Autodata - Diagnostic Trouble Codes Fault locations and probable causes - 2004 edition

Oscilloscope testing
Inductive sensors - Flg. 25
The general procedure is as follows:
Select sensor pin from pin data list with wave form reference.
Connect oscilloscope probe to ECM pin and second probe to earth.
Start engine and observe test conditions.
Compare scope trace with wave form reference.
Raise the engine speed and watch for the voltage (amplitude) display to increase.
ldle air control (IAC) vaive
ldle air control (IAC) valves come in many different types, each with a different wave form.
In each case the duty cycle (or ON time) of the valve should increase when any additional engine load starts to reduce the
idle speed.
If the duty cycle varies but the idle speed is not maintained under load, this indicates a faulty valve.
If the wave form shows a straight line around the zero mark, or the line is constant at the 5 V or 12 V level, this indicates a
fault in the IAC valve circuit or ECM signal output.
The commonly used 4 pin stepper motor type is described below. Two and three pin IAC valves can be tested in a similar
way, but will of course generate very different wave forms.
The stepper motor responds to an oscillating signal from the ECM, enabling small adjustments to be made to the engine
idle speed, in reaction to variations of operating temperature and load.
This voltage signal can be checked by connecting the oscilloscope test probe to each of the four stepper motor ECM pins
in turn.
Ensure engine is at normal operating temperature.
Start the engine and allow idle speed to stabilise.
Increase the load on the engine by switching on the headlights, air conditioning or by turning the steering (power steering
only).
The idle speed should momentarily drop, but then be stabilised by the action of the IAC valve.
Compare scope trace with wave form reference Fig. 26.
Fig. 25 Fig. 26