The purpose of this test is to evaluate the correct operation of the idle speed control valve (ISCV) based on the switching voltage and duty control in response to target idle speeds.
Connection for diagnostic work will vary dependent on application.
Technicians should whenever possible gain access to the test circuit without damage to seals and insulation. If this is not possible then make sure appropriate repairs are completed.
General connection advice
PicoScope offers a range of options within the test kits.
Dependent on difficulty of access, choose from:
Testing sensors and actuators (to include relevant circuit/connectors):
The waveform shows a switched-Earth, pulse-width modulated (PWM) voltage, actuating an idle speed control valve.
The waveform has a cyclic period in which the voltage switches between battery positive (at around 15 volts) and close to battery negative (at just above 0 volts).
The percentage of the total cycle period spent at 0 volts indicates the PWM duty. In this known good example, the waveform is at 0 volts for around half of the cycle, indicating that the PWM duty is around 50%.
One waveform cycle is completed every 10ms, indicating that the cycle frequency is 100 Hz.
An idle speed control valve (ISCV) is used by the Engine Control Module (ECM) to regulate engine idle speed according to engine temperature and load, when there is no driver demand from the accelerator pedal and the throttle is closed.
For example, as an engine is at peak efficiency in normal operating temperatures, during cold-start conditions the ECM will seek to quickly raise engine temperature by increasing engine speed to a fast idle, at around 1200rpm.
The rotary ISCV is an electromechanical device that can have either 2 or 3 connections. One connection will be a constant battery voltage from either the ECM or a control relay, and the others may provide either a single- (2-pin connector) or a double-switched (3-pin connector) earth: a single earth system will open the valve in opposition to a spring force that acts to close the valve, whereas a double earth system will use one circuit to open and the second to close the valve.
The ECM controls the valve opening position by varying the duty cycle of the switched-earth signal. Therefore, the greater the ‘on’ time, the greater the valve opening and the air entering the engine, hence the greater the idle speed.
The ECM can maintain the idle speed regardless of any changes in engine load caused by the air-conditioning, power steering, automatic transmission, or charging systems, etc.
Due to the location of the ISCV it is susceptible to carbon fouling. As such, the ECM can be switching the signal correctly and the waveform can look normal, but the valve can be stuck. In this situation, the valve must be removed for examination and cleaning.
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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