How To Diagnose Brake Light Problems 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.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.With only two terminals, continuity should�show in one position and an open circuit in the other. Some vehicles may have multiple switches 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.Using the wiring diagram, we can determine which leads should have continuity and when. The brake-lamp portion of the switch will show continuity when the brake pedal is depressed.� When we release the brake pedal, it returns to the up-position.� In this position, the brake light switch changes to an open-circuit.� If continuity is indicated when released, or open-circuit shows when depressed, replace the switch.� Other terminals may operate in a different manner.� These are sometimes used by the cruise control

, transmission and other systems. 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.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. Nokian Winter Tires Online CanadaA 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.Repair Cracks In Concrete StatueThe emergency flashers often use the same wiring, bulbs and sockets as the brakes. Walking Shoes For CapsulitisIf 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.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 brake-lights not to work. With a bad turn-signal switch, the third-light should still function. On this type system, the turn signals and brakes use the same circuit. When we use the turn signal switch, it cancels the brake light signal, on the side selected. If it did not, the lights would stay on and not flash. A bad turn-signal switch may interrupt the brake light circuit when it should not.

A broken wire to or from the switch may also cause the lower brake-lights not to work. The third brake light does not flash with turn signals. This is why a bad turn signal switch will not affect it.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.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.

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.In the illustration above, assume the red wire is the power lead for the brake lights and that the black wire is the ground for the circuit.

When we depress the brake, the red wire shows 12 volts, because current is flowing. If it does not, a problem exists with this circuit.With the brakes still applied, less than 0.1 volts should show on the black wire, when checked to the body ground. More voltage shows high resistance in the ground circuit. Full system voltage between the black wire and system ground suggest missing or broken ground. We can also use an ohmmeter to check for continuity between the black lead and body ground.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.