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.
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
Look at the connection end of the switch and hold the same way as below.
The thing to remember about this switch (and the thing that confused me to put the error in
previously) is that the switch is pushed when the brake pedal is NOT pushed. When the brake
pedal is pushed it moves away from the switch. Thanks to Tolsen for pointing out the mistake.
Pins 1 and 4 are connected by a switch and notify the ECU that the brake has been pressed.
Placing a probe on both pins (1 and 4) will show no change on the multimeter until you press the switch. This means that the switch is "normally open" so pressing the switch down will close it.
You shouldn't have to press the switch in more than 3mm to make the contact.
Pins 2 and 3 are connected by a switch and activate the brake lights and ABS controller.
Placing a probe on both pins (2 and 3) will show a change on the multimeter.
This means that the switch is "normally closed" so pressing the switch down will open it.
You shouldn't have to press the switch in more than 1mm to make the contact.