Resonance Flap Control

RESONANCE FLAP CONTROL CIRCUIT DIAGRAM:

This is how it works:

A. RESONANCE FLAP CONTROL

The Resonance Flap Control Unit (1) is located beneath the three relays in the E-Box, between the Motronic control unit and the SRS control unit.

Based on both the full throttle signal and the engine speed signal, it will ground the lead to the changeover valve (2) and thus energize it.

The necessary conditions for such an action are:

- engine speed less than 4,120 RPM and full throttle; or

- engine speed greater than 6,720 RPM and full throttle.

Unless either of the above conditions are met, the changeover valve will not be energized (via grounding by the resonance flap control unit) and the resonance flap (4) remains open.

Energizing the changeover valve (2) will connect the actuator (3) to the vacuum tank
(5) and the flap (4) will be closed.

NOTE: Upon start-up of the engine, the Resonance Flap Control Unit will perform a selfdiagnostic check of the system and close and open the resonance flap twice. This can easily be checked by observing the linkage between the actuator (3) and the flap (4). However, there is no fault storage and a diagnostic check cannot be performed using the MODIC or BMW Tester.

Component Location:

(1) Resonance Flap Control Unit: E-Box beneath relays.

(2) Changeover valve (resonance flap control): mounted to underside of plenum (next to oil separator).

(3) Actuator for resonance flap: bolted to upper side plenum.

(4) Resonance flap: inside plenum.

(5) Vacuum tank: beneath # 5 intake runner.

A built-in hysteresis of 60 RPM (delay of switchpoint depending on whether approaching from lower or higher englne speeds) around each of the two speed values will prevent the flap from constantly switching between open and closed position, should the engine be operated at exactly the speeds of either 4,120 or 6,720 RPM.
Provision of the vacuum tank was found to be necessary, since the pressure in the plenum at full load conditions is not low enough (i.e. there is not sufficient vacuum) to positively move the flap. The vacuum tank was designed to allow for 6 consecutive openings and closings of the flap without replenishment. The check valve (6) prevents a loss of vacuum during engine shutdown or whenever the vacuum level in the intake runners is lower (= the pressure is higher) than in the vacuum tank.