* Failing Brake Light Switch: This is the most common culprit. The switch is activated when you press the brake pedal, completing the circuit to illuminate the brake lights. If it's failing, it might make intermittent contact, causing the light to flicker or stay on dimly before eventually going out completely.
* Wiring Issues: Corrosion, loose connections, or damaged wires in the brake light circuit can cause similar intermittent problems. Check the wiring harness leading from the brake pedal switch to the brake lights and the related fuse. Look for any corrosion, fraying, or loose connections. Pay special attention to the area around the brake pedal.
* Faulty Brake Lights: While less likely to cause the *dash* light to behave this way, a failing brake light bulb can sometimes cause erratic behavior in the circuit. Check all the bulbs (both the brake lights and the turn signals – they share some wiring). A short in a bulb's socket could also be a factor.
* Bad Ground: A poor ground connection can interrupt the circuit, leading to inconsistent illumination. Inspect the ground wires associated with the brake light system.
* ABS Module (if equipped): If your van has anti-lock brakes (ABS), a malfunctioning ABS module *could* contribute to this problem, although it's less likely to cause a dimming light.
Troubleshooting Steps:
1. Check the brake light bulbs: Replace them if they're burnt out or look damaged.
2. Inspect the brake light switch: This is usually located near the brake pedal. You might be able to visually inspect it for damage or looseness. Consider testing it with a multimeter to confirm its proper operation.
3. Examine the wiring: Carefully inspect the wiring harness for any damage, corrosion, or loose connections. Pay particular attention to the connections at the brake light switch and at the lights themselves.
4. Check the fuses: Locate the fuse(s) related to the brake lights in the fuse box and check for any blown fuses.
5. Test the ground connections: Trace the ground wires from the brake light system and ensure they're clean and securely connected.
6. Consult a wiring diagram: A wiring diagram specific to your 1986 Ford E150 van will be invaluable in tracing the circuit and identifying potential problems. You can often find these online.
If you're not comfortable working with car electrical systems, it's best to take your van to a qualified mechanic for diagnosis and repair. Ignoring this issue could lead to unsafe driving conditions.
