Brake Pedal Sensor: How to Bypass/Defeat?

Hi everyone,

Occasionally, one reads about performance issues here, where, after a lengthy investigation, a desynchronization of the switching points between the brake light switch and the brake pedal switch was identified as the cause.

While looking at the wiring diagram (for the Golf 3), I noticed that you could actually short-circuit the two inputs of the engine control unit and connect them together to a single switch.

When the brake is not engaged, the...
- The brake light switch is open, and the resistance of the brake lights pulls input 44 of the engine control unit to ground.
- The brake pedal switch is closed, which also pulls motor computer input 20 to ground.

When the brake is applied.
- There are 12 volts present at the brake lights and at input 44 of the engine control unit.
- the brake pedal switch opens, and an internal pull-up resistor also pulls the motor computer input 20 "high" - conveniently, also to 12 volts instead of 5 volts, as one might expect in a digital system.

So, the voltage at both brake input terminals of the engine control unit (assuming both switches are functioning correctly) is always either 0 or 12 volts simultaneously.

My idea is to disconnect pin 44 of the motor controller, which is connected to the brake light switch, and connect it to pin 20, which is connected to the pedal switch. This way, input 20, which is "not accustomed to receiving signals from the constant positive voltage," will not be exposed to voltage even when the ignition is off (because the brake lights are powered by the constant positive voltage).

Does anyone know of any reasons why this simplification might be problematic, or why the brake pedal monitoring was implemented with such a complex system using two switches?

Hi Ulf.
I believe that this version of the circuit was chosen to minimize power consumption when the motor is off.
What could happen if you connect the two ports as suggested?

I think the only limitation that occurs is that the brake lights only illuminate when the ignition is turned on.
That's exactly where I see a problem.
Suppose your engine stalls (or whatever the reason) and you're stopped at or on a crossroad. Then, the cars behind you won't realize that you've turned off the ignition and are still stopped using your brakes.

I also have no idea what the TÜV (German technical inspection agency) will say about it, or whether this might constitute a modification to the vehicle's approval documents.

What if you inserted a relay that activates when the ignition is turned on?
sends a signal from the brake lights to the engine.
This way, the motor consumes no electricity, and the lights still work.


However, it would be necessary to check whether the motor controller can actually handle 12V, especially since it shouldn't be receiving that voltage in the first place.


It would be interesting to find a solution...
bye
Sure, here is the translation of the text from German to English:

'Micha'

Hi Micha,

In my plan, the brake lights are already connected to a constant power supply.

I would only disconnect the wire from the brake light switch to the engine computer (Pin 44) and connect it together with the pedal switch (Pin 20).

I don't think the TÜV (German technical inspection agency) would notice that.

Okay, then...
go for it
I don't think there will be any reasons to object to it.

If you do it, let me know what happened.
PS: Have you already experienced any problems with this 'VW Doppel' issue?

Please remember that the ABS and automatic transmission control units rely on the correct functioning of the brake light switch, especially the ABS!

CU Gremlin.

Gremlin wrote:

Please remember that the ABS and automatic transmission control units depend on the correct function of the brake light switch. This is especially true for the ABS!

Hi.

Thank you for the tip.
I drive a manual transmission.
My ABS wiring diagram only shows one connection from the brake light switch, but no connection to the pedal switch of the EDC.
If I feed the (correct) signal from the pedal switch to the EDC and leave the ABS connected to the brake light circuit, nothing should happen, right?

@micha: I haven't experienced any stress yet, but I also like to prepare in advance to rule out that possibility .

My ABS wiring diagram only shows one connection for the brake light switch, but no connection to the pedal switch of the EDC.
If I supply the (correct) signal from the pedal switch to the EDC and leave the ABS connected to the brake light circuit, nothing should happen, right?


That should work then.

'The most important thing is that you don't change the functions of the circuit. This plausibility check has its purpose. Since we (like almost all newer gasoline engines) have drive-by-wire, this monitoring is considered safety-relevant.'
As you may have noticed, you can't press the accelerator while the brake is engaged.
'Furthermore, in the event of a failure of the throttle position sensor, the engine control unit will increase the idle speed (and in the case of a failure of the throttle actuator in a gasoline engine, it will be controlled by spring force). However, the brake pedal always overrides the fuel mixture preparation. This means that even if the accelerator pedal fails, you can still limp to the workshop. When the brake pedal is pressed, the engine is immediately and forcefully throttled.'

CU Gremlin.

As you may have noticed, you can't press the accelerator while the brake is engaged.
Furthermore, in the event of a failure of the throttle potentiometer, an increased idle speed is applied (similarly, in the case of a failure of the throttle actuator in a gasoline engine, this is achieved through spring force). However, the brake pedal always overrides the fuel mixture preparation. This allows you to limp to the workshop even if the accelerator pedal fails. When the brake pedal is pressed, the engine is immediately and forcefully throttled.

CU Gremlin
That would fit with the problem I described, which I posted here /viewtopic.php?t=919:!: . However, during one braking maneuver, I experienced a surge in RPM that gave me a huge adrenaline rush. }
Does the issue with the brake pedal also affect the clutch? Lately, I've been experiencing a situation where the engine RPM drops significantly, and then for about half a second to a second, there's no power output, before normal power becomes available again.

Hi everyone,

Regarding the second point mentioned by Micha_Passat, I can already state that it would not be relevant for the TÜV (German technical inspection). (Even though Ulf has already addressed this.)

The brake lights should only function when the ignition is on. It doesn't matter whether they work without the ignition or not. I used to drive a Ford (shame on me...), and they didn't have brake lights that worked without the ignition being on.

Regards,
Private.