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OBJECTIVES After studying Chapter 15, the reader will be able to: • Prepare for ASE Engine Performance (A8) certification test content area “E” (Computerized Engine Controls Diagnosis and Repair). • Discuss how O2S sensors work. • List the methods that can be used to test O2S sensors. • Describe the symptoms of a failed O2S sensor. • List how the operation of the O2S sensor affects vehicle operation.
OXYGEN SENSORSPurpose and Function • Automotive computer systems use a sensor in the exhaust system to measure the oxygen content of the exhaust. • These sensors are called oxygen sensors (O2S). • The oxygen sensor is installed in the exhaust manifold or located downstream from the manifold in the exhaust pipe. FIGURE 15-1 Many fuel-control oxygen sensors are located in the exhaust manifold near its outlet so that the sensor can detect the presence or absence of oxygen in the exhaust stream for all cylinders that feed into the manifold.
OXYGEN SENSORS Construction and Operation • The inner and outer surfaces of the thimble are plated with platinum. • The inner surface becomes a negative electrode; the outer surface is a positive electrode. • Negatively charged oxygen ions are drawn to the thimble where they collect on both the inner and outer surfaces. FIGURE 15-2 A cross-sectional view of a typical zirconia oxygen sensor.
OXYGEN SENSORS Construction and Operation FIGURE 15-3 A difference in oxygen content between the atmosphere and the exhaust gases enables an O2S sensor to generate voltage.
OXYGEN SENSORS Construction and Operation • There are several different designs of oxygen sensors, including: • One-wire oxygen sensor. • Two-wire oxygen sensor. • Three-wire oxygen sensor. • Four-wire oxygen sensor.
OXYGEN SENSORS Construction and Operation FIGURE 15-4 The oxygen sensor provides a quick response at the stoichiometric air-fuel ratio of 14.7:1.
ZIRCONIA OXYGEN SENSORS • The most common type of oxygen sensor is made from zirconia (zirconium dioxide). • It is usually constructed using powder that is pressed into a thimble shape and coated with porous platinum material that acts as electrodes. FIGURE 15-5 A typical zirconia oxygen sensor.
TITANIA OXYGEN SENSOR • The titania (titanium dioxide) oxygen sensor does not produce a voltage but rather changes in resistance with the presence of oxygen in the exhaust. • All titania oxygen sensors use a four-terminal variable resistance unit with a heating element. • A titania sensor samples exhaust air only and uses a reference voltage from the PCM.
WHERE IS HO2S1? FIGURE 15-6 Number and label designations for oxygen sensors. Bank 1 is the bank where cylinder number 1 is located.
WIDE-BAND OXYGEN SENSORS • A wide-band oxygen sensor, also called a lean air-fuel (LAF) ratio sensor or a linear air-fuel ratio sensor, allows engines to operate as lean as 23:1 and still maintain closed-loop operation. • This type of sensor usually uses five wires. • One power wire • One ground wire for the electric heater • Three sensor wires
WIDE-BAND OXYGEN SENSORS FIGURE 15-7 The output of a typical air-fuel mixture sensor showing that the voltage increases as the exhaust becomes leaner, which is opposite from normal oxygen sensors.
CLOSED LOOP AND OPEN LOOP • When the PCM alone (without feedback) is determining the amount of fuel needed, it is called open-loop operation. • As soon as the oxygen sensor (O2S) is capable of supplying rich and lean signals, adjustments by the computer can be made to fine-tune the correct air-fuel mixture. • This checking and adjusting by the computer is called closed-loop operation.
PCM USES OF THE OXYGEN SENSOR • Fuel Control • Fuel Trim
OXYGEN SENSOR DIAGNOSIS FIGURE 15-8 The OBD-II catalytic converter monitor compares the signals of the upstream and downstream oxygen sensor to determine converter efficiency.
OXYGEN SENSOR DIAGNOSIS • Testing an Oxygen Sensor Using a Digital Voltmeter • Testing the Oxygen Sensor Using the MIN/MAX Method • Testing an Oxygen Sensor Using a Scan Tool • Testing an Oxygen Sensor Using a Scope
OXYGEN SENSOR DIAGNOSIS FIGURE 15-9 Testing an oxygen sensor using a DMM set on DC volts. With the engine operating in closed loop, the oxygen voltage should read over 800 mV and lower than 200 mV and be constantly fluctuating. (Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-10 Using a digital multimeter to test an oxygen sensor using the MIN/MAX record function of the meter.(Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-11 A Chrysler DRB III scan tool is an excellent tool to use to test an oxygen sensor(s).
OXYGEN SENSOR DIAGNOSIS FIGURE 15-12 Connecting a handheld digital storage oscilloscope to an oxygen sensor signal wire. The use of the low-pass filter helps eliminate any low-frequency interference from affecting the scope display. (Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-13 The waveform of a good oxygen sensor as displayed on a digital storage oscilloscope (DSO). Note that the maximum reading is above 800 mV and the minimum reading is less than 200 mV. (Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-14 A typical good oxygen sensor waveform as displayed on a digital storage oscilloscope. Look for transitions that occur rapidly between 0.5 and 5.0 Hz. (Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-15 Using the cursors on the oscilloscope, the high- and low-oxygen sensor values can be displayed on the screen. (Courtesy of Fluke Corporation)
OXYGEN SENSOR DIAGNOSIS FIGURE 15-16 When the air-fuel mixture rapidly changes such as during a rapid acceleration, look for a rapid response. The transition from low to high should be less than 100 ms. (Courtesy of Fluke Corporation)
THE PROPANE OXYGEN SENSOR TEST FIGURE 15-17 Adding propane to the air inlet of an engine operating in closed loop with a working oxygen sensor causes the oxygen sensor voltage to read high.
THE PROPANE OXYGEN SENSOR TEST FIGURE 15-18 When the propane is shut off, the oxygen sensor should read below 200 mV.
OXYGEN SENSOR WAVEFORM ANALYSIS • Frequency • The frequency of the O2 sensor is important in determining the condition of the fuel control system. • The higher the frequency the better, but the frequency must not exceed 6 Hz. • Throttle-Body Fuel-Injection Systems. • Port Fuel-Injection Systems. FIGURE 15-19 When the O2S voltage rises above 450 mV, the PCM starts to control the fuel mixture based on oxygen sensor activity.
OXYGEN SENSOR WAVEFORM ANALYSIS FIGURE 15-20 Normal oxygen sensor frequency is from about one to five times per second.
HASHBackground Information • Hash on the O2S waveform is defined as a series of high-frequency spikes, or the fuzz (or noise) viewed on some O2S waveforms, or more specifically, oscillation frequencies higher than those created by the PCM normal feedback operation (normal rich/lean oscillations).
HASHCauses of Hash • Hash on the O2S signal can be caused by the following: • Misfiring cylinders • Ignition misfire • Lean misfire • Rich misfire • Compression-related misfire • Vacuum leaks • Injector imbalance • System design, such as different intake runner length • System design amplified by engine and component degradation caused by aging and wear • System manufacturing variances, such as intake tract blockage and valve stem mismachining
CLASSIFICATIONS OF HASH • Class 1: Amplified and Significant Hash • Class 2: Moderate Hash • Class 3: Severe Hash FIGURE 15-21 Significant hash can be caused by faults in one or more cylinders, whereas amplified hash is not as important for diagnosis.
CLASSIFICATIONS OF HASH FIGURE 15-22 Moderate hash may or may not be significant for diagnosis.
CLASSIFICATIONS OF HASH FIGURE 15-23 Severe hash is almost always caused by cylinder misfire conditions.
HASH INTERPRETATION • Types of Misfires That Can Cause Hash • Other Rules Concerning Hash on the O2S Waveform FIGURE 15-24 An ignition- or mixture-related misfire can cause hash on the oxygen sensor waveform.
HASH INTERPRETATION FIGURE 15-25 An injector imbalance can cause a lean or a rich misfire.
NEGATIVE O2S VOLTAGE • When testing O2S waveforms, some O2 sensors will exhibit some negative voltage. • The acceptable amount of negative O2S voltage is 0.75 mV, providing that the maximum voltage peak exceeds 850 mV. FIGURE 15-26 Negative reading oxygen sensor voltage can be caused by several problems.
LOW O2S READINGS • An oxygen sensor reading that is low could be due to other things besides a lean air-fuel mixture. • False Lean • Ignition misfire. • Exhaust leak in front of the O2S. • A spark plug misfire represents a false lean signal to the oxygen sensor.
HIGH O2S READINGS • An oxygen sensor reading that is high could be due to other things beside a rich air-fuel mixture. • When the O2S reads high as a result of other factors besides a rich mixture, it is often called a false rich indication.
POST-CATALYTIC CONVERTER OXYGEN SENSOR TESTING • The oxygen sensor located behind the catalytic converter is used on OBD II vehicles to monitor converter efficiency. • A changing air-fuel mixture is required for the most efficient operation of the converter FIGURE 15-27 The post-catalytic converter oxygen sensor should display very little activity if the catalytic converter is efficient.
OXYGEN SENSOR VISUAL INSPECTION • Whenever an oxygen sensor is replaced, the old sensor should be carefully inspected to help determine the cause of the failure. • Inspection may reveal the following: • Black sooty deposits • White chalky deposits • White sandy or gritty deposits • Dark brown deposits,
WHAT IS LAMBDA? FIGURE 15-28 The target lambda on this vehicle is slightly lower than 1.0 indicating that the PCM is attempting to supply the engine with an air-fuel mixture that is slightly richer than stoichiometric. Multiply the lambda number by 14.7 to find the actual air-fuel ratio.
OXYGEN SENSOR-RELATED DIAGNOSTIC TROUBLE CODES • Diagnostic trouble codes (DTCs) associated with the oxygen sensor include: