795F AC Off-Highway Truck:Frequency Input Sensors

Traction Motor Speed Sensors


Traction Motor 1 speed sensor connections and Traction Motor 2 speed sensor connections

Note: The speed sensor 1 circuit numbers change when passing through the Inverter Cabinet through wall connector CN1 / CX-C3. The speed sensor 2 power supply circuit numbers change passing through the MC-C20 / CX-C6 connector that comes out of the cab. Use the connection diagram in this procedure or the system schematics in the back of this manual to insure that the correct circuits are followed when passing through these connectors.


Traction Motor 1 speed sensors (view: inside the rear axle housing looking at the non-drive end of Drive Motor 1) (3) Drive Motor Speed Sensor 1 Signal 1(4) Drive Motor Speed Sensor 1 Signal 2

Each traction motor is equipped with two identical speed sensors that are mounted on the non-drive end (NDE) of the motor.

Each speed sensor is a dual signal speed sensor. Each speed sensor provides two signal inputs to the ECM. Two signals enable the ECM to determine the direction of motor rotation in addition to the motor speed.

Since the two traction motors are identical, the location of the speed sensor 1 and speed sensor 2 on the non-drive end are reversed when looking at one motor as compared to the other motor.

For each traction motor, speed sensor 1 will provide two input signal circuits to the respective motor control ECM.

For each traction motor, speed sensor 2 will provide two input signal circuits to the Drivetrain ECM. The speed sensor signals allow the Drivetrain ECM instantly determine the speed of each traction motor and the rear wheel speed for control purposes.

The Drivetrain ECM will use the motor speed and direction information along with other system input information in order to determine the torque commands that will be sent to each motor control ECM.

For each traction motor, both speed sensors are mounted slightly above a gear that is mounted on the non-drive end of the motor shaft. As a gear tooth passes below the sensor, two hall effect elements in the sensor will pull each of the normally high sensor signals low. Two identical square wave outputs are produced. The speed sensor signal B square wave is phase shifted 90 ± 45 degrees in reference to the signal A square wave signal.

Each ECM will monitor the speed sensor signal A in order to determine the rotational speed of the motor. The ECM will monitor the frequency of the square wave signal to calculate the motor speed and the speed of the rear wheel sets.

Each ECM will monitor the speed sensor signal B in order to determine the direction of rotation of the motor. The signal B square wave will either lead or lag the signal A square wave. A leading signal B square wave will indicate rotation in one direction. A lagging signal B square wave will indicate rotation in the other direction.

A test procedure is available that will help to determine if a motor speed sensor has failed. Refer to the Testing And Adjusting, "Motor speed Sensor - Test" section in this manual for the test procedure.

Front Wheel Speed Sensors

Front wheel speed sensor location on the top of each front wheel axle housing

Each front wheel assembly is equipped with a dual signal speed sensor. the speed sensor is mounted on the top side of each front wheel axle housing.

The front wheel speed sensors are identical to the traction motor speed sensors. For an explanation of how the sensors operate, refer to the prior section for the "Traction Motor Speed Sensors".

Each speed sensor will provide two square wave input signals to the Drivetrain ECM.

The Drivetrain ECM will use the wheel speed and direction information to determine the travel speed of the truck.

Each speed sensor provides two signal inputs to the ECM. The two signals enable the ECM to determine the direction of wheel rotation in addition to the wheel speed.

Engine Speed Sensor

Circuit connections for the Engine Speed Sensor


Location of the Engine Speed Sensor (arrow) mounted on the lower rear left-hand side of the Engine

The Engine Speed Sensor is a two wire speed sensor that provides a frequency input to the Drivetrain ECM and the Brake ECM.

The Drivetrain ECM will use the input signal from the speed sensor to determine the rotational speed of the Engine and the Generator.

During sensor operation, as a gear tooth passes below the sensor, a magnetic element in the sensor will induce a voltage across the sensor contacts. The voltage produces one alternating cycle for each gear tooth that passes under the sensor tip. The ECM will monitor the frequency of the alternating cycles to calculate the speed of the Engine and the Generator.

When the truck is in a travel mode, the Drivetrain ECM will use this information to send speed requests to the Engine ECM. The request ensures that the speed of the Engine and the Generator are sufficient to meet the system demands.

The ECM will also use the Engine speed information and the Generator output status to determine the speed and torque commands that will be sent to each motor control ECM for traction motor control.

The speed sensor is a two wire sensor with a screw type mount. When mounted and locked in place with the locking nut, a minimum air gap of 0.5 mm (0.02 inch) must be maintained. Adjust the sensor by screwing the sensor in to contact with the gear tooth. Back the sensor out approximately 1.5 turns. Securely tighten the lock nut to lock the sensor in place.

The DC resistance range of the sensor between contact 2 to contact 1 is 110 ohms to 200 ohms at 25.0° C (77.0° F).

Electric Drive Cooling Fan Speed Sensor


Location of the Electric Drive Cooling Fan Speed Sensor (arrow) located on the hydraulic fan motor that is under the main air duct


Electric Drive Cooling Fan Speed Sensor connections

The Drivetrain ECM will use the Electric Drive Cooling Fan Speed Sensor to monitor and control the speed of the hydraulic motor and the fan.

The speed sensor is a two wire sensor with a screw type mount. When mounted and locked in place with the locking nut, a minimum air gap of 0.9 mm (0.04 inch) must be maintained. Adjust the sensor by screwing the sensor in to contact the gear tooth. Then, back the sensor out approximately 1.75 turns and securely lock the sensor in place.

The DC resistance range of the sensor between contact A and contact B is 1000 ohms to 1200 ohms at 25.0° C (77.0° F).

MARYGAR