Exhaust Flapper Switch

The .:R32 comes with a vacuum operated flap on the exhaust system that allows some of exhaust gas to bypass part of the main muffler under certain operating conditions. The vacuum solenoid operation is controlled by the engine ECU.

In "Quiet" Mode, the exhaust bypass is closed, keepng the exhaust noise level to 75dB. The car must exceed 65 km/h (40mph) for the bypass flap to open. It must also exceed an engine speed of 3400 rpm. When the flap is open ("Loud" Mode), the exhaust noise level is 84dB.

Closure of the bypass flap occurs once the road speed drops below 65 km/h and/or engine speed drops below 3000 rpm.

This modification allows you manual control the operation of the vacuum operated flap independently of ECU programming.

There is an excellent .pdf here describing the installation of a three way (ON/OFF/ON) switch to control the flapper operation. The three way switch allows "Always Quiet - Always Loud - Stock" operation of the flapper.

If you wish to use a two way ON/ON switch (giving "Always Loud" or "Stock" operation), rather than the three way switch descibed in the .pdf, you can use this simple circuit.

All due acknowledgements and thanks to the guys who pioneered the "Flapper Mod" originally!

I was looking for a more OEM+ installation using an illuminated switch. I also wanted the system to reset to "Stock" Mode whenever the ignition was switched off. This page describes the setup I designed to achieve this.

Please note that the Flapper can only have either "Stock" Mode or "Loud" Mode with the modification detailed below.

How Does The Stock Exhaust Flapper Work?

The stock flapper is operated by a rod controlled by a vacuum diaphragm (visible under left rear of vehicle).

An initial vacuum "supply" is stored in a canister mounted under the rear floor on the chassis rail. There is a small electric solenoid mounted near the canister that controls the supply of vacuum to the diaphragm, which is controlled by the ECU.

There is a pipe running from this canister to the rear of the inlet manifold. The vacuum pipe is incorporated into the interior harness.

There is a one-way check valve installed in this line near the solenoid and another in the engine bay as well.

Should the vacuum system fail, the flap will default to the OPEN position.

In "Quiet Mode" vacuum holds the exhaust flap SHUT.

To achieve this in normal operation, the ECU will run this solenoid when it requires the exhaust flap to SHUT.

When the solenoid is therefore ON, it creates vacuum in the diaphragm, which allows the flap to SHUT.

To clarify, if the solenoid is ON (powered), it opens the vacuum line from the diaphragm to the vacuum canister.

When you perform the "Flapper Mod", you are controlling the operation of this solenoid.

The resistor is required in the circuit to avoid the ECU from recording a CEL, as there is a small load from the solenoid motor that the ECU is monitoring.

The function of the switch in my circuit is to trigger the "latching" relay "434" (as the switch I used is only a "momentary action" switch).

This in turn energises the coil in the "79" relay.

This then switches the feed to the ECU from the solenoid motor (Relay Pin 87a) to the new 12V supply through the extra resistor (Relay Pin 87).

You are therefore breaking the circuit of the solenoid so that it does not operate.

The ECU is happily "earthing" what it perceives to be the solenoid, so no CEL is recorded.

Operation of my OEM+ Flapper Switch

Whenever car is started it is always in "Stock" Mode.

If you want "Loud" Mode, you simply press the new switch.

The small yellow LED telltale in the switch face will also then light up as well.

At night, the switch symbol is illuminated red and is dimmable like all the other dashlights, so easily operated at night.

If you press the switch again, it will revert to "Stock" Mode

You can switch between "Stock" and "Loud" Mode at any time whilst the Ignition is ON.

Once ignition is switched OFF, the flapper operation always reverts to "Stock" Mode when you restart (same as stock ESP functions etc).

Here is the "FS" Switch I initially used for my Flapper Switch, mounted on the right in centre console behind ashtray. The switch on the left is for my Footlights:

I have now swapped the switch over to a FUNK Switch. The wiring is identical to the "FS" Switch used previously:

Parts List

1 x 1T0 927 124 9B9     Touran/Passat 3C "FS" Switch (out of Taxi). For other suitable OEM+ switches, see here.

1 x 4D0 971 636            6-pin Plug for Switch.

2 x 443 937 527            9-Pin Clip-in Relay Mount for Additional Relay Plate under dash.

1 x 191 927 841            Relay "79".

1 x 1C0 941 597            Relay "434".

2 x 1K0 937 530            Clip-on Fuseholder (or you can use vacant positions in fusebox, need at least one large fuse space for the Resistor).

2 x 1K0 937 555            Clear plastic cover for above Fuseholder.

1 x N 017 131 11           10 Amp fuse (fits in above Clip-on Fuseholder).

1 x 2000 Ohm 1/4 Watt resistor.

1 x Old fuse with open housing on top (see sample picture in section below titled Mounting Resistor in Fuseholder).

Various repair wires for relay terminals (see Wiring Notes below).

Butt connectors or solder and shrinkwrap.

Wiring Notes

I have quoted metric VW wire sizes. Here are the approximate US equivalent sizes:

0.5mm² ~ 20 gauge

1.0mm² ~ 16 gauge

2.5mm² ~ 12 gauge

Terminal Pins

If you look at the new Relay Plate Mounts, you will note there are three different size terminal pins that they use. The Relay Plate Mount only requires Medium and Large pins to plug in the relays for this installation.

The VW Repair wires for the different terminal sizes are:


Terminal socket numbers 1, 3, 7 and 9.

1.0mm² = 000 979 133

2.5mm² = 000 979 225


Terminal socket numbers 2, 5 and 8.

1.0mm² wire = 000 979 135

2.5mm² wire = 000 979 227


Terminal socket numbers 4 and 6.

0.5mm² wire = You can buy bare terminals from Porsche - Part Number 999 650 330 00

1.0mm² wire = 000 979 114 (or you can buy Porsche bare terminals 999 650 103 12)

2.5mm² wire = 000 979 212

Additional clip-on Fuseholder uses Medium terminal repair wires.

If you use a spare Large Fuse in the main fusebox then you need Medium terminals as well.

If you use a spare small size fuse (Mini Fuse) in the main fusebox, then you will need to use the Small terminal repair wires instead.

The FS Flapper switch itself uses repair wires 000 979 009

When you buy a VW repair wire you get a length of wire with a terminal precrimped on each end.

RHD Flapper Wire Location

The instructions contained in this section for locating the solenoid control wire are only applicable to RHD .:R32's.

If you have a LHD .:R32, please refer to the linked .pdf at the top of this page for the specific instructions to locate the correct wire in your harness.

Easy access to the Right Hand Drive .:R32 solenoid wire is obtained on the passenger side lower A-pillar area behind the side kickpanel. It is very easy to locate the two wires for the vacuum solenoid as they are not bound in the main harness at this point.

You will note two tubes next to the bound harness - one is the rear screen washer supply and the other is the vacuum pipe for the flapper mechanism. Take care not to pierce either of these accidently when you are cutting the flapper solenoid wire.

The two wires for the vacuum solenoid are right next to these tubes. One wire is 1.0mm² yellow/black and the other is 1.0mm² grey/blue.

The 1.0mm² grey/blue wire is the only one that requires cutting.

Cut the 1.0mm² grey/blue wire and butt connect a wire to each loose end. You may need to unwrap the harness slightly to get a sufficient working length of wire.

On a RHD car these wires need to be long enough to extend right across the dashboard to reach your new "79" Relay mounted in the Additional Relay Plate located above the driver's feet.

Take very careful note of your connection at the "79" Relay.

The 87a connection must join to the cut 1.0mm² grey/blue wire that runs DOWN the loom towards the rear of the vehicle.

The 30 connection must join to the cut 1.0mm² grey/blue wire that runs UP the loom. This wire runs through to the engine bay area.

The rest of the installation just requires mounting the switch and connecting the other wires in the new circuit, such as would also be required in LHD vehicles.

Mounting Resistor in Fuseholder

I wanted to have a "plug in" Resistor, rather than just solder one into the wiring somewhere. This is how I made up a mount out of an old fuse. BMW sell these type of fuses if you can't find one.

Old Fuse dismantled and resistor soldered to blades:

Resistor mounted in fuseholder:

Testing Resistor for correct resistance after assembly:

Plugged into Fuseholder:

With clear plastic cover cap installed:

There is a vacant spot on the side of the Additional Relay Plate that this assembly will clip onto.

Wiring Components

Here is the wiring schematic of my "Flapper Mod" (OEM wiring indicated by blue lines):

Power Fuse Wiring Connections

Run new 2.5mm² wire from - 87F- Screw (located under Relay Plate) to your new 10 Amp fuse (unfused side).

Run new 1.0mm² wire from your new 10 Amp fuse (fused side) to a new wiring junction point that you need to create.

I have designated this new point the "- 87F- Connection Point" in descriptions below.

I just used a VW Butt connector to create this new join point.

You need to run four more wires from the - 87F- Connection Point:

1.0mm² to Flapper Switch Pin 1

1.0mm² to Relay "434" Pin 2

1.0mm² to Relay "434" Pin 5

1.0mm² to Pin 1 on 2000 Ohm Resistor mounted in Additional Fuse Holder on Additional Relay Plate.

Relay "79" Wiring Connections

2 / 30 = Butt connect new 1.0mm² wire to original 1.0mm² grey/blue wire you cut in A-pillar that runs UP the loom.

3 / 30a = Not used in this application.

4 / 86 = 0.5mm² to Relay "434" Pin 8 and also splice to "FS" Flapper Switch Pin 5

5 / 87a = Butt connect new 1.0mm² wire to original 1.0mm² grey/blue wire you cut in A-pillar that runs DOWN the loom towards the rear of the vehicle.

6 / 85 = 0.5mm² to Earth.

8 / 87 = 1.0mm² to Pin 2 on 2000 Ohm Resistor on Relay Plate

Relay "434" Wiring Connections

1= Not used in this application

2 / 30 = 1.0mm² wire to - 87F- connection point

4 / 31 = 1.0mm² wire to Earth

5 / 56 = 1.0mm² wire to - 87F- connection point

6 / LU = 1.0mm² wire to "FS" Flapper Switch Pin 2

8 / 56a = 0.5mm² wire to Relay "79" Pin 4 / 86 and also splice to "FS" Flapper Switch Pin 5

"FS" Flapper Switch Wiring Connections

The wiring connections noted here for this "FS" Switch are identical only to these other OEM+ Switches:

1T0 927 235 - Interior Light Switch; 1T0 919 241 - "ALARM" Switch; 1T0 927 227 - "TAXI" Switch; and the "FUNK" Switch.

Pin 1 & 2 are the feeds for the on/off momentary contact in switch

Pin 3 & 4 are the feeds for the red illumination LED in switch

Pin 5 & 6 are the feeds for the yellow telltale LED in switch

Pin 1 = 1.0mm² to new - 87F- Connection Point

Pin 2 = 1.0mm² to Relay "434" Pin 6 in Relay Plate

Pin 3 = dimmer positive - join to dimmer rheostat circuit

Pin 4 = dimmer earth - join to dimmer rheostat circuit

Pin 5 = 0.5mm² to Relay "434" Pin 8 and Relay "79" Pin 86 in Relay Plate

Pin 6 = 0.5mm² to Earth

I ran all the required earth wires noted above to a common Earthpoint on the steering column which is directly in front of the Relay Plate.

I tapped into the dimmer rheostat circuit for the Switch illumination at the OEM in-dash CD player feed wire. You may use any dash switch illumination or the Cigar Lighter illumination for a dimmed feed. You may also, of course, use a direct connection to the rheostat switch itself.

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