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Brake Switch Test

The brake light switch failing is a common problem but other things can cause the same symptoms. Makes sense to test your switch before blindly replacing it.

Modification Details




 It's fairly common for people to experience problems selecting reverse in the 450
fortwo and the Roadster. The first thing people say is "check to see if the brake lights
work". If they don't come on then the brake light switch is usually broken.

Unfortunately this isn't a 100% accurate test as the brake light switch is actually
2 separate switches in 1 package. Usually, when the switch fails, it is both parts that
become faulty but not always. It is very possible for the ECU switching side to become
faulty but the brake lights still function as expected.

One part of the switch activates the brake lights and the ABS controller.
The other part of the switch tells the ECU the state of the switch, on or off.

Removing The Brake Light Switch

Removal of the fortwo brake light switch can be found
here.
Removal of the Roadster brake light switch is even easier, it is situated on
the top of the brake pedal. Just reach it, give it a quarter turn twist and it
will come free of the holder, then just disconnect the wiring.

The Brake Light Switch

The picture below shows the brake light switch as used in the fortwo. The Roadster
version is the same but without the black rubber bellows over the switch slider.



Using A Multimeter

You will need a multimeter to test the switch. There is no excuse not having a
multimeter these days as you can buy them for as cheap as £5. Even the cheapest
one you can buy will do what you want throughout the car.

You need to put the multimeter to Ohms or continuity setting. It will give you a level of
resistance or beep at you if there is a suitable connection between the 2 multimeter probes.



If your multimeter doesn't do the beeping thing then you will need to put it on
the lowest Ohm value and keep an eye on the screen as you do the test. What
you are looking for is a suitable low number. There will be an obvious change in
number as you press the brake light switch if it is working correctly.



Testing The Brakelight Switch

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