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Idle Speed Control Valve - Electromagnetic Diagnostics

The Electro-magnetic Idle Speed Control Valve (ISCV) will have 2 electrical connections, with 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 prerequisite speed according to its programming. The valve will form an air by-pass around the throttle butterfly, to form a controlled air bleed within the induction tract. If the engine has an adjustable air by-pass and an ISCV, it may require a specific routine to balance the two air paths.

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 it's temperature and different load conditions.

When the engine is first started from cold, the engine management Electronic Control Module (ECM) will give the engine cold start enrichment and will increase the engine's idle speed to approximately 1200 rpm: it is the ISCV that is responsible for this increase. As the engine reaches operating temperature the enrichment will be eliminated and the idle speed reduced to a predetermined speed; this speed will be 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 electro-mechanical device that has a supply voltage either from the ECM or a control relay.

The component will have 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 prerequisite speed according to it's programming. An ISCV can be either a rotary device or an electro-magnetic device, both of which are popular with the rotary being the most common. The valve will form an air by-pass past the throttle butterfly, to form a controlled air bleed within the induction tract and is therefore susceptible to dirt and carbon like deposits impeding it's 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 by-pass 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 producing a square wave, and a 'sawtooth' waveform from the electro-magnetic device.

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


An electro-magnetic idle speed control valve.


Idle Speed Control Valve - Rotary Diagnostics


The rotary Idle Speed Control Valve (ISCV) will have 2 or 3 electrical connections, with a voltage supply at battery voltage and either a single or a double switched earth path.

The rate at which the earth path is switched is determined by the Electronic Control Module (ECM) to maintain a prerequisite speed according to it's programming.


The valve will form an air by-pass past the throttle butterfly, to form a controlled air bleed within the induction tract. If the engine has an adjustable air by-pass and an ISCV, it may require a specific routine to balance the two air paths. The rotary valve will have the choice of either single or twin earth paths, the single being pulled one way electrically and returned to its closed position via a spring; the double switched earth system will switch the valve in both directions. This can be monitored on a dual trace oscilloscope.

Probing onto the supply side will produce a straight line at charging voltage and when the earth circuit is monitored a square wave will be seen. The frequency can also be measured.

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 it's temperature and different load conditions.

When the engine is first started from cold, the engine management Electronic Control Module (ECM) will give the engine cold start enrichment and will increase the engine's idle speed to approximately 1200 rpm: it is the ISCV that is responsible for this increase. As the engine reaches operating temperature the enrichment will be eliminated and the idle speed reduced to a predetermined speed; this speed will be 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.

The component will have 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 prerequisite speed according to it's programming. An ISCV can be either a rotary device or an electromagnetic device, both of which are popular with the rotary being the most common. The valve will form an air by-pass past the throttle butterfly, to form a controlled air bleed within the induction tract and is therefore susceptible to dirt and carbon like deposits impeding it's performance. It is recommended that they are cleaned at the manufacturers service intervals with a spray solvent to maintain their efficiency.


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

The rotary valve will have the choice of either single or twin earth paths, the single being pulled one way electrically and returned to it's closed position via a spring; the double switched earth system will switch the valve in both directions.

Here is a rotary idle speed control valve.