/h (2 mph)
Torque Limiting:  The torque limiting strategy protects the power train from excessive torque loads during stall conditions in FIRST GEAR and REVERSE.  When the 797B is operated in FIRST GEAR or REVERSE, the Transmission ECM tells the Engine ECM to limit engine speed to 1600 ± 65 rpm.  The Torque Converter Output Speed (COS) is directly proportional to the torque transmitted through the torque converter.  Thus, as the torque converter output speed increases, desired engine speed, or torque converter input speed, is reduced.  Limiting the actual engine speed has the effect of reducing the torque developed by the engine and torque transmitted through the torque converter
Speed Limiting:
- Transmission cold mode: Limits the top speed until the oil temperature is high enough to ensure proper lubrication.  It also reduces the risk of simultaneously latching three or more clutches when operating with cold oil.  Shifting is limited to FIRST GEAR CONVERTER DRIVE if the cold mode is active.  An upshift to SECOND GEAR CONVERTER DRIVE will occur to prevent an engine overspeed.  If the cold mode goes active while above FIRST GEAR, then the transmission will drop to CONVERTER DRIVE and will not limit to FIRST GEAR until the transmission downshifts to FIRST GEAR.  The cold mode is triggered when transmission lube oil temperature goes below 35°C (95°F) and goes inactive when transmission oil temperature goes above 40°C (104°F).  The torque converter oil temperature sensor is used as backup.

- Differential cold mode: Limits the top speed until the oil temperature is high enough to ensure proper lubrication. Shifting is limited to FIFTH GEAR when the differential oil temperature is below -4°C (25°F).  Shifting is limited to SIXTH GEAR when the differential oil temperature is greater than -4°C (25°F) and below 24°C (75°F).

NOTE:  The programmable Top Gear Limit and Body Up Gear Limit functions can also be considered as Speed Limiting functions.

Fail In Gear Protection: Prevents shifts to a gear that is not appropriate for the current ground speed (engine overspeed protection).  There is a latching valve located below each clutch supply valve. The latching valves are used to keep the transmission in the current gear if there is a loss of electrical power to the Transmission ECM or a clutch valve solenoid.  The lockup solenoid will also be de-energized if the transmission latches in a gear.

Coasting In Neutral

Coasting In Neutral:  If the operator coasts in NEUTRAL at speeds above 19.3 km/h (12 mph), engine speed will increase to 1300 rpm and an event will be logged by the Transmission ECM as a "Coasting in Neutral" event.

Transmission Abuse:  If the operator shifts from NEUTRAL to FIRST or REVERSE with the engine speed above 1500 rpm, the "Desired Engine Speed" will be momentarily reduced to low idle, and an event will be logged by the Transmission ECM as a "Transmission Abuse" event.

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