Products suited to this guided test*
  • Back-pinning Probe Set

  • *At Pico we are always looking to improve our products. The tool used in this guided test may have been superseded and the product above is our latest version used to diagnose the fault documented in this case study.

Idle speed control valve - rotary solenoid

The purpose of this test is to evaluate the idle speed control valve (ISCV) control signal from the Engine Control Module (ECM).

How to perform the test

View connection guidance notes.

  1. Use manufacturer's data to identify the ISCV switched earth circuit.
  2. Connect PicoScope Channel A to the switched earth circuit.
  3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
  4. Start the engine and allow it to idle.
  5. Start the scope to see live data.
  6. With your waveform on screen stop the scope.
  7. Turn off the engine.
  8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

This known good waveform has the following characteristics:

  • A switched earth, pulse width modulated (PWM) voltage having a cyclic period in which it switches between battery positive at around 15 V and battery negative at around 0.5 V.
  • One cycle is completed every 10 ms indicating that the cycle frequency is 100 Hz.
  • The percentage of the total cycle spent at 0.5 V indicates the PWM duty. This waveform is at 0.5 V for around half of the cycle, indicating that the PWM duty is around 50%.

The PWM duty will change with an additional engine load (e.g. from the power steering, air-conditioning or alternator systems etc.).

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select Idle Speed Control Valve ISCV voltage.

Further guidance

An 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, during cold start conditions, the ECM will seek to quickly raise engine temperature by increasing engine speed to a fast idle, at around 1200 rpm.

With an ISCV, the ECM can maintain and adjust the idle speed for changes in engine load caused by the air conditioning, power steering, automatic transmission, or charging systems, etc.

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 providing a single or double switched earth.

The single switched earth system will open the valve in opposition to a closing spring, whereas a double switched 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 further the valve opens allowing more air into the engine resulting in higher idle speed.

Due to its location, the ISCV is susceptible to carbon fouling. As such the valve may be electrically functioning with a normal waveform but mechanically faulty. In this situation, the valve must be removed for examination, cleaning or replacement.

Symptoms of a faulty idle control valve may be uneven running or cutting out at idle, or fuel trim related diagnostic trouble codes.

Diagnostic trouble codes

Selection of component-related Diagnostic Trouble Codes (DTCs):

P0505 Idle Control System Malfunction

P0506 Idle Control System RPM Lower Than Expected

P0507 Idle Control System RPM Higher Than Expected

P0508 Idle Air Control System Circuit Low

P0509 Idle Air Control System Circuit High

P050A Cold Start Idle Air Control System Performance

P0511 Idle Air Control Circuit

P0518 Idle Air Control Circuit Intermittent

P0519 Idle Air Control System Performance


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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Guided test: Idle speed control valve - rotary solenoid