Idle speed control valve - rotary solenoid

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 guidance

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:

  1. Breakout leads.       
  2. Back-pinning probes.

Testing sensors and actuators (to include relevant circuit/connectors):

  • When testing a sensor, it is desirable to gain access at the control module.
  • When testing an actuator, it is desirable to gain access at the actuator. 

How to perform the test

  1. Use the vehicle circuit diagram to identify the ISCV switched earth circuit.
  2. Connect PicoScope Channel A to the switched earth circuit.
  3. Minimise the help page and with the example waveform on your screen PicoScope has already selected suitable scales for you to capture a waveform.
  4. Start the engine and allow it to idle.
  5. Start the scope.
  6. With your live waveform on screen stop the scope.
  7. Turn off the engine.
  8. Use the Waveform Buffer and Zoom 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 just above 0 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 V indicates the PWM duty. This waveform is at 0 V for around half of the cycle, indicating that the PWM duty is around 50%.

The PWM duty should 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.

Example rotary-type idle speed control valve.

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.

Suitable accessories

  • Premium 6-way breakout lead set


  • Back-pinning Probe Set


  • PicoScope Battery Clip


Help us improve our tests

We know that our PicoScope users are clever and creative and we’d love to receive your ideas for improvement on this test. Click the Add comment button to leave your feedback.

Add comment

Your email address will not be published. Required fields are marked *

Guided test: Idle speed control valve - rotary solenoid