Test Point 20 - Fuel Tank Venting

TEST POINT 20: Fuel tank venting (V)

When the engine is at regular operating temperature and the throttle valve is opened (idle speed contact), the solenoid valve is activated by the LH control unit with an intermittent signal to ground. The greater the airflow at the air mass sensor, the longer the solenoid valve remains open.

a) Checking solenoid valve activation
- Connect terminals 17 and 21 with test leads at the LH control unit plug. Use another test lead to connect terminal 27 to ground (on body): the solenoid valve is energized.

If the valve does not open:
- Check power supply and continuity of circuit from terminal 27 to solenoid valve according to the wiring diagram.

b) Checking power supply (V)
- At the control unit plug, connect terminals 17 and 21 with the test leads. Disconnect the solenoid valve plug and connect the voltmeter between the plug connection and ground with the aid of test leads.

Display: battery voltage

No display:
- Check power supply through LH relay according to wiring diagram.




c) Checking control signal with oscilloscope
- Connect DME test lead (Bosch No. 1 684 463 093) between solenoid valve and plug connection. Connect the engine tester in accordance with the manufacturer's instructions and adjust it.

- Start the engine and open the throttle slightly (the idle speed contact must open, engine temperature must be > 60°C). This display is then obtained:

As air flow increases, the signal broadens.