ABSTRACT
The various types of air-flow sensors used in automobile systems produce an electrical signal that is transmitted to the ECU to represent the amount of air that is flowing to the combustion chambers. As an example to demonstrate the principles involved in validating a DTC, I have chosen a potentiometer-type sensor. These sensors are normally supplied with a voltage of 5 V d.c. The potentiometer part of the sensor then outputs a voltage between zero and 5 V, which can be examined on an oscilloscope trace, as shown in Fig. 29.2. If the test shows the sensor to be functioning correctly and the fault persists, it becomes necessary to
check the circuit between the sensor and the ECU input connections. Using the same procedure at the ECU end of the air
sensor cable (Fig. 29.3) should produce an identical result to that obtained at the sensor. A material difference between the two results indicates that there is a defect in the circuit between the sensor and the ECU. The types of test that can be deployed from there on depend on the nature of the circuit, and it will be necessary to examine circuit diagrams and other workshop manual data. The example shown here indicates that there is
a general procedure for fault diagnosis that can be applied to any ECU-controlled system that provides a range of DTCs.
