G3520B Industrial Engine Caterpillar

The oxygen sensor is used by the oxygen buffer in order to measure the concentration of oxygen in the engine exhaust.

The oxygen buffer receives power from terminal P2-13 on the Electronic Control Module (ECM). The return circuit is through terminal P2-23. The buffer provides a power supply of 1.5 amperes and 10.5 volts between the positive VH and negative VH terminals in order to power a resistive type of heater in the oxygen sensor. After this heater raises the temperature of the sensing element to 700 °C (1292 °F), the buffer regulates the current from the positive IP terminal to the negative IP terminal. This keeps the voltage between the positive VS terminal and negative VS terminal relatively constant.

The amount of current is a measure of the concentration of oxygen in the exhaust gases. The current will vary from 0 mA at 0 percent of oxygen to approximately 8.7 mA at 21 percent of oxygen. The oxygen buffer measures this current. The buffer converts the current to a Pulse Width Modulated signal (PWM) with a nominal frequency of 500 Hz. This signal is supplied to terminal P2-67.

The duty cycle of the PWM signal has a range of 30 to 60 percent. The 30 percent corresponds to 0 percent of oxygen and the 60 percent corresponds to 21 percent of oxygen. These values vary slightly between different buffers and sensors. Calibration is required for every new buffer. Calibration of the buffer is also required after every 1500 service hours. Some applications may require more frequent calibration or less frequent calibration.

The oxygen buffer is equipped with a diagnostic circuit that detects the current which is provided to the heating element of the oxygen sensor. If this current is too low, the buffer assumes that the oxygen sensor has failed or the sensor is disconnected from the oxygen buffer. This will cause the PWM signal from the oxygen buffer to change to a duty cycle of approximately 15 percent. The ECM interprets this condition as a diagnostic condition. The ECM will generate a diagnostic code that can be viewed with the Caterpillar Electronic Technician (ET).

The sensor provides a wet reading that is slightly lower than a dry reading. The wet reading is multiplied by a conversion factor of approximately 1.25 in order to obtain the reading that is displayed on Cat ET. This allows oxygen readings from Cat ET to be compared to dry readings from a meter such as a Teledyne meter. The ECM uses the signal of the percent of oxygen for two functions:

• Make corrections for variation of fuel BTU.

• Maintain desired emissions levels.

If an incorrect signal is detected by the ECM, a diagnostic code is generated and compensation for fuel quality is disabled. Problems with the circuit for the oxygen signal must be repaired as soon as possible because of the effects on engine performance and emissions.

Illustration 1	g01090820
Schematic of the circuit for the oxygen sensor and the oxygen buffer

Test Step 1. Inspect the Electrical Connectors and Wiring

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During operation, the oxygen sensor may reach temperatures that exceed 700 °C (1292 °F). Severe personal injury and property damage will result from contact with a hot oxygen sensor. Do not touch the sensor during engine operation, calibration, or testing. Allow the sensor to cool before moving the sensor. Wear gloves that resist heat. Do not place the sensor on or near any flammable material or any surface that can be damaged by high temperatures.

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Illustration 2	g01090824
Terminal box (1) Master ECM J2/P2 connectors (2) J8/P8 connectors for the harness from the oxygen buffer

1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

Note: For the following steps, refer to Troubleshooting, "Inspecting Electrical Connectors".

2. Thoroughly inspect the following connections:
   • J2/P2 connectors

   • J8/P8 connectors on the terminal box

   1. Check the torque of the allen head screw for the ECM connector. The proper torque is 6 ± 1 N·m (55 ± 9 lb in).
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   Illustration 3	g01075280
   Harness side of the P2 connector (P2-13) +24 VDC for the buffer (P2-23) Return (P2-53) Shield (P2-67) Oxygen signal (PWM)

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   Illustration 4	g01090933
   Harness side of the P8 connector (P8-C) +24 VDC for the buffer (P8-H) Oxygen signal (PWM) (P8-P) Return (P8-Z) Shield

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   Illustration 5	g01075432
   Harness connector for the oxygen sensor (A) VH+ (B) VH- (C) Current input + (E) Output V+ (F) Output V-

   2. Perform a 45 N (10 lb) pull test on each of the wires that are associated with the circuit for the oxygen sensor.
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   Illustration 6	g01090834
   Left side view (3) Oxygen sensor (4) Connectors for the oxygen sensor (5) Oxygen buffer (6) P651 connectors for the harness from the terminal box (7) P652 connectors for the harness to the oxygen sensor

   3. Check the harness and wiring for abrasion and for pinch points from the oxygen sensor to the ECM.

Expected Result:

All of the connectors, pins, and sockets are connected properly. The connectors and the wiring do not have corrosion, abrasion, or pinch points.

Results:

• OK - All of the connectors, pins, and sockets are connected properly. The connectors and the wiring do not have corrosion, abrasion, or pinch points. The components are in good condition with proper connections. Proceed to Test Step 2.

• Not OK - At least one of the connectors, pins, or sockets are not connected properly. At least one of the connectors and/or the wiring has corrosion, abrasion, and/or pinch points.

  Repair: Perform the necessary repairs and/or replace parts, if necessary.

  Stop.

Test Step 2. Measure the Supply Voltage for the Heater at the Harness Connector for the Oxygen Sensor

1. Connect Cat ET to the service tool connector.

2. Disconnect the oxygen sensor from the engine harness.

3. Restore the electrical power to the engine. Set the engine control to the STOP mode.

4. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".

5. Select "Change" in the lower left corner of the screen.

6. Select a "New Value" of "On" and then select "OK".

   The oxygen buffer is now ON. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.

   Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START mode.

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Illustration 7	g01075432
Harness connector for the oxygen sensor (A) VH+ (B) VH-

7. Measure the voltage between terminals A and B on the harness connector for the oxygen buffer.

Expected Result:

The voltage is greater than 9 VDC.

Results:

• OK - The voltage is greater than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is present at the harness connector. Proceed to Test Step 3.

• Not OK - The voltage is less than 9 VDC. The supply voltage for the heater of the oxygen sensor is not present at the harness connector. Proceed to Test Step 6.

Test Step 3. Check for Diagnostic Codes

1. Access the "Logged Diagnostic Codes" screen on Cat ET. If the following codes are logged, clear the codes:
   • 1086-09 Oxygen Sensor Element not communicating on link

   • 1087-03 Oxygen Buffer Module short to +batt

   • 1087-08 Oxygen Buffer Module noisy signal

   • 1088-05 Oxygen Sensor Power Supply open circuit

   • 1088-06 Oxygen Sensor Power Supply short to ground

2. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".

3. Select "Change" in the lower left corner of the screen.

4. Select a "New Value" of "On" and then select "OK".

   The oxygen buffer is now ON. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.

   Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START mode.

5. Access the "Logged Diagnostic Codes" screen again. Look for the following diagnostic codes again: 1086-09, 1087-03, 1087-08, 1088-05 and 1088-06.

Expected Result:

At least one of the above diagnostic codes is logged.

Results:

• Logged 1086-09 - A 1086-09 diagnostic code is logged.

  Repair: Perform the following steps:

  1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

  2. Inspect the connections and the harness from the oxygen buffer to the oxygen sensor. Look for corrosion and other damage. Refer to Troubleshooting, "Inspecting Electrical Connectors".

  3. Make repairs, as needed. If the connectors and the harness appear to be OK, install an oxygen sensor that is known to be good. Calibrate the oxygen sensor. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".

  4. Verify that the condition is resolved with the new oxygen sensor.

     If the condition is not resolved, replace the oxygen buffer. Be sure to calibrate the oxygen sensor after you replace the buffer. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".

  Stop.

• Logged 1087-03 or 1087-08 Diagnostic Code - The PWM signal is noisy or the signal is missing. Proceed to Test Step 4.

• Logged 1088-05 or 1088-06 Diagnostic Code - There is a problem with the supply voltage for the oxygen buffer.

  Repair: Refer to Troubleshooting, "Oxygen Sensor Buffer Supply".

  Stop.

• No Codes - The circuit for the oxygen sensor is OK. Stop.

Test Step 4. Check the Harness for Open Circuits and Short Circuits

1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

2. Disconnect the J2/P2 connectors and the J651/P651 connectors. Thoroughly inspect the connectors for corrosion and/or for moisture. Make repairs, if necessary.

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Illustration 8	g00895813
P651 harness connector for the terminal box to the oxygen buffer (P651-B) Return (P651-C) Oxygen signal (PWM)

3. Install the ends of a jumper wire into terminals P651-B and P651-C.

4. Check for continuity between the terminals that are listed in Table 1.
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   Table 1

   Points to Check for Continuity
   P2-67 Oxygen signal (PWM)	P2-23 Return
   P2-67 Oxygen signal(PWM)	Ground strap for the ECM
   	All of the remaining terminals in the P2 connector

Expected Result:

The wiring between terminals P2-67 and P2-23 has continuity. All of the other continuity checks indicate an open circuit.

Results:

• OK - The wiring between terminals P2-67 and P2-23 has continuity. All of the other continuity checks indicate an open circuit. Proceed to Test Step 5.

• Not OK - At least one of the continuity checks is incorrect. There is a problem with the harness between the terminal box and the oxygen buffer.

  Repair: Repair the harness, when possible. Replace parts, if necessary.

  Stop.

Test Step 5. Isolate the Oxygen Signal and Check the Signal at the ECM

1. Remove the jumper wire from the P651 connector. Reconnect the J2/P2 and J651/P651 connectors.

2. Use a 151-6320 Wire Removal Tool to remove terminal 67 from the P2 connector.

3. Use a multimeter that is capable of measuring the duty cycle and the frequency of a signal. Connect one lead of the multimeter to the terminal that was removed from P2-67.

4. Insert a 7X-1710 Multimeter Probe along terminal P2-23 of the ECM P2 connector. Connect the other multimeter lead to the 7X-1710 Multimeter Probe .

5. Restore the electrical power to the engine. Set the engine control to the STOP mode.

6. Access the "Diagnostic Override" screen of the Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".

7. Select "Change" in the lower left corner of the screen.

8. Select a "New Value" of "On" and then select "OK".

   The oxygen buffer is now ON. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.

   Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START mode.

9. Measure the signal between the wire that was removed from terminal 67 and terminal P2-23.

Expected Result:

The duty cycle of the signal is between 20 and 90 percent.

The frequency of the signal is between 375 and 625 Hz.

Results:

• OK - A valid oxygen signal is present at terminal P2-67.

  Repair: Perform the following steps:

  1. Set the engine control to the OFF/RESET mode.

  2. Reinstall terminal 67 into the P2 connector. Pull on the wire in order to verify proper installation of the terminal.

  3. Remove the 7X-1710 Multimeter Probe from terminal P2-23.

  4. Set the engine control to the STOP mode. Clear all of the logged diagnostic codes.

  5. Turn ON the oxygen buffer. Use Cat ET to check for logged 1087-03 and 1087-08 diagnostic codes.

  6. If either of the codes are present, replace the ECM according to Troubleshooting, "Replacing the ECM".

  Stop.

• Not OK - A valid oxygen signal is not present at P2-67.

  Repair: Perform the following steps:

  1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

  2. Reinstall terminal 67 into the P2 connector. Pull on the wire in order to verify proper installation of the terminal.

  3. Remove the 7X-1710 Multimeter Probe from terminal P2-23.

  4. Install an oxygen buffer that is known to be good.

  5. Verify that the condition has been resolved. If the condition is not resolved, replace the oxygen sensor. Be sure to calibrate the oxygen sensor after you replace the buffer and/or the sensor. Refer to Troubleshooting, "Oxygen Sensor - Calibrate".

  Stop.

Test Step 6. Measure the Supply Voltage for the Heater at the Oxygen Buffer

1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

2. Disconnect the P652 connector from the oxygen buffer.

3. Restore the electrical power to the engine. Set the engine control to the STOP mode.

4. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".

5. Select "Change" in the lower left corner of the screen.

6. Select a "New Value" of "On" and then select "OK".

   The oxygen buffer is now ON. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.

   Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START mode.

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Illustration 9	g01090945
J652 connector on the oxygen buffer (J652-A) VH+ (J652-B) VH-

7. Measure the voltage between terminals J652-A and J652-B on the oxygen buffer.

Expected Result:

The voltage is greater than 9 VDC.

Results:

• OK - The voltage is greater than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is present at the oxygen buffer. There is a problem with the harness between the oxygen buffer and the oxygen sensor.

  Repair: Repair the harness, when possible. Replace parts, if necessary.

  Stop.

• Not OK - The voltage is less than 9 VDC. The correct supply voltage for the heater of the oxygen sensor is not present at the oxygen buffer. Proceed to Test Step 7.

Test Step 7. Check the Supply Voltage for the Oxygen Buffer

1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

2. Disconnect the P651 connector from the oxygen buffer.

3. Restore the electrical power to the engine. Set the engine control to the STOP mode.

4. Access the "Diagnostic Override" screen of Cat ET. Select "Diagnostics/Diagnostic Tests/Override Parameters".

5. Select "Change" in the lower left corner of the screen.

6. Select a "New Value" of "On" and then select "OK".

   The oxygen buffer is now ON. The status of the "Oxygen Sensor Override" should change to "On". Wait for 65 seconds.

   Note: After you exit the "Override Parameters" screen, the status of the "Oxygen Sensor Override" is automatically changed to "OFF" when the engine control is not in the START mode.

7. Measure the voltage between terminals P651-A and P651-B.

Expected Result:

The voltage is greater than 18 VDC.

Results:

• OK - The oxygen buffer is receiving the correct supply voltage. However, the buffer is not creating the voltage for the oxygen sensor.

  Repair: Perform the following procedures:

  Reconnect the J651/P651 connectors. Repeat Test Step 2. If the condition is not resolved, replace the oxygen buffer. Be sure to calibrate the oxygen sensor after you replace the buffer.

• Not OK - The oxygen buffer is not receiving the correct supply voltage. Proceed to Test Step 8.

Test Step 8. Check the Supply Voltage at the ECM

1. Set the engine control to the OFF/RESET mode. Remove the electrical power from the engine.

2. Use a 151-6320 Wire Removal Tool in order to remove terminals 53 and 67 from the P2 connector.

3. Set the engine control to the STOP mode. Restore the electrical power to the engine.

4. Measure the voltage between terminals J2-53 and J2-67.

Expected Result:

The voltage is greater than 18 VDC.

Results:

• OK - The voltage is greater than 18 VDC. The ECM is creating the correct voltage. However, the voltage is not at the oxygen buffer. There is an open circuit between the ECM and the oxygen buffer.

  Repair: The open circuit may be inside the terminal box. Alternatively, the open circuit may be in the engine harness. There may be a problem with the J8/P8 connectors. Repair the harness, when possible. Replace parts, if necessary.

  Stop.

• Not OK - The voltage is less than 18 VDC. The ECM is not creating the correct voltage.

  Repair: It is unlikely that the ECM has failed. Exit this procedure and perform this procedure again. If the problem is not resolved, perform the following steps:Temporarily install a new ECM. Refer to Troubleshooting, "Replacing the ECM".If the problem is resolved with the new ECM, install the original ECM and verify that the problem returns. If the new ECM operates correctly and the original ECM does not operate correctly, replace the original ECM. Refer to Troubleshooting, "Replacing the ECM".

  Stop.