ALL EXCEPT 8N6120, 8N6121, 8N6122, 6V0221 - Caterpillar

System Operation Description: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 bufferTest Step 1. Inspect the Electrical Connectors and Wiring
During operation, the oxygen sensor may reach temperatures that exceed 700 °C (1292