Brake lights seem extremely simple. Press the brake pedal and they come on, release, and they go off. Many things can and do go wrong. When they do, this guide can help find the problem.
A volt ohm meter or a LED test light will be needed for some tests. Using these tools properly is very important. The point of the tool probe should only touch the contact, when checking. Pushing the probe into a light socket will spread the contacts and cause the socket to fail. Please read, "Tips on Automotive Electrical Testing," for more information.
Brake lights will not go off
Only a few things normally account for brake lights staying on. One of the more common is a bad brake-light switch. If the contacts stick, the switch may continue supplying power to the lights, though it is released. Before replacing a switch check to see if it can be tested. Analog switches are very straight forward. Some modern vehicles may use digital sensors instead of a switch. These are far more difficult and may require professional service. Service data for the vehicle in question will normally identify the type.
To test an analog brake-light switch for staying on, remove the switch and test for continuity between the terminals. A wiring diagram and ohmmeter are needed. The wiring diagram shows which terminals to check.
Some vehicles may have more than one switch on the brake pedal and some switches may have more than one set of contacts. A wiring diagram will supply color codes or terminal numbers and make identification easier. The ohmmeter should show continuity when the switch is pressed and open circuit when it is released. If continuity is indicated when released, replace the switch.
The brake pedal may also fail to contact the switch. Many vehicles have small plastic pads, where the switch contacts the pedal, when released. If these pads break or fall out, the pedal may not push the switch enough when released. Honda, Toyota and many other vehicles use these rubber pads and have this problem.
A switch may also slip out of position, increasing the space between the plunger and the pedal. With the pedal released, the brake-light switch should be pushed to its full off position. Pressing the pedal should cause the switch to move. If the plunger of the switch does not contact the pedal, check for an adjustment.
In rare instances, the brake booster may fail, and the pedal will not rise enough to contact the switch. Pull up on the brake pedal and if the lights go out, switch mis-alignment or pedal position error is the likely cause.
The final possibility is a wire shorted to power. Unplug the brake-light switch and if the lights stay on, a short circuit is the case. Finding such a shorted-circuit, is best left to a professional.
Brake lights will not come on
When the brake lights will not come on, the first thing to check is the third-brake light. If it too is not working, the brake-light switch, a bad fuse or an unplugged harness is likely. A quick test is to try the emergency-flashers, on vehicles that use the brake lights as flashers. On vehicles where the flashers are separate from the brake lights, this will not apply.
The emergency flashers often use the same wiring, bulbs and sockets as the brakes. If all lights come on, when the emergency flashers are turned on, this shows the bulbs, wires and sockets are good. Check for a blown fuse and be careful, as many vehicles have more than one fuse box. The owner's manual or service data will show fuse box location.
Good fuses mean the most likely cause is the brake-light switch. Test the switch, as described above, except there should be continuity when the switch activated. If the switch does not show continuity when pressed, replace the switch.
Lower-lights out, but third light works
When the lower lights do not come on and only the third-brake light works, the brake-light switch may be ruled out. Again try the emergency-flashers to check for wiring, socket and lamp problems. If the flashers work, check for blown fuses as a few vehicles use separate circuits for the lower and third brake lights.
On vehicles that use the same lights for brakes and turn signals, a bad turn-signal switch can cause the lower-lights not to work, and still allow the third-light to function. On systems where turn signals and brakes are the same bulb, the turn signal must cancel the brake light, on the side selected, in order to flash. A bad turn-signal switch or a broken wire can cause the lower brake-lights not to work. The third brake light does not flash with turn signals, so it is not affected.
Third brake-light out but lower lights work
If only the third light is out and the lowers still work, see if it will flash with the emergency-flashers. If the inoperative third light flashes, with the emergency flashers, a fuse is the most likely cause. If it does not operate with the flashers, bad bulbs or sockets in the third-light are likely. Many modern third-brake lights use an LED and do not have bulbs that can be replaced. These must normally be replaced as an assembly and are not serviceable.
Brake lights out on one side
A brake light out on one side only is most often a bulb or socket problem. Try the emergency-flashers and if the lamp still does not light, bulbs or sockets are the very likely cause. Inspect the bulb filament and replace any found bad. Substitute a known good bulb, from the other side, to verify bulbs that cannot be confirmed otherwise.
Checking light sockets and connectors
Inspect sockets closely for signs of discoloration, distortion or corrosion on the contacts. With the brakes depressed, lightly touch the terminals with a voltmeter, to check for voltage. No voltage at the terminals means the wires behind the socket should be back-probed if possible. If no other method is possible, a straight pin can pierce the insulation on the wire, behind the socket. Be certain to seal any area check in this manner as the damaged wire may cause future problems.
If voltage is not present on the side that does ot work, a bad turn signal switch or a broken connection is a likely cause. A voltage present between the wires and a ground means continuity on the ground wire should be checked. Check for continuity with an ohmmeter, between the ground wire and a known good ground. If the ground is good, the socket should be replaced. Be sure to repair any damage to insulation, caused by testing.
Finding broken wires and bad connectors
Finding a broken circuit or bad connector requires a wiring-diagram for the system being checked. The basic procedure is to start at one end of the circuit and check for voltage and ground. If either cannot be found, go to the other end of the circuit. Voltage and ground at this point show the need to select a midway point. Power and ground here, means the problem is in the second half of the circuit. A lack of power or ground shows the first half holds the problem. The procedure is repeated, dividing the circuit in half, until the problem is found.
Most brake light problems are fairly simple to find. For tougher problems give AGCO a call. We specialize in electrical diagnosis, minor or major.