G3520B Industrial Engine Caterpillar

The oxygen buffer uses an oxygen sensor in order to measure the concentration of oxygen in the engine exhaust.

The oxygen buffer receives power from terminal P2-13 on the 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 65 percent. The 30 percent corresponds to 0 percent of oxygen and the 65 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 an emissions analyzer. 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 activated 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 on emissions.

Illustration 1	g01090820
Schematic of the circuit for 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	g01090907
   Harness side of the P2 connector (P2-13) +24 VDC for the buffer (P2-23) Return (P2-53) Shield

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   Illustration 4	g01090828
   Harness side of the P8 connector (P8-C) +24 VDC for the buffer (P8-P) Return (P8-Z) Shield

   2. Perform a 45 N (10 lb) pull test on each of the wires that are associated with the circuit for the oxygen buffer.
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   Illustration 5	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 - The components are in good condition with proper connections. Proceed to Test Step 2.

• Not OK - The components are not in good condition and/or at least one connection is improper.

  Repair: Perform the necessary repairs and/or replace parts, if necessary. Ensure that all of the seals are properly installed. Make sure that the connections are properly fastened. Verify that the repair has eliminated the problem.

  Stop.

Test Step 2. Check for Diagnostic Codes for the Oxygen Buffer

1. Connect Cat ET to the service tool connector.

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

3. Access the "Logged Diagnostic Codes" screen on Cat ET. If the following codes are logged, clear the codes:
   • 1088-05 Oxygen Sensor Power Supply open circuit

   • 1088-06 Oxygen Sensor Power Supply short to ground

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

5. Select "Oxygen Sensor Override" under the "Description" column.

6. Select "Change" in the lower left corner of the screen. A popup window with the title "Change Parameter Value" will appear.

7. Select a "New Value" of "On" and then select "OK". Allow a minimum of ten seconds for the diagnostic codes to be activated.

   The oxygen buffer is now ON. The "Value" of the "Oxygen Sensor Override" should change to "On".

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

8. Look for the 1088-05 or 1088-06 diagnostic codes again.

Expected Result:

A 1088-05 or 1088-06 diagnostic code is activated.

Results:

• 1088-05 diagnostic code - There is a problem with the circuit for the supply voltage to the oxygen buffer. Proceed to Test Step 3.

• 1088-06 diagnostic code - There is a problem with the circuit for the supply voltage to the buffer. Proceed to Test Step 5.

• No codes - The supply for the buffer is OK. Stop.

Test Step 3. Check the Supply Voltage at 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.

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Illustration 6	g00894897
P651 harness connector for the oxygen buffer (P651-A) +24 VDC for the buffer (P651-B) Return

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

Expected Result:

The supply voltage for the oxygen buffer is greater than 18 VDC.

Results:

• OK - The supply voltage for the oxygen buffer is greater than 18 VDC. The buffer is receiving the correct supply voltage.

  Repair: Reconnect the connectors. Determine whether the diagnostic code recurs. If the problem is not resolved, set the engine control to the OFF/RESET mode and remove the electrical power from the engine. Replace the oxygen buffer. After you replace the buffer, calibrate the oxygen sensor. Follow the instructions in Troubleshooting, "Oxygen Sensor - Calibrate".

  Stop.

• Not OK - The supply voltage is less than 18 VDC. Proceed to Test Step 4.

Test Step 4. 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.

Test Step 5. Check the Supply Voltage at 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.

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Illustration 7	g00894897
P651 harness connector for the oxygen buffer (P651-A) +24 VDC for the buffer (P651-B) Return

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

Expected Result:

The supply voltage for the oxygen buffer is greater than 18 VDC.

Results:

• OK - The supply voltage for the oxygen buffer is greater than 18 VDC. The buffer is receiving the correct supply voltage.

  Repair: Reconnect the connectors. Determine whether the diagnostic code recurs. If the problem is not resolved, set the engine control to the OFF/RESET mode and remove the electrical power from the engine. Replace the oxygen buffer. After you replace the buffer, calibrate the oxygen sensor. Follow the instructions in Troubleshooting, "Oxygen Sensor - Calibrate".

  Stop.

• Not OK - The supply voltage is less than 18 VDC. Proceed to Test Step 6.

Test Step 6. 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 a short circuit between the ECM and the oxygen buffer.

  Repair: The short circuit may be inside the terminal box. Alternatively, the short 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.