As kindly posted by Pimpernell, here's what I found on my car.

The full thread is here:

EGR/Resonance Control Module

My engine is a OM606.910, naturally aspirated. To do this check, I connected LEDs to the solenoid valves which are fitted under the inlet manifold, ran the wires into the car, and watched the LEDs as I drove along.


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Here's how the resonance flaps work on my car;

The valve in the crossover pipe, I'll call X/O, and the valve in the manifold itself, I'll call 3/6.

The solenoid for the X/O valve is connected to the Yellow / Brown wire pair, and is the upper of the two solenoids when in the as installed position.

The solenoid for the 3/6 valve is connected to the Grey / Brown wire pair.

In the relaxed position with no vacuum, the X/O valve is closed - i.e. the crossover pipes remain independent for their full length.

With no vacuum, the 3/6 valve is closed, and the manifold behaves as an independent pair of 3 cylinder manifolds.

<2400 rpm 3/6 valve Closed, X/O valve Closed

>2400 rpm &
<3500 rpm 3/6 valve Closed, X/O valve Open

>3500 rpm 3/6 valve Open, X/O valve Open

I didn't reliably obtain any switching above 3800 rpm as has been suggested elsewhere, but, for me, that's quite academic; it's not an area of the rev range that I use often, or for any duration.

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