Test Point 19 - Resonance Flap

TEST POINT 19: Resonance flap

In certain engine speed ranges and at not less than one-third of full engine load, the resonance flap in the intake system is opened by vacuum. It is also opened whenever the engine is started (provided that vacuum is present in the reservoir).

a) Checking function of flap
- Remove the resonance flap shaft cover (rubber cover) attached above the intake pipe cluster. Using a felt marker or similar, mark a central line on the surface of the flap shaft (this makes it easier to detect opening movement).
- Start the engine: the flap will open and close again as the engine is started.
- When the engine is running, depress the accelerator pedal to the full throttle position suddenly: the flap will open and close (However, this does not happen unless the engine is running at more than 3500 rpm and at not less than one-third of its full load).

b) Checking operation of resonance flap without running the engine: vacuum side
- Detach the vacuum supply pipe from the vacuum reservoir at the solenoid valve. Check vacuum with a vacuum tester: Vacuum approx. 0.6 bar (17.7 in Hg).

After this, build up a vacuum of approx. 0.4 bar (11.8 in Hg) at the pipe to the diaphragm valve (below the intake air pipe cluster). The resonance flap should open.

If the flap does not open:
- Check diaphragm valve and supply pipe.

c) Power supply (V)
- Use the test leads to bridge terminals 17 and 21 at the LH control unit plug. Use other test leads to connect terminal 34 of the control unit plug to ground = the resonance flap will open.

If the flap does not open:
- At the LH control unit plug, connect terminals 17 and 21 with the test leads. Separate the plug connection at the solenoid valve. Connect voltmeter to plug socket (2) and ground.

Display: battery voltage
No display: Check power supply to solenoid valve by way of LH relay XXV according to the wiring diagram. Check that there is no break in the circuit from LH control unit terminal 34 to the solenoid.




d) Checking the control signal with the oscilloscope
- Attach the LH control unit plug. Disconnect the plug at the solenoid valve. Connect the engine tester with the test leads to the sleeve side of the solenoid valve plug in accordance with the manufacturer's instructions, and adjust (with Bosch MOT 300/400, but select screen range of 20 V). While starting the engine and if the throttle is opened suddenly when it is running, this picture must be visible on the tester: