Automotive Sensor Testing (Diagnostic Strategies of Modern Automotive Systems Book 1)
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The titanium O 2 sensor was used throughout the late s and early s on a limited basis. This sensor's semiconductor construction makes its operation different from that of the zirconia O 2 sensor.
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Instead of generating its own voltage, the titanium O 2 sensor's electrical resistance changes according to the exhaust oxygen content. As with the zirconia sensor, the titanium O 2 sensor is also considered a narrow-band O 2 sensor. As mentioned before, the main problem with any narrow-band O 2 sensor is that the ECM only detects that the mixture is slightly richer or leaner than the stoichiometric ratio. The ECM does not measure the operating air-fuel ratio outside the stoichiometric range. In effect it only detects that the mixture is richer or leaner than stoichiometry.
An O 2 sensor voltage that goes lower than mV will cause a widening of injector pulse and vice versa. The resulting changing or cycling fuel control closed-loop O 2 signal is what the technician sees on the scope when probing at the O 2 sensor signal wire.
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These sensors are often called by different names such as continuous lambda sensors lambda representing air-fuel ratio , AFR air-fuel ratio sensors , LAF lean air-fuel sensor and wide-band O 2 sensor. Such control is needed on new lean burning engines with extremely low emission output levels. Tighter emission regulations and demands for better fuel economy are driving this newer fuel control technology. The wide-band O 2 sensor looks similar in appearance to the regular zirconia O 2 sensor. Its inner construction and operation are totally different, however.
The wide-band O 2 sensor is composed of two inner layers called the reference cell and the pump cell. The AFR sensor uses dedicated electronic circuitry to set a pumping current in the sensor's pump cell. The pump cell then discharges the excess oxygen through the diffusion gap by means of the current created in the pump-cell circuit. The ECM senses the current and widens injector pulsation accordingly to add fuel. The pump cell then pumps oxygen into the monitoring chamber by way of the reversed current in the ECM's AFR pump cell circuit.
The ECM detects the reversed current and an injector pulsation-reduction command is issued bringing the mixture back to lean. The ECM is constantly monitoring the pump cell current circuitry, which it always tries to keep at a set voltage. For this reason, the techniques used to test and diagnose the regular zirconia O 2 sensor can not be used to test the wide-band AFR sensor.
These sensors are current-driven devices and do not have a cycling voltage waveform.
The testing procedures, which will be discussed later, are quite different from the older O 2 sensors. The sensing part at the tip of the sensor, is always held at a constant voltage depending on manufacturer. If the mixture goes rich, the ECM will adjust the current flowing through the sensing tip or pump cell circuit until the constant operating voltage level is achieved again.
The voltage change happens very fast.
Although the ECM varies the current, it tries to maintain the pump circuit at a constant voltage potential. It is possible to monitor the actual AFR sensor varying current, but the changes are very small in the low milliamp range and difficult to monitor. A second drawback to a manual AFR current test is that the signal wire has to be cut or broken to connect the ammeter in series with the pump circuit. Today's average clamp-on ammeter is not accurate enough at such a small scale.
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For this reason, the easiest but not the only way to test an AFR sensor is with the scanner. However, on some vehicles and scanners it will show up as "lambda" or "equivalence ratio.
The reference voltage varies from car to car, but is often 3. When the fuel mixture becomes richer on a sudden, quick acceleration , the voltage should decrease. Under lean conditions such as deceleration the voltage should increase. If the scanner PID displays a "lambda" or " equivalence ratio ," the reading should be 1.
Numbers above 1. The ECM uses the information from the sensors to adjust the amount of fuel being injected into the engine, so corresponding changes in the short-term fuel trim PID s should also be seen. Lean mixture readings from the AFR sensor will prompt the ECM to add fuel, which will manifest itself as a positive or more positive short-term fuel trim percentage. Some technicians will force the engine to run lean by creating a vacuum leak downstream from the mass airflow sensor, and then watch scanner PIDs for a response.
The engine can be forced rich by adding a metered amount of propane to the incoming airflow. In either case, if the sensor does not respond, it likely has a problem. However, these tests do not rule out other circuitry problems or ECM issues. Thorough, systematic diagnosis is recommended.
The ECM controls the heater circuit. The wide operating range coupled with the inherent fast acting operation of the AFR sensor, puts the system always at stoichiometry, which reduces a great deal of emissions. If the mixture goes slightly rich the ECM adjusts the pump circuit's current to maintain the set operating voltage. The current is detected by the ECM's detection circuit, with the result of a command for a reduction in injector pulsation being issued.
As soon as the air-fuel mixture changes back to stoichiometry, because of the reduction in injector pulsation, the ECM will adjust the current respectively. The end result is no current 0. In this case a light negative hump is seen on the ammeter with the reading returning to 0. The fuel correction happens very quickly.
A narrow-band sensor has a nonlinear output, with ranges from 0. Narrow-band sensors are temperature-dependent. If the exhaust gases become warmer, the output voltage in the lean area will rise, and in the rich area it will be lowered.
Consequently, a sensor, without pre-heating, has a lower lean-output and a higher rich-output, possibly even exceeding 1 volt. The influence of temperature on voltage is smaller in the lean mode than in the rich mode. The engine control unit ECU when operating in "closed loop" tends to maintain zero oxygen thus a stoichiometric balance , wherein the air—fuel mixture is approximately This ratio maintains a "neutral" engine performance lower fuel consumption yet decent engine power and minimal pollution.
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