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- The frequency of the control speed of the lambda sensor thus becomes smaller as the sensor ages.

- The amplitude of the sensor signal becomes smaller. This becomes evident in the case of a lean mixture, where the 100 mV of an intact sensor (difference in voltage approx. 900 mV -100 mV = 800 mV) is exceeded by several hundred mV (difference in voltage approx. 900 mV - 400 mV = 500 mV).

Fig. 7.11 Comparison between a good sensor and a sluggish one and a sluggish, or aged, one. The 'good' sensor pattern is the continuous line and the sluggish one is the broken line.

Obtaining these scope patterns is but part of the diagnostic process. It is in fact, the collecting evidence step of the six-steps approach. Analyzing the evidence is another step and in this case the main features to note are the lower peak voltages and lower frequency of the sluggish one. Among the factors to consider when seeking a cause of the sluggishness would be the age of the vehicle, the hours of running, type of fuel used, the maintenance record, and the condition of the exhaust system, for example, is it damaged? are there leaks or obstructed pipes?

Dual oxygen sensors for catalyst monitoring

Figure 7.12 shows the general arrangement of the sensors and catalyst in a system that uses a second oxygen sensor to monitor the performance of the catalyst. The second oxygen sensor downstream of the catalyst is a requirement of the OBD II standard.

If the catalyst is operating efficiently, the control voltage of the downstream oxygen sensor is smoothed, as shown in Fig. 7.13. This difference in voltage patterns is the method by which the efficiency of the catalyst is monitored by

1 Control unit

2 Lambda sensor upstream of catalytic converter

3 Lambda sensor downstream of catalytic converter

4 Catalytic converter

1 Control unit

2 Lambda sensor upstream of catalytic converter

3 Lambda sensor downstream of catalytic converter

4 Catalytic converter

Fig. 7.12 Dual oxygen sensors as used on OBD II type systems

Fig. 7.12 Dual oxygen sensors as used on OBD II type systems

Fig. 7.13 Comparison of voltage patterns for upstream and downstream oxygen sensors the ECM. If the catalytic converter is not working, the voltage patterns of the two sensors will not show the marked difference and this is the basis of the fault monitoring procedure.

The downstream oxygen sensor is less likely to age than the oxygen sensor upstream of the catalyst. This permits the downstream sensor to be used as a 'guide' signal to allow the fuelling ECM to compensate for any ageing in the upstream sensor.

Features of oxygen sensor performance that are monitored by the ECM include:

• output voltage

• short circuits

• internal resistance

• speed of change from rich to weak

• speed of change from weak to rich.

Testing the upstream and downstream oxygen sensors

Figure 7.14 shows the principle for the use of dual-trace portable oscilloscope. The leads are equipped with electrical filters for this test and the test allows the two sensor patterns to be observed simultaneously. The general form of the voltage traces is shown in Fig. 7.15.

Fig. 7.14 The set-up for testing the two oxygen sensors
Fig. 7.15 The voltage traces from the upstream and downstream oxygen sensors

A similar test could be conducted with the aid of two digital voltmeters but, as you will appreciate, this test would be more difficult to perform. It must be noted that the test must be performed when the system is hot and this, coupled with the fact that the exhaust system is often not in the most accessible place, means that care must be taken in preparing for the test. A vehicle lift is virtually essential, precautions must be taken to prevent personal injury through burns etc., and the instruments and leads must be kept away from the hot exhaust.

Titania oxygen sensors

These are described in Chapter 5. Because they operate on the voltage changes that are produced by changes of resistance which in turn are caused by changes in oxygen levels in the exhaust, they produce a high voltage when the mixture is weak and a low output when the mixture is rich. Quite the opposite of the zirconia sensor. The response time is somewhat faster, the voltage trace is similar, but the voltages can be measured in volts rather than millivolts.

Do It Yourself Car Diagnosis

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