938H Wheel Loader and IT38H Integrated Toolcarrier Power Train Caterpillar

Illustration 1	g01433857
(1) Transmission direction and speed control lever (2) Intermediate speed sensor (3) Intermediate speed sensor (4) Output speed sensor (5) Output speed sensor (6) Transmission oil temperature sensor (7) Transmission input speed sensor (8) Autoshift switch (9) Parking brake switch (10) Downshift switch (11) Ride control switch (12) Brake accumulator pressure switch (13) Torque converter oil temperature sensor (14) Transmission direction control switch (15) Power train ECM (16) Data link (17) Lead that goes to the service connector (18) CAN data link (19) Engine ECM (20) Dash panel cluster (21) Key start switch (22) Left brake pedal position sensor

Switches

Switches provide information to the ECM (15). Switches are two state devices. The switches are either open or the switches are closed.

Closed - The switch closes the circuit to ground or the switch closes the circuit to the battery.

Open - the switch opens a circuit to the ECM input.

Key Start Switch

Illustration 2	g01434255

The key start switch is an input of the power train electronic control module (ECM). The key start switch informs the power train ECM of an attempt to start the engine. The power train ECM then initiates the neutral start function.

During normal machine operation, the start terminal of the key start switch is open. When the key start switch is turned to the START position, the start terminal closes to the +battery and the +battery voltage is present at connector contact J2-1 of the power train ECM. When all starting conditions are satisfied, the power train ECM sends a +battery signal to the connector contact J2-7 for the start relay and engine cranking begins.

Note: When the key start switch is in the ON position, a +battery signal is sent to connector contact J1-1. This activates the power train ECM.

Note: After the key start switch is initially turned to the START position, the switch will not return to the START position from the ON position. The switch must be turned to the OFF position first. Then, the switch can be turned to the START position.

ReferenceFor more information on the neutral start function, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Downshift Switch

Illustration 3	g01420575

The downshift switch (10) tells the ECM (17) that the operator wants the transmission to downshift one gear. The downshift switch has one input connection to the ECM.

The downshift switch signals the power train electronic control module (ECM) to downshift the transmission by one speed. To downshift, the operator depresses the downshift switch. The downshift switch has one input connection to the power train ECM on connector J1: downshift N/O at contact 63. When the operator presses the downshift switch, the downshift N/O circuit is closed to ground. When the downshift switch is not activated, the downshift N/O circuit is open or floating.

ReferenceFor more information on the function of the transmission downshift switch, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Rotary Switch (Autoshift Control)

Illustration 4	g00779624

Illustration 5	g01420580
Autoshift Switch (Conventional Steering)

The autoshift switch tells the power train electronic control module (ECM) of the desired shift mode, AUTO mode or MANUAL mode. The switch has three input connections to the power train ECM on connector J1. Two switch contacts are used for diagnosing a failure of the switch. A circuit failure is present if all three of the switch contacts are open at the same time or if any two switch contacts are grounded at the same time. The valid combinations for each switch position are shown in Table 1 that follows.

ReferenceFor more information on the function of the autoshift control, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

The power train ECM (15) will use the following inputs in order to engage the proper clutches:

• Transmission direction and speed control lever

• Intermediate speed sensors

• Output speed sensors

• Torque converter output speed sensor

• Transmission neutralizer override switch

• Variable Shift Control switch

• Autoshift switch

• Downshift switch

• Transmission direction control switch

• Left brake pedal position sensor

• Parking brake switch

• Engine ECM

The power train ECM (15) uses input from the service connector via the CAT data link (16). The power train ECM (15) also uses input from the engine ECM (19) via the CAT data link.

Transmission Direction and Speed Control Lever

Illustration 6	g01420572
Transmission direction and speed control lever

Illustration 7	g00887024

Direction switches

In normal operation, the switches for the transmission direction and speed control lever (1) close the direction inputs on the ECM to ground. The closed switches tell the ECM the direction that is selected by the operator. The ECM then activates the appropriate directional solenoid.

Note: Because three input lines are used for each direction, the ECM can diagnose problems with the circuits for the direction lever. A solenoid for direction is not activated and the transmission remains in neutral if the following actions occur:

• The direction lever does not ground any of the forward inputs or reverse inputs of the ECM.

• The direction lever grounds more than one input at the same time.

Speed switches

In normal operation, the switches for the speed close the speed inputs on the ECM to ground. The closed switches tell the ECM the speed that is selected by the operator. The ECM then activates the appropriate solenoid for the speed that is selected.

ReferenceFor more information on the function of the transmission direction and speed control lever, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Ride Control Switch

Illustration 8	g01424528
Ride Control Switch (7) + 24 volts DC (9) − Ground (8) − Ground (10) + 24 volts DC

The ride control switch signals the mode that is selected by the operator to the ECM. The operator may select the AUTO mode, the ON mode or the OFF mode.

Note: A double-pole single-throw switch is used for diagnostic purposes. If the two poles are ever tied to + battery voltage, the control knows that a circuit failure is present. This fault is then shown on the diagnostic indicator.

The ride control switch informs the power train electronic control module (ECM) of the ride control mode that is selected. There are three ride control modes: AUTO, ON and OFF.

The ride control switch has two input connections to the power train ECM at connector J2: input 1 at contact 46 and input 2 at contact 45. When the operator selects the AUTO position, input 1 is closed to ground and input 2 is open. When the ON position is selected, input 1 is open and input 2 is closed to ground. When the OFF position is selected, input 1 and input 2 are closed to ground. The valid combinations for each switch position are shown in Table 2 that follows.

ReferenceFor more information on the ride control function, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Parking Brake Switch

Illustration 9	g01424587
Parking Brake Switch (1) Common (2) Normally closed (3) Normally open

The parking brake switch is used in order to signal the status of the parking brake to the ECM. If the parking brake is applied and the machine is in first speed, the machine remains in neutral. If the machine is shifted into first speed and the parking brake is applied, the machine will stay in neutral.

Note: If the machine is shifted into any speed twice the machine will go into the selected speed with the parking brake engaged.

When the parking brake is applied and the machine speed is above a preset speed, the transmission will automatically downshift in order to slow the machine.

The parking brake pressure switch is an input of the power train electronic control module (ECM). This switch informs the power train ECM when the parking brake is engaged.

The parking brake pressure switch is a double-pole, single-throw pressure switch. When the operator disengages the parking brake, the normally open (N/O) pole opens and the normally closed (N/C) pole closes to ground. This tells the power train ECM that the parking brake is disengaged. When the parking brake is engaged, the N/O pole is closed and the N/C pole is open. This tells the power train ECM that the parking brake pressure switch circuit is operating correctly and that the parking brake is engaged. No action is required. The N/C pole connects to connector contact J2-53 of the power train ECM. The N/O pole connects to connector contact J2-52 of the power train ECM. The common terminal of the switch connects to ground. The valid combinations for each switch position are shown in Table 3 that follows.

Note: The normally closed (N/C) circuit and normally open (N/O) circuit of the parking brake pressure switch are used for diagnostic purposes. A circuit failure is present if the two circuits are open at the same time or if the two circuits are grounded at the same time. This fault is then shown on the display of the power train ECM.

ReferenceFor more information on the diagnostic operations, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the parking brake function, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Pressure Switch (Primary Steering)

Illustration 10	g00287701
Primary Steering Pressure Switch (A) Common (B) Normally open (C) Normally closed

The primary steering pressure switch is an input of the power train electronic control module (ECM). This switch informs the power train ECM of the status of the primary steering system.

The primary steering pressure switch is a double-pole, single-throw pressure switch. When the primary steering pressure drops below the threshold pressure of 700 ± 100 kPa (101.5 ± 14.5 psi), the primary steering pressure switch will change. The power train ECM will activate the warning indicator for the primary steering. The ECM will engage the secondary steering system. When the primary steering system functions correctly, the N/O pole is closed and the N/C pole is open. No action is required. The N/C pole connects to connector contact J2-38 of the power train ECM. The N/O pole connects to connector contact J2-37 of the power train ECM. The common terminal of the switch connects to ground. The valid combinations for each switch position are shown in Table 4 that follows.

Note: The normally closed (N/C) circuit and normally open (N/O) circuit of the primary steering pressure switch are used for diagnostic purposes. A circuit failure is present if the two circuits are open at the same time or if the two circuits are grounded at the same time. This fault is then shown on the display of the power train ECM.

ReferenceFor more information on the diagnostic operations, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the primary steering system, refer to Systems Operation, "Steering System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Pressure Switch (Secondary Steering)

Illustration 11	g00287701
Secondary Steering Pressure Switch (A) Common (B) Normally open (C) Normally closed

The secondary steering pressure switch is an input of the power train electronic control module (ECM). This switch informs the power train ECM of the status of the secondary steering system.

The secondary steering pressure switch is a double-pole, single-throw pressure switch. When the secondary steering pressure switch indicates pressure in the secondary steering system, the secondary steering pressure switch will change. This will send a signal to the power train ECM. The power train ECM will illuminate the warning indicator for the secondary steering and the power train ECM will set the EMS warning level to Category 3. When the secondary steering system functions correctly, the N/O pole is closed and the N/C pole is open. The N/C pole connects to connector contact J2-61 of the power train ECM. The N/O pole connects to connector contact J2-40 of the power train ECM. The common terminal of the switch connects to ground. The valid combinations for each switch position are shown in Table 4 that follows.

Note: The normally closed (N/C) circuit and normally open (N/O) circuit of the secondary steering pressure switch are used for diagnostic purposes. A circuit failure is present if the two circuits are open at the same time or if the two circuits are grounded at the same time. This fault is then shown on the display of the power train ECM.

ReferenceFor more information on the diagnostic operations, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the secondary steering system, refer to Systems Operation, "Secondary Steering System" for the machine that is being serviced.

ReferenceFor more information on the function of the power train ECM, refer to Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Sensors

Sensors provide information to the power train electronic control module (ECM) about changing conditions. The sensor signal changes proportionally to the changing conditions. The following type of sensor signals are recognized by the power train ECM:

• Frequency signals: The frequency (Hz) of the sensor signal varies as the condition changes.

• Pulse width modulated signals (PWM): The duty cycle of the sensor signal varies as the condition changes. The frequency of this signal is constant.

Illustration 12	g00288430
Pulse Width Modulated Signal

Illustration 13	g01434622
(2) Intermediate speed sensor (3) Intermediate speed sensor (4) Output speed sensor (5) Output speed sensor (6) Transmission oil temperature sensor (7) Transmission input speed sensor

Speed Sensors (Transmission and Torque Converter Output)

Illustration 14	g00288428
Speed Sensor

There are two transmission speed sensors and one torque converter output speed sensor on the machine. The speed sensors are inputs of the power train ECM. These speed sensors are frequency sensors. Frequency sensors produce a signal (Hz) which varies as the condition changes. The sensor generates an approximate sine wave signal from the gear teeth as the gear teeth pass the sensor. The sensor produces a signal that equals one pulse per gear tooth. This signal is sent to the power train ECM. The power train ECM measures the frequency of the signal in order to determine the speed. In the case of dual sensors, the power train ECM receives signals from the speed sensors and the ECM uses the difference in input from the speed sensors in order to determine the direction of the system. The power train ECM uses the input from the speed sensors in order to regulate transmission shifts. Each speed sensor has two connections to the power train ECM (+ and −).

The power train ECM sends the speed information to other electronic control modules via the CAT Data Link. For all of the speed sensors, connector contact 2 is the signal line and connector contact 1 is the return line. Two intermediate speed sensors (if equipped) measure intermediate transmission speed. Two transmission speed sensors measure transmission output speed. The power train ECM uses the pairs of speed sensors to determine the direction of rotation.

Note: The speed sensors are used to diagnose each other during normal operation. The power train ECM periodically checks the value from the speed sensor. If an incorrect value is found, the power train ECM will log a service code that indicates a fault for a speed sensor circuit.

ReferenceFor more information on diagnostic operations for the power train electronic control system, refer to Systems Operation, "Power Train Electronic Control System" for the machine that is being serviced.

ReferenceFor more information on the CAT Data Link, refer to the Service Manual module Systems Operation, "CAT Data Link" for the machine that is being serviced.

ReferenceFor more information on the power train electronic control module (ECM), refer to the Service Manual module Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Temperature Sensor (Transmission Oil )

Illustration 15	g01420614
Transmission Oil Temperature Sensor (1) Signal wire (2) Ground wire

The transmission oil temperature sensor is a passive sensor. As the temperature of the oil changes, the resistance in the sensor will change. This change in resistance changes the voltage that is seen by the power train ECM for the sensor. The power train ECM measures transmission oil temperature from the voltage of the transmission oil temperature sensor. The power train ECM uses the oil temperature reading in order to adjust the shift times of the transmission. The power train ECM adjusts the shift times of the transmission in order to provide smooth shifts over the various ranges of the transmission oil temperature.

ReferenceFor more information on the operation of the machine, refer to the Service Manual module Operation and Maintenance Manual for the machine that is being serviced.

ReferenceFor more information on the power train electronic control module (ECM), refer to the Service Manual module Systems Operation, "Electronic Control Module (Power Train)" for the machine that is being serviced.

Position Sensor (Left Pedal)

Illustration 16	g01420617
Position Sensor (Left Brake Pedal)

The position sensor (left brake pedal) sends an input to the ECM. The sensor will continuously update the power train ECM on the position of the left pedal. The left pedal position sends a change in the input signal to the power train ECM. The ECM records the change of position. Then, the ECM activates the appropriate solenoid valve.

The position sensor (left brake pedal) is a PWM sensor. The sensor continuously generates a PWM signal. The duty cycle varies in proportion to the position of the lever. The ECM receives the PWM signal from the position sensor. The ECM measures the duty cycle in order to determine the position of the lever. The frequency of the sensor signal is constant at 500 Hz. The machine electrical system provides +battery voltage to the lever position sensor for operating power.

Other Inputs

The power train ECM also receives information from the service connector (19) via the CAT data link (18) and engine ECM (21) via the CAT data link.