Volkswagen Front Wheel Drive 1974-1989 Repair Guide

Digifant II Engine Controls



Airflow Sensor

See Figure 1

This unit converts air flow to a voltage signal. Air enters and moves a spring-loaded vane which is attached to a potentiometer. The potentiometer then modulates a voltage sent from the control unit and the return signal represents the mass of air flowing to the intake manifold. Air temperature is also measured and reported to the control unit. Adjustment of the air/fuel mixture is accomplished by means of a bypass screw which is also contained in this unit. This screw allows a certain amount of air to bypass the vane and enter the engine unmeasured. More unmeasured air means a leaner mixture.

  1. With the ignition OFF , unplug the sensor connector and measure the resistance across the of the sensor terminals. At 60°F (15°C), there should be about 3000 ohms. At 80°F (27°C), there should be about 1900 ohms.
  3. The resistance between terminals 3 and 4 should be 500-1000 ohms when the flap is in the rest position.

Click image to see an enlarged view

Fig. Fig. 1: Airflow sensor terminal identification

  1. Connect the ohmmeter between the center terminals 2 and 3, then move the vane inside the sensor. The resistance change should be smooth and linear as the vane is moved. There is a strong return spring on the flap but it should move smoothly with no binding.

Throttle Position Switches

There are 2 switches on the throttle body, one above and one below. The lower switch signals the ECU when the throttle is at idle and the upper switch signals full throttle. This information is used to calculate fuel shut-off and ignition timing during deceleration, idle stabilizer valve operation and full throttle enrichment. If there is a problem with any of these functions, check these switches first.

  1. Locate the switches on the throttle body and unplug the connector.
  3. Connect an ohmmeter to the idle switch and make sure it is closed when the throttle is against the stop. Open the throttle, position a 0.024 inch (0.60mm) feeler gauge against the stop and let the throttle close on the gauge. The switch must remain open. DO NOT adjust the throttle stop screw.
  5. Connect the ohmmeter to the full throttle switch. Open the throttle all the way to the stop and make sure the switch closes. The switch should open when the throttle is allowed to close 10 degrees from the stop.
  7. Turn the ignition switch ON and use a voltmeter to check for 5 volts at each switch connector. This signal comes directly from the ECU.

Idle Stabilizer Valve

See Figure 2

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 meters 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 (available at your dealer) is required which allows connection of a multi-meter that reads milliamps while the wiring is still connected to the valve.

  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.
  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.
  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.

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Fig. Fig. 2: A special adaptor is used to test the idle stabilizer valve

  1. 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 .
  3. 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.

Temperature Sensors

There are two coolant temperature sensors mounted in the upper radiator hose-to-engine flange. The black connector is for the gauge on the instrument panel. The blue connector is for the ECU.

There is also an air temperature sensor at the inlet end of the air flow sensor. Both the coolant and air temperature sensors operate with the same resistance values.

  1. Unplug the sensor connector.
  3. Using an ohmmeter, check the sensor resistance.
  5. Compare the test values to the specifications listed. If resistance is out of specification, replace the sensor.

    55°F (13°C)-3000-3800 ohms
    65°F (18°C)-2200-3000 ohms
    75°F (24°C)-1800-2500 ohms
    85°F (29°C)-1500-2100 ohms
    150°F (65°C)-400-550 ohms
    200°F (93°C)-200-270 ohms