Volkswagen Golf/Jetta/Cabriole 1990-1999

Idle Stabilizer Valve

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The idle speed is controlled electronically on vehicles equipped with the Mono-Motronic fuel control system. This system also controls the idle speed by adjusting the engine timing. The Electronic Control Unit (ECU) monitors all engine loads and adjusts the idle speed accordingly.

Operation





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Fig. Typical VW idle stabilizer valve

Motronic

The idle stabilizer is a motorized rotary valve that is operated by the ECU to control the amount of air that bypasses the throttle. This design allows very precise control of idle speed regardless of engine temperature or load. The valve is spring loaded towards the minimum opening position. The voltage that holds the valve open is applied in variable length pulses called a duty cycle. The duty cycle ranges from 5-95 percent. Because of this design, there is no idle speed adjustment.

Digifont

The idle stabilizer is a linear motor solenoid valve that is operated by the ECU. The linear motion moves a plunger to control an opening in the valve which controls the amount of air that bypasses the throttle. This design allows very precise control of idle speed regardless of engine temperature or load. The voltage supplied to the valve can't really be measured because it is not constant. To test the duty cycle of the valve in operation, a special adapter is required that allows connecting a multi-meter that reads milliamps while the wiring is connected to the valve.

TESTING




NOTE
On vehicles with high mileage on them, carbon deposits may form on the throttle plates and restrict air flow. This will affect the operation of the IAC system.

Motronic
  1. With the valve installed, turn the ignition switch ON but do not start the engine. It should be possible to hear or feel the valve hum and/or vibrate.
  2.  
  3. Turn the ignition switch OFF . Unplug the connector from the idle stabilizer valve and connect an ohmmeter across the valve terminals. There should be 7-11 ohms resistance. Resistance may be higher on a warm engine.
  4.  
  5. Disable the ignition system so the engine will not start. Connect a voltmeter or an LED test lamp between terminal 1 of the connector and ground. When the starter is operated, there should be voltage at terminal 1.
  6.  
  7. Remove the valve and check for visual signs of scoring or binding on the rotating portion. Do not lubricate the valve. If it is receiving voltage but does not operate, it must be replaced.
  8.  

Digifont
  1. With the ignition ON but the engine not running, the valve should vibrate to the touch. If not, make sure the idle switch on the throttle body is working properly and that the throttle is fully closed.
  2.  
  3. If there is no vibration at the valve, turn the ignition OFF and unplug the connector. Use an ohmmeter to check the resistance across the terminals on the valve. There should be about 2-10 ohms resistance.
  4.  
  5. Connect the adapter so a multi-meter can be connected. With the engine at operating temperature and idling, the current to the valve should fluctuate from 390-460 milliamps. With the blue temperature sensor wiring disconnected, the current should be steady.
  6.  
  7. If the current is not correct, remove the valve and check for visual signs of sticking. Do not lubricate the valve. If no other problem is found, check the continuity of the wiring between the valve and the ECU with the ignition OFF .
  8.  
  9. If the idle stabilizer valve seems to work properly but engine idle is out of specification, check for a vacuum leak, a faulty coolant temperature sensor or some other problem with the engine control system.
  10.  

 
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