1. Visual Inspection:
* Bulbs: Start with the simplest check. Turn on the brake lights (either by pressing the brake pedal or using a helper) and visually inspect all three brake lights (two on the rear and one on the center high-mounted stop lamp (CHMSL) or third brake light). Look for burned-out filaments. If a bulb is burnt out, replace it with the correct wattage bulb. (Always refer to your owner's manual for the correct bulb type).
* Wiring: Examine the wiring harness leading to each brake light for any obvious damage, such as fraying, cuts, or corrosion. Look for loose connections at the bulb sockets. Gently wiggle the connectors to see if there's an intermittent connection.
2. Testing the Bulbs and Sockets:
* Multimeter (Recommended): A multimeter is the best tool for this job. It allows you to test for voltage and continuity.
* With the Brake Pedal Depressed:
* Voltage Test: Set your multimeter to measure DC voltage (usually indicated by a symbol like "V"). Touch the positive probe to the metal contact on the bulb socket and the negative probe to the metal casing of the light fixture (ground). You should read 12V (or close to it) if the circuit is working correctly. If you don't, the problem is either before the bulb or the ground is faulty. Repeat this for each brake light.
* Continuity Test: With the brake pedal *not* depressed, set the multimeter to measure continuity (usually indicated by a symbol like "Ω"). Touch the probes to the two contacts in the bulb socket. You should read near zero ohms (a closed circuit) indicating a good connection through the socket and wiring. Repeat for each bulb and socket.
* Without the Brake Pedal Depressed:
* Continuity Test: Repeat the continuity test above, but you should have infinite resistance (an open circuit) indicating the circuit is open when the brake pedal is not pressed. If you have continuity, there is a short circuit somewhere in the brake light wiring.
3. Checking the Brake Light Switch:
The brake light switch is located near the brake pedal. It's a small switch that closes the circuit when you press the brake pedal.
* Access: You may need to reach under the dashboard to access the brake light switch. Consult a repair manual for the exact location and access procedures.
* Visual Inspection: Check the switch for any obvious damage or loose connections.
* Testing (Multimeter): With the multimeter set to continuity, check the continuity between the two terminals on the switch. One terminal should be always connected to ground; the other should only make contact when the brake pedal is depressed. A continuity test across the switch should give you infinite resistance with the pedal released, and continuity when the pedal is pressed. If not, the switch is faulty and needs replacement.
4. Checking the Fuse:
Your owner's manual will show you the fuse box location and the fuse for the brake lights. Check to make sure the fuse hasn't blown. If it has, replace it with a fuse of the same amperage. If the fuse blows immediately after replacing it, there's a short circuit somewhere in the system that needs further investigation.
5. Checking the Ground:
A poor ground connection can also cause brake light issues. Inspect the ground wires for corrosion or loose connections at each brake light assembly and on the chassis. Clean any corrosion and ensure tight connections.
6. Wiring Harness Issues:
If none of the above solves the problem, you might have a damaged wire somewhere within the wiring harness. This can be difficult to troubleshoot without a wiring diagram. A professional mechanic may be necessary at this point.
Important Considerations:
* Safety First: Always disconnect the battery's negative terminal before working on the electrical system.
* Repair Manual: A repair manual specific to your 1999 Ford Explorer will be invaluable in this troubleshooting process. It will provide wiring diagrams, fuse box layouts, and component locations.
* Professional Help: If you're uncomfortable working with electricity or cannot identify the problem, consult a qualified mechanic.
By following these steps systematically, you should be able to pinpoint the cause of your brake light problem. Remember safety and take your time; rushing can lead to further damage.
