794 AC Off-Highway Truck VIMSTM 3G and Truck Payload Measurement System (TPMS) Caterpillar

Sensor Signal (PWM) - Test

Usage:

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Illustration 1	g03597477
Sensor circuit

Table 1 contains PWM sensor codes.

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Table 1
Communication Gateway #2 ECM (VIMS Application ECM) MID 162
DTC	Code Description	System Response
838-3	Left Front Strut Pressure Sensor - Voltage Above Normal	The Truck Payload Measurement System (TPMS) feature will not work properly.
838-4	Left Front Strut Pressure Sensor - Voltage Below Normal	The TPMS feature will not work properly.
838-8	Left Front Strut Pressure Sensor - Abnormal Frequency, Pulse Width, or Period	The TPMS feature will not work properly.
839-3	Right Front Strut Pressure Sensor - Voltage Above Normal	The TPMS feature will not work properly.
839-4	Right Front Strut Pressure Sensor - Voltage Below Normal	The TPMS feature will not work properly.
839-8	Right Front Strut Pressure Sensor - Abnormal Frequency, Pulse Width, or Period	The TPMS feature will not work properly.
840-3	Left Rear Strut Pressure Sensor - Voltage Above Normal	The TPMS feature will not work properly.
840-4	Left Rear Strut Pressure Sensor - Voltage Below Normal	The TPMS feature will not work properly.
840-8	Left Rear Strut Pressure Sensor - Abnormal Frequency, Pulse Width, or Period	The TPMS feature will not work properly.
841-3	Right Rear Strut Pressure Sensor - Voltage Above Normal	The TPMS feature will not work properly.
841-4	Right Rear Strut Pressure Sensor - Voltage Below Normal	The TPMS feature will not work properly.
841-8	Right Rear Strut Pressure Sensor - Abnormal Frequency, Pulse Width, or Period	The TPMS feature will not work properly.

Pulse Width Modulation (PWM) is a technique for controlling analog circuits with digital outputs. PWM is employed in various applications ranging from measurement to communication with the Electronic Control Module (ECM). The sensor varies the output frequency with a change in pressure. The pressure is programmed into the ECM software.

One of the advantages of the PWM sensor is that the signal is digital from the ECM to the controlled system. No digital-to-analog conversion is necessary. By using a digital signal, noise effects are minimized.

Possible FMI 03 causes are:

Failed sensor
Open sensor supply or ground circuit
Signal circuit shorted to +Battery
Signal circuit open or sensor disconnected
Failed ECM

Possible FMI 04 causes are:

Failed sensor
Signal circuit shorted to ground
Failed ECM

Possible FMI 08 causes are:

Failed sensor
Intermittent or poor connections

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Table 2
Troubleshooting Steps	Values	Results⁽¹⁾
1. Check for Diagnostic Codes A. Connect Cat^® Electronic Technician (Cat ET) to the machine. B. Check for diagnostic codes.	Code is not active	OK - The code is not active. STOP Not OK - The code is active. Go to Step 2.
2. Check Connectors A. Inspect the circuit harness connectors.	Connectors are: * Clean * Secure * In good condition	OK - Connectors are OK. FMI 03 diagnostic code, go to Step 3. FMI 04 diagnostic code, go to Step 7. FMI 08 diagnostic code, go to Step 9. Not OK - Connectors are not OK. Repair: A. Correct any connection problem. B. Verify that the problem is corrected. STOP
Begin FMI 03 Troubleshooting Here
3. Check The Control And The Harness A. Turn on the key start switch and disconnect switch. B. Disconnect the harness from sensor. C. Measure voltage between sensor supply and ground contacts at the sensor harness connector.	Voltage is correct⁽²⁾	OK - The voltage is correct. Go to Step 4. NOT OK - The voltage is not correct. Repair - Verify the code status.⁽³⁾ STOP
4. Check For A Sensor Circuit Open A. Verify that the sensor is disconnected from the harness. B. Turn the key start switch and the disconnect switch OFF. C. At the sensor harness connector, place a jumper wire between the ground contact and the signal contact. D. Disconnect J1 and J2 harness connectors from the ECM. E. At machine sensor harness connector, measure the resistance from the signal contact to the return contact.⁽⁴⁾ F. Repeat the process for the sensor connector.	< 5Ω	OK - The resistance is < 5Ω. The signal circuit and the ground circuits of the sensor are not open. Go to Step 5. NOT OK - The resistance is ≥ 5Ω. The circuit is open. Repair: Correct the open. STOP
5. Check Signal Circuit For A Short To +Battery A. Verify that the sensor and ECM are disconnected from the harness. B. Turn off the disconnect switch and the key start switch. C. Measure the resistance between the signal and +Battery contacts of the sensor harness connector. D. At harness connector J1 and J2, measure the resistance from signal contact to all possible sources of +Battery.	≥ 5KΩ	OK - The resistance is ≥ 5KΩ. The circuit is correct. Go to Step 6. NOT OK - The resistance is < 5Ω. A short exists in the harness between the +Battery and signal circuit. Repair: Repair or replace the harness. STOP
6. Check For An Open A. Verify that the disconnect switch and key start switch are OFF. B. Verify that the sensor and ECM are disconnected from the harness. C. Connect a jumper wire from the end of the signal wire at the ECM to a known ground. D. Measure the resistance of the signal wire at the ECM.	< 5Ω	OK - The resistance is < 5Ω. Go to Step 15. NOT OK - The resistance is ≥ 5KΩ. The signal wire is open. Repair: Repair or replace the harness. STOP
Begin FMI 04 Troubleshooting Here
7. Check The Sensor A. With FMI 04 active, disconnect the sensor from the harness. B. Observe Cat ET or the operator monitor for a code change as the sensor is disconnected and connected to the harness.	The code remains active	OK - The code remains active when sensor is disconnected. The sensor is not the cause of the problem. Go to Step 8. Not OK - The code changed to FMI 03 when the sensor was disconnected. Repair: Replace the sensor. STOP
8. Check The Signal Circuit For A Short To Ground A. Turn off the key start switch and the disconnect switch. B. Disconnect the J1 harness connector from the ECM. C. Measure the resistance between the sensor signal contact and frame ground.	≥ 5KΩ	OK - The resistance is ≥ 5KΩ. The harness circuit is correct. Go to Step 15. NOT OK - The resistance is < 5Ω. A short exists between frame ground and the signal circuit. Repair: Repair or replace the harness. STOP
Begin FMI 08 Troubleshooting Here
9. Check For An Open A. Turn off the disconnect switch and the key start switch. B. Verify that the sensor and ECM are disconnected from the harness. C. Connect a jumper wire from the end of the signal wire at the ECM to a known ground. D. Measure the resistance of the signal wire at the ECM.	< 5Ω	OK - The resistance is < 5Ω. Go to Step 10. NOT OK - The resistance is ≥ 5KΩ. The signal circuit is open. Repair: Repair or replace the harness. STOP
10. Check The Signal Circuit For A Short To Ground A. Turn off the key start switch and disconnect switch. B. Disconnect J1 harness connector from ECM. C. Measure the resistance between the signal contact for the sensor and frame ground.	≥ 5KΩ	OK - Resistance is ≥ 5KΩ. Go to Step 11. NOT OK - Resistance is < 5Ω. The signal circuit is shorted to ground. Repair: Repair or replace the harness. STOP
11. Check The Sensor For A Short To Case A. Turn off the key start switch and the disconnect switch. B. At the harness connector for the sensor, measure the resistance between each sensor pin and a good ground.	≥ 5KΩ	OK - Resistance is ≥ 5KΩ. Go to Step 12. NOT OK - A resistance is < 5Ω. The sensor has failed. Repair: A. Replace the sensor. B. Confirm that the new sensor corrects the problem. STOP
12. Check The Sensor For A Short To +Battery A. Disconnect the harness at the sensor. B. Turn on the key start switch and the disconnect switch. C. At harness connector for the J2 connector, measure the voltage between the sensor signal pin and frame ground.	0 VDC	OK - Voltage is 0 VDC. Go to Step 13. NOT OK - Voltage is present in circuit. Repair: Repair or replace the harness. STOP
13. Check The Harness For A Short To +Battery A. Turn on the key start switch and the disconnect switch. B. At the harness connector for the sensor, measure the voltage between each sensor pin frame ground.	0 VDC	OK - Voltage is 0 VDC. Go to Step 14. NOT OK - Voltage is present in circuit. The circuit is shorted to +Battery. Repair: A. Replace the sensor. B. Verify that the new sensor corrected the code. STOP
14. Check The Sensor A. Turn on the key start switch and the disconnect switch. B. Use Cat ET or the monitor to confirm the diagnostic code. C. Disconnect the sensor from the harness.	The code is not active	OK - The code is not active. The sensor has failed. Repair: A. Replace the sensor. B. Verify that the new sensor corrected the code. STOP NOT OK - The code remains active. Reconnect the sensor. Go to Step 15.
15. Check If The Diagnostic Code Remains A. Turn on the key start switch and the disconnect switch. B. Clear all diagnostic codes. C. Operate the machine. D. Stop the machine and engage the safety lock lever. E. Check the code.	The code is not active	OK - The code is not active. The initial code may have been caused by a poor connection or a short. Resume machine operation. STOP NOT OK - The code is active. Repair: Contact your Technical Communicator.⁽⁵⁾ STOP

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⁽¹⁾	In the following procedures: Resistance ≥ 5Ω but < 5KΩ indicates a loose or corroded connection, while ≥ 5KΩ indicates an open.
⁽²⁾	See the machine Electrical Schematic to determine the proper voltage.
⁽³⁾	See Troubleshooting, "Sensor Supply - Test".
⁽⁴⁾	Gently move the wires at the ECM connector while observing the resistance readings.
⁽⁵⁾	If ECM replacement is required, see Testing and Adjusting, "ECM - Replace".