Idle speed control valve (solenoid type)

The purpose of this test is to investigate the operation of a solenoid type Idle Speed Control Valve (ISCV) based on switching voltage and frequency under engine operating conditions.

How to perform the test

View connection guidance notes.


  1. Use the vehicle circuit diagram 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. Whilst observing the screen switch on electrical loads, (AC, heating, lights etc.) in order to affect the idle speed.
  7. With your waveform on screen stop the scope.
  8. Turn off the engine.
  9. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Example waveform

Waveform notes

The electromagnetic Idle Speed Control Valve (ISCV) has 2 electrical connections: a voltage supply at battery voltage and a switched earth path.

The rate at which the earth path is switched is determined by the Electronic Control Module (ECM) to maintain a pre-programmed speed. The valve forms an air bypass around the throttle butterfly, to form a controlled air bleed within the induction tract. If the engine has an adjustable air bypass and an ISCV, it may require a specific routine to balance the two air paths.

The example waveform shows the earth path being switched. Probing the supply side will produce a straight line at charging voltage, and probing the earth circuit shows a 'sawtooth' waveform.

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

The function of the Idle Speed Control Valve (ISCV) is, as the name implies, to control the engine's idle speed according to its temperature and different load conditions.

When the engine is first started from cold, the engine management Electronic Control Module (ECM) gives the engine cold start enrichment and increases the engine's idle speed to about 1200 rpm. It is the ISCV that is responsible for this increase. As the engine reaches operating temperature the enrichment is eliminated and the idle speed reduced to a predetermined speed. This speed is maintained regardless of electrical loads on the alternator and to a certain extent mechanical loads, for example, when an automatic gearbox has drive selected.

The ISCV is an electromechanical device that has a supply voltage either from the ECM or a control relay. It has 2 or 3 electrical connections, with the aforementioned voltage supply and either a single or a double switched earth path. The rate at which the earth path is switched is determined by the ECM to maintain a pre-programmed speed. An ISCV can be either a rotary or an electromagnetic type, both of which are popular with the rotary being the most common. The valve forms an air bypass around the throttle butterfly to deliver a controlled air bleed within the induction tract, and is therefore susceptible to dirt and carbon like deposits impeding its performance. It is recommended that they are cleaned at the manufacturer's service intervals with a spray solvent to maintain their efficiency.

If the engine has an adjustable air bypass and an ISCV, it may require a specific routine to balance the two air paths.

The switching of the earth path can be monitored on an oscilloscope with the rotary type producing a square wave, and the electromagnetic type a 'sawtooth' waveform.

The rotary valve may have either single or twin earth paths, the single being pulled one way electrically and returned to its closed position by a spring; the double switched earth system switching the valve in both directions. This can be monitored on a dual trace oscilloscope.


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


  • Large Dolphin/Gator Clips


  • PicoScope Battery Clip


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Guided test: Idle speed control valve - electromagnetic