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The Power Train Electric Drive System will contain hazardous voltage levels during machine operation and for a short period of time after engine shutdown. Do not remove any covers that will expose energized high voltage electrical components while the engine is operating. Any type of maintenance on the following components can only be performed after the Power Train Electrical System Service Shutdown procedure has been followed:
Failure to follow these instructions could result in personal injury or death. |
This procedure will instruct the user about the steps required to check the operation of the "Electric Drive Traction Motor Speed" sensors. the speed sensors are used to monitor the speed and direction of the traction motors. This procedure can also be used to check the operation of the "Front Wheel Speed" sensors. Both applications use the same model of dual input speed sensor.
This procedure should be performed when a dual input speed sensor is suspected to be faulty. Perform the procedure when a speed sensor-related diagnostic code is active.
Note: This procedure will not require the truck to be operated to check the status of an active speed sensor diagnostic code. However, when a diagnostic code for the speed sensor is activated and a reset is required, the truck must be driven at a travel speed greater than
If this procedure is performed and the speed sensor operation is verified to be correct, refer to the appropriate troubleshooting procedure that referred you to this test procedure. The following diagnostic codes are assigned to dual input speed sensors:
- CID 3271 - Drive Motor 1, Speed Sensor Signal 1 (A). Can be activated by either a motor ECM ("Motor Speed Sensor 1") or the Drivetrain ECM ("Motor Speed Sensor 2").
- CID 3272 - Drive Motor 1, Speed Sensor Signal 2 (B). Can be activated by either motor ECM ("Motor Speed Sensor 1") or by the Drivetrain ECM ("Motor Speed Sensor 2").
- CID 3273 - Drive Motor 2, Speed Sensor Signal 1 (A). Can be activated only by the Drivetrain ("Motor Speed Sensor 2") ECM.
- CID 3274 - Drive Motor 2, Speed Sensor Signal 2 (B). Can be activated only by the Drivetrain ECM.
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| Illustration 1 | g01824374 |
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Typical view of the traction motor speed sensors (view: inside the rear axle housing looking at the non-drive side (rear) of the left-hand side "Drive Motor 1") (1) Electric Drive Traction Motor 1, Speed Sensor 1 (2) Electric Drive Traction Motor 1, Speed Sensor 2 | |
The left and right front wheel speed sensors are active frequency sensors. The signal indicates the rotational speed of the rotor in the motor. The signal is generated by a gear passing in front of the sensor. One full pulse is generated per tooth of the gear. The signal is low when a gear tooth is in front of the sensor and high when in a valley.
The following diagnostic codes are assigned to dual input speed sensors:
- CID 3488 - "Left Front Wheel Speed Sensor Signal" Line 1 (A). Can be activated only by the Drivetrain ECM.
- CID 3489 - "Left Front Wheel Speed Sensor Signal" Line 2 (B). Can be activated only by the Drivetrain ECM.
- CID 3490 - "Right Front Wheel Speed Sensor Signal" Line 1 (A). Can be activated only by the Drivetrain ECM.
- CID 3491 - "Right Front Wheel Speed Sensor Signal" Line 2 (B). Can be activated only by the Drivetrain ECM.
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| Illustration 2 | g06360916 |
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Location of the "Front Wheel Speed" sensor (with the wheel cap removed) | |
Note: Facing the non-drive end (rear) of either traction motor, the "Speed Sensor 1" is always at the left-hand position. The "Speed Sensor 2" is always at the right-hand position.
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| Illustration 3 | g06361019 |
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Front and side view of "Front Wheel Speed" sensors for the 794F AC. The "Allen" screws (top and bottom in image) secure the sensor inside the hub. | |
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| Illustration 4 | g06364446 |
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Image of the "Amphenol" male and female connectors. (1) Male Connector (2) Female Connector (A) Voltage (B) Ground (C) Signal A (D) Signal B | |
The front wheel speed sensors are connected to the wiring harness using "Amphenol" connectors. The connectors are joined by threaded rings that secure the pins inside the sockets.
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| Illustration 5 | g01392482 |
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The electrocution hazard safety message is present at all compartment covers and component covers where high voltage could be present. The "Power Train Electrical System" Service Shutdown procedure must be performed before a cover is removed from any component or compartment where this safety message is present. | |
Note: The Electrical System Service Shutdown procedure is not required if this procedure is used to check the operation of the front wheel speed sensors.
Note: This procedure may require entries into the rear axle housing to access the rear motor speed sensors. Before this test procedure can begin, the service shutdown procedure must be performed to verify that the voltage in the power train system components is discharged.
Refer to the Troubleshooting, "Electrical Shutdown and Voltage Discharge" section of this manual to perform the Power Train Electrical System Service Shutdown procedure.
In this test, the suspect speed sensor will be removed from the mount on the rear of the motor or the mount on the front tire. System voltage will be supplied to the sensor while monitoring the sensor signals to determine if the sensor signal performs as expected.
Once the "Power Train Electrical System Service" Shutdown procedure has been performed and the voltage in the "Inverter Cabinet" is verified as properly discharged, proceed with this test.
- Verify that the key start switch and the battery disconnect switch are in the OFF position.
- Open the access cover for the rear axle housing or access the wheel speed sensor.
- Use the high-voltage meter to measure for voltage between the three-phase motor-high voltage connections at the rear of each of the motors. Verify that there is less than 50.0 VDC or no voltage present. Once the voltage level is verified, continue to the next step.
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Illustration 6 g01877265 Typical view of the rear electric drive traction motor speed sensor
(3) Sensor mounting bolts
(4) Connector bracket retaining bolt
(5) Machine harness connector for the speed sensor - Disconnect the four contact machine harness connector from the suspect speed sensor and remove the retaining bolt that secures the connector bracket to the housing.
- Remove the two 5 mm speed sensor mounting bolts.
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Illustration 7 g01877266 Removal of the motor speed sensor - Carefully remove the speed sensor from the mount by pulling evenly and out of the mounting hole.
Note: For the rear wheel sensors pull straight up and out of the mounting hole.
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Illustration 8 g01879439 Traction motor speed sensor removed - the sensor tip should be clean and undamaged - Examine the sensor measurement tip for any foreign material that could interfere with the proper operation of the speed sensor. The sensor tip should be clean.
- Examine the sensor measurement tip for damage. For traction motors, damage to the sensor tip can be caused by a motor bearing failure or other mechanical failures inside the motor housing. If the speed sensor is damaged, the cause of the damage must be investigated. Refer to the Disassembly and Assembly manual to check the motor shaft bearing runout and end play.
- If no obvious problems with the speed sensor, connect the sensor connector to the machine harness connector. The speed sensor will remain removed from the mount.
Note: In the next step, when the battery disconnect is in the ON position and the engine lockout control switch is moved to the ON position (lock out), the 24 volt control system voltage will be energized. The system voltage will energize regardless of the status of the key start switch.
- Energize the 24 volt control system by moving the battery disconnect to the ON position and the engine lockout control switch to the ON position. Lock and tag the switches accordingly.
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Illustration 9 g01877264 Electric Drive traction Motor Speed Sensor harness connections for rear speed sensors Show/hide table
Illustration 10 g06364446 "Amphenol" connectors used to connect the front wheel speed sensors to the wiring harness. Note: In the next step, if a speed sensor power supply problem is discovered, refer to the diagnostic troubleshooting procedure for the involved ECM speed sensor power supply. Investigate the cause of the power supply problem. To determine which ECM power supply is involved, refer to the "Systems Schematics" in the back of this manual. Then, refer to the Troubleshooting, "Diagnostic Code List" section of this manual to determine the CID of the ECM power supply troubleshooting procedure.
- Verify that system power is being supplied to the sensor. Using a digital multimeter with the "spoon" type 7X-1710 probes, insert the positive probe along the harness connector contact 1 or A (+ power supply). Insert the negative probe along the harness connector contact 2 or B (- power supply). The ECM power supply voltage should be 12.0 +/- 1.0 VDC for motor speed sensors and 10.0 +/- 0.7 VDC for wheel speed sensors.
- Remove the positive multimeter probe from contact 1 or A and insert the probe along the connector contact 3 or C (signal A). The negative multimeter probe remains inserted along connector contact 2 or B (- power supply).
- While observing the multimeter display, place a piece of ferrous (magnetic) metal on the tip of the sensor. Move the metal away and back to the tip of the sensor several times. The sensor signal should toggle between:
- 2.3 VDC and 0.5 VDC for a rear motor speed sensor.
- 3.5 VDC and 1.0 VDC with a 10K ohm circuit resistance for a front motor speed sensor circuit.
- 5.0 VDC and 0.5 VDC with a 2K ohm circuit resistance for a front motor speed sensor circuit.
- Remove the positive multimeter probe from contact 3 or C (signal A) and insert the probe along the connector contact 4 or D (signal B). The negative multimeter probe remains inserted along connector contact 2 or B (- power supply).
- Repeat step 13.
Expected Result:
The important detail is whether the sensor signal voltage changes between greater than the high voltage and less than the low voltage as the metal is moved in proximity of the sensor tip for each of the sensor signal circuits. The point at which the sensor registers a specific voltage is not important. The change of voltages indicates that the speed sensor is detecting the metal as the metal is passed close to the tip. The operation of the speed sensor is correct.
Result: - OK
The speed sensor changes voltage levels of greater than 8.5 VDC and less than 0.5 VDC as the metal is moved to the sensor tip and away from the sensor tip for each of the sensor signal circuits. The operation of the speed sensor is correct. The speed sensor is most likely not causing a problem.
Repair:
Turn the battery disconnect to the ON position and turn the engine lockout control switch to the OFF position (not locked out).
Install the speed sensor back on the original mount. The orientation of the speed sensor is important for proper operation. The bolt holes on the sensor mount are offset. The speed sensor bolt holes will line up properly when the speed sensor orientation is correct.
If a speed sensor diagnostic code is still active for the sensor under test, refer to the diagnostic code troubleshooting procedure for the speed sensor. Investigate other conditions that could be causing the active diagnostic code.
STOP.
Result: - NOT OK
Either one or both of the speed sensor signal circuits are not performing as expected. The speed sensor signal did not change as expected when the metal is moved in proximity of the sensor tip. The operation of the speed sensor is faulty.
Repair:
Replace the speed sensor. Install the new speed sensor where the faulty speed sensor was mounted. The orientation of the speed sensor is important for proper operation. The bolt holes on the sensor mount are offset. The speed sensor bolt holes will line up properly when the speed sensor orientation is correct.
Verify that the new speed sensor is securely mounted. Verify that the connector is connected to the harness connector. Verify that the connectors are properly fastened to the motor housing before checking the operation of the speed sensor.
STOP.