Jeff's DIY

Advice on doing your own diagnosis and repair

Advice from the book Can I Do It Myself?


How To Change a Light Bulb--In Your Car

You suspect the bulb is burned out, but how do you get at it?

First, make sure you buy the right bulbs for your socket. Using the wrong bulb can damage your wiring or blow a fuse. Take the old bulb out and make sure it looks like the right one (fits and has contacts that align with the contacts in the socket). You can always have the parts store look up your car and tell you which bulbs to buy.

Ok, back to getting at your bulbs. The first choice is always to check your owner's manual, if you have one.
If you don't have one, see if you can find one through this portal:

Still no manual? There are millions of different designs, so the best I can do is to give you some ideas about how it is generally done. Let me break it down by the standard groups.

1. Head Lights
2. Parking, turn, and Brake Lights
3. Interior Lights

1. Head Lights:
Bulbs (most modern cars) usually go in thru the back (through the engine compartment) of the lens housing. The bulb/socket is usually held in by a twist of the socket. Twist it loose, pull it out, and remove the bulb from the socket. Some sockets are held in by clips instead of twisting. Make sure you understand yours before forcing it. Also note, some sockets are permanently attached to the bulb, but these always have a connector that unplugs at the socket.
Old style sealed beams are held in by a ring with 3 screws. Pull the connector off the back and take the 3 screws out of the front. Be careful that you are NOT turning the headlight aim adjuster screws!! The ring screws are smaller and there are 3 versus only 2 adjusters. You may find it too tight to access the screws. If there is a trim piece you can take off to get better access, do so. Otherwise just sneak a small screwdriver into the crack. Take the ring and bulb out. The new bulb only goes in one way--it should lock in when you have it turned upright. Put the ring back on and reconnect the bulb.

2. Parking, turn, and brake lights:
Sockets for these lights usually come out with a simple twist--the trouble usually comes in getting to the socket. The front lights may be accessed from the engine compartment. If not, try reaching them from under the car. You may need to remove a splash panel or a wheel well splash shield to get at them. Once you have the socket/bulb out of the lens, the bulb comes out of the socket either with a twist or by pulling straight out.
The rear lights are usually accessible from the inside. If not, such as in the case of a pick-up truck, they will have screws on the outside of the lens so you can remove the whole lens. Otherwise, from the inside, pull back or remove whatever is covering the sockets. In some cases, there are just butterfly nuts or screws holding the lens with no way to access the bulb from inside. In these cases, you have to remove the lens to get at the socket. Once you have the socket/bulb out of the lens, the bulb comes out of the socket either with a twist or by pulling straight out.
If the car has separate backup lenses or fog lights, they may be mounted in the bumper, and some are sealed beams; but there are too many variations to cover here.
The license plate light is usually held in by screws or tabs on the cover similar to courtesy lamps.

3. Interior Lights:
An interior light with a cover on it can usually be accessed with a small screwdriver. The cover is either held in by screws or small tabs. In the latter case, use a very small flat blade screwdriver to push the small tabs one at a time while pulling to release the cover. Be careful not to break the plastic, which may be brittle from the heat of the bulb.
The bulbs come out of the socket either with a twist or by pulling straight out.

So you've changed your bulb--is it working? No?
If not, check my other tips on light out.


Lights Out, Lights Not Out, Bulbs OK

You say your lights aren't behaving? Not coming on even though they should? Not going off and draining your battery?

Let's break this down by type of light so you can jump to the section that applies to your current issue.

1. Brake Lights
2. Parking Lights
3. Head Lights
4. Interior Lights

1. Brake Lights:
The first thing to note about brake lights on cars and trucks is that they are always hot--that is, the key usually doesn't need to be on for them to work. This is a safety feature, but it can also present problems when the brake switch sticks on. The brake switch is usually a mechanical switch mounted at the brake pedal arm that closes the circuit when the pedal is pushed just a little. Some cars use a hydraulic pressure switch on the brake master cylinder to actuate the brake lights, and there may or may not be a relay in the circuit. Either type sends a signal (ground or 12 VDC) to whatever circuit controls the brake lights. Most designs have the switch close when the brake is applied. If you remove the connector from the switch, you have simulated the brake pedal up. If you use a paper clip to join the terminals in the connector, you are simulating the brake pedal down. This is how we test the switch. Note that many brake light switches have additional terminals for either releasing the automatic transmission shifter or canceling cruise control. This may make troubleshooting more challenging as far as figuring out which terminals to connect to make the lights come on. If you can't figure it out, look for a wiring diagram at the library or a repair manual.

Bottom line on the brake switch:
1. If your brakes are staying on, pull the connector off the switch, and if the lights go out you have a bad switch.
2. If your brakes are not coming on, pull the connector off the switch and short the connector with a paper clip. If the lights come on you have a bad switch.

If the switch works and is adjusted such that the pedal actuates it properly, move on to the next troubleshooting step--the relay.
Almost all brake lights use a relay to limit the current through the switch. The relay is actuated by the brake switch itself or by a more complicated circuit in the case of some systems that use the same bulb filaments for both turn signals and brakes. In these latter cases, the brake circuit must allow the signal for the commanded bulb to blink rather than light continuously. Troubleshooting the signal light circuitry is a bit challenging for the average do-it-yourselfer, and therefore is not covered in this tip. Either way, there will be a brake light fuse and brake light relay in the relay box (usually under the hood). The relay socket can be interrogated with a 12 volt test light for the presence of a command signal from the brake switch circuit. If the command signal is present, the relay should click once when plugged into its socket. If not, you've got a bad relay. The relay can be tested using my relay tips or by simply swapping the relay with a similar one from the horn or A/C circuit. If the command signal is absent at the relay socket and your brake switch is good, you have a problem with the brake light wiring or possibly the circuitry in the turn signal switch.

Bottom line on the brake light relay: If there are two hot terminals at the relay socket when the pedal is depressed, your brake switch and circuitry is telling the lights to be on; and if there is only one hot terminal, the switch and circuitry is telling the lights to stay off.

If your switch, circuitry, and relay are all working, the only thing left is the wiring and bulb sockets. With the brake depressed, you can check the wiring and sockets using a test light on the socket. If the power is not getting from the relay to the socket, the wire is open. Check any intermediate connectors for a good connection and look for broken wires.

2. Parking Lights:
The parking and marker lights may be actuated manually or automatically. In either case, there is often a relay; but in some cases the relay may be incorporated in a relay module and thus cannot be easily interrogated or purchased separately. If you are good with electronics, you may have some luck testing the light switch itself for proper operation. If you can find a parking light relay, you can test the relay socket for the presence of the command signal and you can test the relay itself using my relay tips. Note that some cars have running light circuitry to turn the front parking lights on whenever the key is on. This is called daytime running lights or DRL. In these cases, there is usually a separate relay for the front and rear parking lights, but it may be incorporated into a relay module. In most DRL designs, the rear parking lights and instrument panel lights are both controlled by the PARK setting of the manual light switch.

If your switch, circuitry, and relay are all working, the only thing left is the wiring and bulb sockets. With the switch on, you can check the wiring and sockets using a test light on the socket. If the power is not getting from the relay to the socket, the wire is open. Check any intermediate connectors for a good connection and look for broken wires.

3. Head Lights:
The head lights may be actuated manually or automatically. In either case, there is a fuse and a relay or possibly two or more relays. There are almost always four bulb filaments among the headlights--two high beams and two low beams. The left and right bulbs may also have separate relays. There is obviously a High-Low switch that switches between the filaments. If the high and low beams have their own relays, the relays and sockets can be checked using my relay tips.

A simpler design is to feed the headlight switch signal through the High-Low switch and on to a double-pull relay that powers the High beams, Low beams, or neither. Such a relay and socket cannot be interrogated using my simple relay troubleshooting tips; however, if you can access the wiring diagrams for the car, you may be able to devise your own troubleshooting procedure. Also, if you are good with electronics, you may be able to test your headlight switch or automatic circuitry (including the daylight sensor, if your car is so equipped).

Bottom line on Head Lights: check the fuse in the engine compartment first. Then check the relays, if possible. If you aren't sure the relays are working, it's a relatively small investment to buy new ones just to be sure. Alternately, try swapping a suspect relay with the horn or A/C relay to see if it's bad.

4. Interior Lights:
The interior lights are controlled by a combination of door switches and manual switches. There is usually a centralized controller that receives ground signals from switches in each door, when opened. If any of the switches sends a ground signal, the controller turns on the lights. Any given light may also have a manual switch that can turn the light on and perhaps off, though there are usually some lights that cannot be turned off without closing all doors. Many controllers incorporate a delay and perhaps also a dimmer to allow the light to continue for some time after the last door is closed.

The first thing to check when the lights are staying on is the instrument lights dimmer (if it is also used as an interior lights switch) or an interior lights switch that may have been turned on by accident.

If not that, the most likely cause is one of the door switches stuck on (closed). This can be caused by a failure of the switch or a consequence of the door lock being adjusted too loose. A good test for this is to have someone go around the car and push in on each door to see if one of them will turn the light off. Any door that can turn the light off when pushed must have its lock adjusted in tighter. If this test doesn't fix it, check the individual switches. If you can locate the switches, pull them out of the car body. If the lights go out, the switch is bad. In case the switch has two wires rather than one, pulling the connector off the switch should have the same effect. Test each switch in this way. If you have all switches disabled and the lights are still on, the problem is likely to be either one of the switch wires grounding out to the car body or the interior lights control module shorted on.

If your interior lights are not coming on, first check the fuse. Next, try other doors to see if one of the switches may have failed. If neither of these is the problem, the issue is likely the controller.

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