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t e c h o t a l k Oxygen Sensors Written by Clive Ward, www.motorcyclebiz.com.au The way they work, the effect they have on the engine management system and a way of testing them. In most Oxygen Sensors there is a Zirconia element that, when heated to temperatures above approximately 300 deg C, produce voltages that are based on the oxygen content surrounding the sensor tip, as compared to the oxygen in the atmosphere. Oxygen sensors perform this comparison through an atmospheric reference orifice in the sensor housing. If the atmospheric reference orifice is blocked with road grime, mud or in some cases, dried crankcase oil, the performance of the sensor will be impaired to a point that will cause poor fuel economy, excessive exhaust emissions and in severe cases, motorcycle rideability problems. Always visually inspect the oxygen sensor. If the sensor is externally covered with any form of contaminant there is a very good likelihood that the sensor will not perform to optimum capacity due to the atmospheric reference orifice being blocked. Pay particular attention to oxygen sensors that are located on off-road Motorcycles as they are prone to mud contamination. Another easy to perform physical test, is to determine whether or not the sensor is securely tightened in the exhaust system. Because any electrical circuit has to be complete before the current can flow, poor contact between the sensor securing thread (sensor grounding point) and the exhaust system will cause erratic, poor, or incorrect oxygen sensor signals to be sent to the ECU, also check that the exhaust is tight. Engine air/fuel mixtures. It is easy to determine what air/fuel mixture is being used by an engine by observing the output of the Oxygen Sensor. Connect a labscope or high quality multimeter to the signal wire. An engine that is running continually lean will cause the Oxygen Sensor to produce a low voltage on the signal wire. This voltage will be between 0.45 V and 0.00 V. The cause of this symptom could be unmetered air entering the intake manifold, aerated fuel, low fuel pressure or restricted injectors. Leaking exhaust header pipe will also cause additional oxygen to be present in the exhaust gases. An engine that is running continually rich will cause the Oxygen Sensor to produce a high voltage. The cause of this could be high fuel pressure, faulty coolant temperature circuit, faulty MAP (manifold absolute pressure) sensor circuit, air temperature sensor circuit, or faulty TPS (throttle Position Sensor) circuit. An engine management system that is functioning correctly and thus has a correct air/fuel ratio will cause the oxygen sensor to produce a voltage that will transition rapidly back and forth across the 0.45 volt mark. See Figure: 1, snapshot view of a correctly operating O2 sensor on a labscope. Important note: For those of you who think that you’ll improve the performance of a motorcycle by disconnecting the O2 sensor, think again. The O2 sensor only works up to 80% throttle, after that the ECU enters WOT mode (Wide Open Throttle), ignores the 02 sensor and delivers the higher quantity of fuel needed to produce maximum performance, the only thing you’ll achieve by disconnecting the 02 sensor is that you’ll dramatically increase the fuel consumption and cause over-rich spluttering through the low and mid-range. SINGLE WIRE OXYGEN SENSOR. This sensor is mounted in the exhaust with the sensing portion exposed to the exhaust gas stream. When the sensor has reached an operating temperature of more than 300 deg C, it acts as a voltage generator, capable of producing a rapidly changing voltage of between 10 and 1,000 millivolts. This voltage output is dependent on the oxygen content in the exhaust gas, as compared to the oxygen in the sensor’s atmospheric oxygen reference cavity. This reference cavity is exposed BIKEBusiness 32 Nov - Dec 2009 b u s i n e s s t o b u s i n e s s
t e c h o t a l k to the atmosphere through a small “vent” hole located under the sensor’s external metal vent cover. The single wire Oxygen sensor uses the engine as a ground reference and has a single signal wire going back to the Motorcycles ECU. When the sensor is cold, it produces either no voltage, or an unusable slowly changing one. At this point the ECU is operating in “open loop” using a preprogrammed fuel map. Also when the sensor is cold, its internal electrical resistance is high. ECU’s that are associated with this type of Oxygen Sensor usually supply a steady 450 millivolt, very low current “bias” voltage to the Oxygen Sensor circuit. When the sensor is cold and not producing any voltage, the ECU “sees” only the steady bias voltage. As the sensor begins heating, its internal resistance decreases and it begins producing a rapidly changing voltage that will overshadow the ECU-supplied steady bias voltage. When the ECU “sees” the changing voltage it knows the sensor is hot and its output is ready to be used for the job of “fine-tuning” the engine’s fuel requirements. The ECU monitors the changing voltage transition above and below a mid range of approximately 450 millivolts, to determine when to operate in the “closed-loop” mode. When the fuel system is correctly operating in the closed-loop mode, the sensor voltage output is rapidly changing, going above and below a rich/lean midpoint range of between 250 to 450 millivolts. The ECU monitors the change in voltage and corrects the air/ fuel ratio by lengthening or shortening the time injectors are open to provide a richer or leaner air/fuel mixture. To measure the amount of time the injectors are open, place your labscope tool on any injector and measure the pulse width in milliseconds. With the Oxygen sensor sending a rich signal, i.e. a high voltage, the millisecond reading at the injector will fall. Conversely with the Oxygen sensor sending a lean signal to the ECU, i.e. a low voltage, the millisecond reading at the injector will rise. OXYGEN SENSORS WITH HEATERS (3 wire sensors) Some Oxygen Sensors utilise a Zirconia element and operate the same way as described for single wire Oxygen Sensors but they also have the added advantage of a heater element. This heating element allows the Oxygen Sensor to stay at operating temperature even though the exhaust gas temperature is below 300 deg C. In these systems the ECU remains in closed-loop when the engine is idling, i.e. in the closed throttle position. The advantage of this is the Oxygen Sensor signal is always available to the ECU as after initial Oxygen Sensor warm-up as it never cools down until the engine is switched off. These sensors have a 12 volt positive and negative supply from the ECU as well as the signal wire, hence the 3 wire reference. Never short any of the 3 wires together. Sensor failures Normally, the lifetime of an unheated sensor is about 30,000 to 50,000 kilometres, but some can last up to 100,000kms. Failure of an unheated sensor is usually caused by the buildup of soot on the ceramic element, which lengthens its response time and may cause total loss of ability to sense oxygen. For heated sensors, normal deposits are burnt off during operation and failure occurs due to catalyst depletion, similar to the reason a battery stops producing current. The probe then tends to report a constant lean mixture, the ECU enriches the mixture, the exhaust gets rich with carbon monoxide and hydrocarbons, and the fuel comsumption increases. Leaded petrol contaminates the oxygen sensors and catalytic converters. Most oxygen sensors are rated for some service life in the presence of leaded gasoline but sensor life will be shortened to as little as 10,000kms depending on the lead concentration. Lead-damaged sensors typically have their tips discolored light rusty. Another common cause of premature failure of lambda probes is contamination of fuel with silicones, used in gasket sealing operations In this case, the deposits on the sensor are colored between shiny white and grainy light gray. To prevent this use sensor safe silicons on Motorycles with O2 sensors. An overly rich mixture causes buildup of black powdery deposit on the probe. This may be caused by failure of the probe itself, or by a problem elsewhere in the fuel management system. Applying an external voltage to the zirconia sensors, e.g. by checking them with some types of ohmmeters, may damage them. So never do it. Only test by viewing voltage. Symptoms of a failing oxygen sensor includes: • Sensor Light on dash indicates problem • Increased tailpipe emissions (possibly even black smoke) • Increased fuel consumption • Hesitation on acceleration • Stalling • Rough idling Need More Copies of Bike Business Visit: www.bikebusiness.com.au BIKEBusiness 33 Nov - Dec 2009 b u s i n e s s t o b u s i n e s s
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