Idle speed control valve - electromagnetic

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

  1. Plug a BNC test lead into Channel A on the PicoScope.
  2. Fit a black clip on the black test lead.
  3. Fit a Back-pinning Probe or  multimeter probe on the colored test lead.
  4. Place the black clip onto the battery negative terminal.
  5. Probe the idle speed control valve's earth return circuit with the Back-pinning Probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout box or lead if you have one available.

The electronic component is controlled by switching the earth path to ground under specified conditions. The idle speed control valve has a 12 volt supply and its switching can be seen in the example waveform. It may be possible to see a slight frequency shift as the valve opens to maintain the engine idle speed under high electrical demand.

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.


Figure 2 - Electromagnetic idle speed control valve

Technical information

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

  • Back-pinning Probe Set


  • Large Dolphin/Gator Clips


  • PicoScope Battery Clip


  • Connector breakout lead kit A


  • Multimeter Probes


  • Premium Test Lead: BNC to 4 mm, 3 m


  • Premium Test Leads: Set of four leads 3 m (TA125 - TA128)


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Guided test: Electromagnetic