I have changed out many brake light switches on various cars over the years, with most of them being on high mileage vehicles primarily driven in stop and go city traffic. With the brakes being used so many times in that kind of traffic it is no wonder these switches occasionally fail. Many models actually have two micro-switches inside this switch, the first one is for the brake lights, and the second one unlatches the cruise control if it is turned on when you press the brake pedal.
At the heart of these switches are tiny mechanical “micro-switches” which can only cycle so many times before the internal contacts and tiny springs begin to wear out. What I have seen most often is the switch fails in the “ON” position where your brake lights will not turn off. People find this out when they go to start the car and find the battery has drained overnight. You can test these with the car off by slowly pressing the brake pedal towards the floor, carefully listening for the click that these make when the switch closes the circuit. Bear in mind that occasionally the switch fails in such a way that makes your cruise control stop working!
With the car backed up close to another car, a wall or garage door, you should be able to see the reflection of your lights when the switch works correctly. Pressing the brake should light the tail lights, and releasing the brake pedal must turn off those same lights. I have seen some work ten or twenty times in a row, and then fail. If there is any doubt at all, it is cheap insurance to just put in a new switch.
So once you know if the switch is staying on or off, then you will need to go to the parts store and get an exact replacement. Many times the new one will look similar, but the wiring connector will not fit, so you may have to return the first part and get another one. There are numerous switches made for brakes on cars and trucks, and I find it is wise to take the old one out first and take it to the parts store. Once you get the new one, you should compare the old with the new.
Now comes the hard part, getting the old switch out from under the dashboard. Most times these are buried really high up, and some are even above the brake pedal arm. The wiring connector is usually out of sight so that it is not easily kicked or broken. You will have to contort your body upside down to even see up under the dash then you have to use a small flat screwdriver to lift the latching tab on the wiring connector. If you are not sure how this connector works, check with your parts store and see if they have any information on removing the wiring harness connector on your vehicle and switch.
Half the battle is getting the wiring connector off, now comes the fun part, many switches have a spring retainer that the barrel pushes through, and this only lets you push the ribbed stem of the switch in until it stops against the end of the mounting bracket. It is really hard to pull the switch back out of this retainer, and I usually have to destroy this metal retainer holding the switch in place in order to remove the switch. Don’t worry about saving the old retainer since the new switch will come with a new one. I have had to put a tiny pair of needle nose pliers between the mounting bracket and this silver metal retaining ring to force it off the barrel of the old switch.
Once the old switch is out of the bracket, you can pull down on the wires in order to gain access to the wiring connector on the switch. Use a small screwdriver to lift the latch on this connector, and pull the connector out of the switch body. You may have to wiggle this back and forth to get it to break free.
Now connect the new switch to your wiring connector. Now would be a great time to test how far out the switch button has to be prior to the switch making a connection, do this before you mount the switch into it’s mounting bracket. If the brake lights go on when the button on the switch is out, then go forward and insert the barrel of the switch through the mounting bracket that holds the switch up against the brake pedal. Next press the ring clip over the barrel until it is pressing tightly against the bracket holding the switch firmly in place. If you are lucky, then the new switch will not require any adjustments, but it is always wise to go ahead and check the operation of the new switch before you plan a big road trip.
In the event that you have to adjust this switch for proper operation, this is done by loosening the bolts or screws holding the switch mounting bracket to the metal dashboard framework. The proximity of this switch to the brake pedal must be open enough that a slight tap of the brake pedal gives you a brake light, but the brake light should not be able to go on randomly without you pressing the pedal. If you have the switch set with the gap open too far, then It can bounce and vibrate which may cause the brake lights to accidentally go on and off by themselves.
On the other hand if it is set too close, you could be braking, and yet the brake lights will not go on to warn people behind you. I usually set these by pressing the pedal with my hand while lying under the dash so that I can see and hear when this switch engages. As the gap between the switch and the brake pedal opens up, the on point becomes more sensitive, and moving the switch back to press more tightly against the brake pedal makes the switch less likely to turn the brake lights on.
The point here is you need the brake lights to go on as the pedal moves towards the floor about an eighth to a quarter of an inch. This is usually enough of a gap to get these switches to work properly. You will find that many newer vehicles use switches that are mounted into a “self adjusting” bracket, and they will click into the correct place the first few times you press the brake pedal.
Please be very careful when setting this switch since a failure to light your brake lights can cause catastrophic problems for you. Any decent aftermarket service manual specific to your model vehicle will have information on setting this switch. Worst case scenario you can call the dealership which sells your model, and ask the service department if they can give you some information on this.
This information is intended for those who have some automotive repair skills, and is presented as a Do It Yourself project guide, and the risk is all on you in this kind of deal. Please be safe and smart, if you cannot get this to work correctly, call in an expert!