Looking for the Right Mixture (GM Systems) Over 25 years ago, most manufacturers introduced closed loop fuel control systems in their vehicle fleets. At the time, this change was considered state of the art technologically and the advent of the closed loop system was a huge leap for some technicians to make. A core component in the closed loop system is the oxygen sensor. The oxygen sensor was introduced in the late 1970s as a way of monitoring the rich/lean status for the engine and fuel control system. Theoretically, the oxygen sensor monitors the amount of unburned oxygen in the exhaust stream. By monitoring the amount of oxygen in the exhaust stream, the computer has a good idea if the fuel mixture was rich or lean. Air and fuel mix to create a combustible mixture. A certain amount of air is required to burn all of the fuel charge while creating low emissions output. The correct ratio was determined to be 14.7 pounds of air to every 1 pound of fuel. This ratio is known as stoichiometric or sometimes lambda. When lambda equals 1, the ratio is considered stoichiometric or 14.7:1. When the ratio is lean or above 14.7:1 lambda will be greater than 1. If the ratio is rich or less than 14.7:1 lambda will be less than 1. Lean air/fuel ratios can lead to improvements in fuel economy but they can also cause a sharp rise in oxides of nitrogen (NOx) and even hydrocarbon (HC) emissions. Conversely if the mixture gets too rich, carbon monoxide (CO) and hydrocarbon (HC) emissions can rise. In addition, engine and catalytic converter damage may occur if the air/fuel ratios aren t correct. by Steve Garrett Five types of oxygen sensors have been used over the years. They include: 1. Unheated thimble (conventional) 2. Heated thimble (conventional) 3. Planar (conventional) 4. Wide band (WRAF) 5. Titania The conventional oxygen sensor was introduced in an attempt to monitor the engine s air/fuel ratio indirectly. For a conventional sensor to operate correctly, its temperature must typically exceed 315ºC (625ºF). Some oxygen sensors are heated while others are unheated. The heated oxygen sensor was introduced in an attempt to reduce emissions output during warmup and 42 GEARS July 2007
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Looking for the Right Mixture (GM Systems) extended idle conditions by keeping the sensor at the proper operating temperature at all times. Thimble-style sensors use a round sensing element, which is coated in zirconia, while the electrodes are covered in platinum. Known as a solid-state, electrochemical fuel cell or Nernst cell, the conventional sensor typically measures the difference in oxygen content between the exhaust stream and the outside air. It senses the outside oxygen content either though a hole in the body of the sensor or between the insulation and wires of the sensor pigtail. The sensor generates an output voltage in proportion to the difference. The range on this type of sensor is typically between 0-1000 millivolts. A rich exhaust creates a high voltage (about 800mV). If the exhaust is lean, it develops a low voltage (about 200mV). A conventional sensor will typically toggle between high and low voltage as the system makes corrections. The ECM provides a bias voltage to the sensor at all times (about 450mV). The ECM uses the bias voltage to verify the circuit when the sensor isn t operating. The oxygen sensor isn t the main input for the fuel delivery system. Instead, it acts as a scorekeeper for the fuel system. If the exhaust indicates a rich condition, the high voltage input to the ECM from the oxygen sensor will bias the fuel delivery system lean. Conversely, if the exhaust is 1/3 lean, Horiz the ECM Herewill bias the fuel delivery system rich. The conventional sensors are known as narrow band sensors, as they operate in a narrow range of air/fuel ratios. Conventional design sensors are fairly slow to react to changes in exhaust oxygen content. This means the sensors are better at determining the direction of the correction needed than the amount of correction needed. Planar-type sensors were introduced in the mid 90s and function like a conventional sensor, but their construction is different. The sensing element uses a flat ceramic zirconia strip. This type of sensor is more compact, lighter, and is more resistant to contamination. The operation of the planar-type sensor is the same as the heated thimble, conventional sensor. The heater systems used in this design are typically superior to that of the heated thimble sensor. The planar sensor has become common with most OEMs over the past decade. The Titania-type sensor isn t a common sensor in the automobile marketplace. In fact, some estimates suggest only 0.5% OEM applications use a titania sensor. The titania sensor is a little different than the other sensors we just discussed. With this type of sensor, the ceramic sensing element is coated with titanium dioxide. The titania sensor doesn t generate voltage; instead, it changes its internal resistance. A typical sensor will have about 20 kilohms resistance when the exhaust is lean and about 1 kilohms resistance when the exhaust is rich. The ECM will typically provide a 5-volt, low current voltage to the sensor. The ECM then measures the voltage drop across the sensor to determine the air/fuel ratio. The advantage of this type 44 GEARS July 2007
of sensor is it s quicker to respond to changes in the A/F ratio than the other style sensors. The disadvantage is its cost. One of the newest sensors on the scene is the Wide Band oxygen sensor, also called a WRAF or Wide Range Air Fuel sensor. In the GM family, it was first used on the 1999 Cadillac Cater. The wide band oxygen sensor was introduced because of three basic disadvantages with the previous design systems: 1. Slow response time to rapid changes in exhaust oxygen content. A typical V-6 engine will produce about 62 firing pulses per second, per cylinder bank when the engine is operating at 2500 RPM. That means during one rich/lean, lean/rich transition of the oxygen sensor, about 31 firing pulses have occurred. Each switch is simply an average of several cylinder pulses. 2. The inability to determine how much correction was needed. Earlier systems used very sophisticated software algorithms (integrator/short term fuel trim, block learn/long term fuel trim) to estimate the necessary correction, but at best it could simply average the correction required. The GM wide band sensor is accurate at A/F ratios as high as 16.0:1 and as low as 11.0:1, with the ability to calculate the actual ratio anywhere in between. 3. Excessive cold start emissions. The previous sensor designs don t do a good job during cold starts, even when they re equipped with heaters. New emissions requirements were on the horizon and the older design sensors simply couldn t do the job. These new requirements include: NLEV National Low Emission Vehicle Standard ULEV Ultra Low Emission Vehicle Standard SULEV Super Low Emission Vehicle Standard The wide band sensor was introduced to help meet these new standards. The sensor is a planar design that incorporates an electrochemical gas pump. The wide band sensor doesn t operate like the sensors we ve discussed, nor are the diagnostic techniques the same. These sensors have an oxygen-sensing cell (it works like a conventional sensor), an oxygen-pumping cell, and a heater. The ECM provides a signal voltage to the sensor. The ECM contains a control circuit called a wide band controller. Known as a PID (proportional integral differential) feedback controller, the control circuit monitors and controls the amount of current within the sensor circuits. The wide band controller circuit controls the amount of current flow through the pumping cell, which controls the voltage variation in the sensing cell. The exhaust gases pass through a gap located between the pumping cell and the sensing cell. To aid in this process the oxygen pump uses a heated anode and cathode to pull oxygen ions from the exhaust through the gap. The oxygen pump requires a specific amount of current flow to maintain the required oxygen concentration level in the gap. The amount of current required to main- Ready to Shift to a Better Insurance Program? Get into Gear with Heffernan Insurance Brokers. Offering property, liability, workers comp and more for your transmission business. Contact us today Brant Watson 800.234.6787 BrantW@heffgroup.com www.heffgroup.com License #0564249 ATRA program facilitated by Heffernan Insurance Brokers and Brant Watson. GEARS July 2007 45
Looking for the Right Mixture (GM Systems) tain the correct oxygen balance is directly proportional to the oxygen level in the exhaust. The ECM monitors the current flow to determine the air/fuel ratio. A lookup table within the ECM software determines the air/fuel ratio. In simple terms, the lookup table works like this: A lambda value of 1 = a 14.7:1 actual air/fuel ratio divided by the stoichiometric desired air/fuel ratio of 14.7:1. 14.7 divided by 14.7 = 1 lambda. If lambda is less than 1, the air/fuel ratio is rich. For example, 13.0:1 A/F ratio: 13.0 divided by 14.7 = 0.884 lambda. 0.884 lambda would be displayed on your scan tool if the car were running at a 13.0:1 air/fuel ratio. If lambda is greater than 1, the A/F ratio is lean. For example, 16.0:1 A/F ratio: 16.0 divided by 14.7 = 1.0884 lambda. The 1.08884 lambda is what you would see displayed on your scan tool if the car were running at a 16.0:1 A/F ratio. You can also work this process in reverse to determine your exact A/F ratio. If your scan tool displayed a lambda value 46 GEARS July 2007
of 1.0884, multiply the value by 14.7: 14.7 x 1.0884 = 15.999:1 A/F ratio If your scan tool displayed a value of 0.884 lambda, multiply the value by 14.7:1. 14.7 x 0.884= 12.994:1 A/F ratio This process allows the ECM to determine the exact A/F ratio, so it can command fuel trim corrections. NOTE: Some GM applications will indicate lambda numbers at extreme values such as 0.750 or 3.999 during some driving conditions, such as decel fuel cutoff. The controller software doesn t have the ability to display infinity, so it displays the software clamp value instead. This is normal. NOTE: Some GM import applications, such as the Pontiac Vibe (Toyota Matrix) display their wide band sensor values in voltage rather than lambda. On the Pontiac Vibe, the sensor and scan data values range from 1-5 volts. 3.2-3.3 volts indicates a 14.7:1 air/fuel ratio. A reading lower than 3.2 volts indicates a rich exhaust; a higher reading indicates a lean exhaust. On other import applications, you may see the sensor scan value indicating range between 0-1 volts. The actual sensor voltage value is actually much higher, but to meet the previous OBD-II standard, the ECM divides the actual value by 5 for the display. The heater circuits used on GM wide band oxygen sensors is PWM controlled. The heater performs the same function as the heater on other GM oxygen sensors. The heater circuit on the wide band sensors will keep the sensor operating temperature between 350ºC (1112ºF) and 900ºC (1652ºF). Unlike some other manufacturers wide band oxygen sensors, the GM sensor incorporates a trimming resistor into the connector. The resistor is laser etched at the end of the assembly process to calibrate the sensor. This process will shift the sensor output response to the appropriate level for the application. Diagnosis for the wide band oxygen sensor is also a little different when compared to a conventional type sensor. Some of the typical diagnostic strategies you used will still apply; others won t. GM requires the use of the scan tool rather than a scope or DMM for diagnosis. A DMM is simply used to establish the signal and heater voltage being fed to the sensor rather than to check the actual sensor operation. This means the lambda values will be used to check sensor operation. Voltage checks will be limited to checking the engine harness side of the circuits with the sensor disconnected. The wide band software calibrations are tighter, so typically a DTC will set if the sensor has a problem. The wide band sensor can be fooled just like the conventional sensor by things such as exhaust leaks and misfire. And it can be damaged by silicone contamination (antifreeze, sealers). All GM wide band sensors currently use 5 or 6 wires. This new kid on the block will take a little getting used to. As before, get some vehicles that use the wide band sensor and play with them a little. Once you become familiar with how they operate normally, you ll have little trouble diagnosing one that s acting up. Until next time, keep the shiny side up and the rubber side down. GEARS July 2007 47