Idle speed controller

Here you will find useful basic information and important tips relating to idle speed controllers and idle speed actuators in vehicles.

The idle speed controller ensures that the idle speed of the engine remains constant in all load conditions. On this page you can find out about how the idle speed actuator regulates the intake air mass of the engine, and how the failure of this actuator impacts on the engine running. Practical tips for troubleshooting and assembly instructions help to quickly rectify defects relating to the idle speed controller.

Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.



The idle speed actuator is a bypass air valve. The idle speed actuator shown as an example consists of a closed cast housing with flange-mounted solenoid valve servo unit. Attached to this is a nozzle holder, which releases different air cross sections by moving the servo unit, and can therefore control the air mass flow when the throttle valve is closed.


The idle speed actuator is responsible for regulating the engine speed as part of the overall idle-speed control of the engine management system. If there is a sudden change to the engine load condition when in idle mode (e.g. air-conditioning system being switched on, creep speed in 1st gear, or activation of another electrical consumer), air and fuel are also required in order to prevent the engine from stopping. If the engine speed falls below a certain critical value, which is stored as a constant in the control unit memory, the solenoid valve is activated and an increased air flow rate is achieved. The opening time of the injection valves is simultaneously extended and adapted to the engine's requirements.


A faulty idle speed actuator can manifest itself as follows:

  • Fluctuating idle speed
  • Engine stops running at idle speed
  • Engine stops running at idle speed and activation of additional consumers
  • Engine indicator lamp comes on


Causes for failure of the idle speed actuator can be:

  • Contamination / resin formation
  • Short circuits in the coil
  • Electric magnetic drive becomes stuck
  • No voltage supply from the engine management control unit


The following test steps should be considered during troubleshooting:

Checking the voltage supply

Check the voltage supply with the ignition switched on.

  • Measured value: 11 – 14 V.


An Ohm measurement must only be carried out in a de-energized state (plug connection removed).

Measuring the coil resistance

Use the multimeter to measure the coil resistance between the two connection pins of the idle speed actuator.

  • Reference value = approx. 10 ohms (observe manufacturer's specifications).

Checking the coil for winding short circuit

Check the coil for a winding short circuit between the two connection pins.

  • Measured value in the event of a short circuit = 0 ohms.

Checking the coil for winding interruption

Check the coil for a winding interruption between the two connection pins.

  • Measured value in the event of an interruption = >30 MOhm.

Checking the coil for short circuit to frame

Check the coil for short circuit to frame – between pin 1 and component housing, and between pin 2 and component housing.

  • Measured value in the event of an unimpaired coil = >30 MOhm.

Mechanical check

Unscrew the servo unit from the housing. Visual check to determine whether the bypass opens and closes when the valve rod is actuated.

Read out fault code

Read out fault code.


Before mounting the idle speed actuator, the supporting surface must be cleaned and, if necessary, the flange seal must be replaced.


Observe the tightening torque!

  • Fixing screws 12 – 15 Nm


Please observe the specifications provided by the respective vehicle manufacturer.