1. Identify the Problem:
* Which lights aren't working? Headlights, taillights, brake lights, turn signals, interior lights, dome light, etc.? Knowing this narrows down the potential causes significantly.
* Are *some* lights working, or are *all* lights out? This helps determine if the problem is a single component or a more widespread issue (like a dead battery or blown fuse).
* When did the problem start? Did it happen suddenly, or gradually? This can provide clues about the cause. For instance, a sudden failure might indicate a blown fuse or damaged wire, while a gradual dimming might point to a failing battery or alternator.
2. Basic Checks:
* Battery: Check the battery voltage with a multimeter. It should be around 12.6V when fully charged. A low voltage indicates a charging system problem (alternator or voltage regulator) or a dead battery. Jump start the truck if the battery is low to see if the lights work; if they do, you know the problem is the battery or charging system.
* Fuses: Check the fuse box (usually under the hood or inside the cab). Consult your owner's manual to identify the fuses responsible for your lights. Look for blown fuses (broken filaments). Replace any blown fuses with fuses of the *same amperage*. Don't use a higher amperage fuse – this is a fire hazard.
* Light Bulbs: Check the bulbs themselves. Remove them and inspect the filaments. Replace any burned-out bulbs with the correct type and wattage (again, consult your owner's manual). Make sure they're properly seated in their sockets.
* Ground Connections: Poor ground connections are a common cause of electrical problems. Check the ground wires connected to the lights and chassis. They should be clean, tight, and free of corrosion. Clean any corroded connections with a wire brush and reconnect them securely.
3. More Advanced Troubleshooting (If the above steps don't solve the problem):
* Wiring: If specific lights are out, trace the wiring harness to those lights. Look for broken, frayed, or corroded wires. Repair or replace any damaged wiring. A wiring diagram (available online or in a repair manual) will be essential here.
* Switches: Test the light switches (headlight switch, turn signal switch, etc.). A multimeter can help determine if they're functioning correctly.
* Multi-meter: Use a multimeter to test for voltage at the light sockets. If you have voltage, the problem is likely the bulb or the ground. If you don't have voltage, the problem is further upstream in the circuit – possibly a fuse, switch, or wiring.
* Relay: Some lighting circuits use relays. Check the relays for proper operation.
* Alternator/Voltage Regulator: If the battery is constantly low despite charging, the alternator or voltage regulator may be faulty.
4. Resources:
* Owner's Manual: Your owner's manual will have diagrams and information specific to your 1986 Chevy truck.
* Repair Manual: A Haynes or Chilton repair manual provides detailed wiring diagrams and troubleshooting guides.
* Online Forums: Online forums dedicated to classic Chevy trucks can be a valuable resource for finding solutions to specific problems.
Safety First: Always disconnect the negative battery cable before working on any electrical system. Be careful when working with electrical components; improper handling can lead to injury or damage. If you're not comfortable working with automotive electrical systems, it's best to take your truck to a qualified mechanic.
