1979 Chevy Truck Turn Signal Wiring Schematic and Color Codes Guide
Locate the main harness connector behind the instrument cluster–the three-wire plug handles left and right flasher circuits. Green (left) and yellow (right) feeds run directly to the bulb sockets; splice in new 18-gauge wire if corrosion has compromised the originals. The hazard flasher relay sits on the firewall near the brake master cylinder–test continuity between the red/white striped wire and chassis ground when the switch is engaged.
Replace the original turn signal switch if the canceling cam wears down–aftermarket replacements include a pigtail with color-coded connections. Connect brown (tail lights) last; improper grounding here causes bulbs to glow dimly when brakes are applied. Use a multimeter to verify 12V across the flasher terminal while the lever is in either direction–any reading below 11V indicates a voltage drop from corroded contacts at the trailer connector.
Install in-line fuses (7.5A) on both output wires before routing them to auxiliary lamps–factory circuits lack protection and shorting risks melting the column wiring harness. Route wires along the frame rail on the driver’s side, securing every 6 inches with nylon straps. Solder all splices and cover with heat-shrink tubing; electrical tape degrades over time under exposure to oil and road salt.
Test operation with the parking brake engaged to confirm emergency flashers activate both directions simultaneously. If the relay clicks but no lights illuminate, inspect the ground strap under the left kick panel–factory welds often rust through, requiring a new #10 ring terminal screwed into the chassis.
For vehicles with factory towing packages, ensure the heavy-duty flasher (square relay) is installed–standard relays overheat with trailer loads. Match the new flasher’s load rating (typically 16-21 candlepower per bulb) to the existing one; excessive current trips the circuit breaker built into the switch.
Lighting Circuit Schematics for Classic C/K Models
Locate the flasher relay beneath the dash–it’s typically mounted near the steering column on the left side. For C/K series pickups built before 1980, the relay has two spade terminals marked “X” and “L.” The “X” terminal connects to the hazard switch via a 16-gauge brown wire, while “L” joins the turn stalk via a 14-gauge light green lead. If the stalk doesn’t cancel after a lane change, trace the canceling cam behind the steering wheel–adjust the cam’s three plastic teeth so they engage the stalk’s metal tab at 45-degree wheel rotation.
- Front bulbs: amber lenses use dual-filament 1157 bulbs–high filament (brighter) for braking, low filament (dimmer) for signaling; ground each bulb through the headlight bucket.
- Rear bulbs: red 1157 bulbs share ground with the taillight socket–replace corroded sockets with pigtail connectors pre-loaded with dielectric grease.
- Neutral switch: on column-shift models, the backup light wire (purple, 18 AWG) branches off the neutral safety switch–verify switch continuity with a multimeter set to 200 Ω.
- Fuse: a single 15 A fuse in the instrument panel protects both signaling lanes–upgrade to a blade-style fuse with an inline holder for easier replacement.
Finding the Flasher Circuit Bundle in a Classic 1978 GMC Pickup
Begin under the dashboard on the driver’s side–look above the fuse panel where a black plastic conduit spanning left to right houses the flasher wires. The bundle splits near the steering column’s base into two 16-gauge leads: a light green/yellow stripe (left circuit) and a solid dark green (right). Trace these back to the firewall grommet; they emerge alongside the brake light harness before terminating at the rear lamp assemblies.
Check behind the instrument cluster for an inline connector–usually a three-prong plug linking the hazard relay to the main harness. Disconnecting it reveals color-coded terminals: the center (power) carries 12V from the ignition switch, while outer prongs switch to ground when the indicator stalk engages. Inspect for brittle insulation, especially around the column’s tilt mechanism where repeated movement frays copper strands.
If relays aren’t clicking, peel back the kick panel’s plastic trim below the glove box to expose the flasher unit–a silver cylindrical device screwed into the firewall’s lower brace. Its wiring mates with three spade connectors: red (battery feed), brown (output to lamps), and black (ground). Swap with a known-good unit (3-prong 12V, typically 4- to 6-ohm resistance) before probing deeper.
How to Locate and Trace Indicator Circuit Paths Behind the Instrument Panel
Begin by disconnecting the battery’s negative terminal to prevent short circuits while probing connections. Locate the fusebox beneath the left side of the dash–older models often house flasher relays here, identifiable by a cylindrical metal canister with two or three prongs. Use a multimeter set to continuity mode to verify the relay’s functionality; if defective, replace it before proceeding.
Identify the color-coded leads from the steering column switch. Most classic rigs use a standard scheme: the left-side flasher wires are typically light blue with a black stripe, while right-side leads are dark green with a white stripe. Trace these wires from the column connector toward the firewall, noting any junctions or splices where corrosion or broken strands may occur. A frayed or discolored wire at these points often indicates failure.
- Remove the lower dash panel screws–usually 7/16″ or 10mm fasteners–and gently pull the panel away to expose the harness. Label each connector with masking tape and a permanent marker to simplify reassembly.
- Follow the wires into the firewall grommet, where they transition into the engine bay. Inspect the grommet for cracks or brittleness; a compromised seal allows moisture ingress, accelerating wire decay.
- Use a non-contact voltage tester to confirm power absence before manipulating exposed conductors. Probe suspect wires with a test light or multimeter to confirm signal presence–expect 12V on the hot side when the switch is activated.
If the signals operate erratically, focus on ground integrity. Locate the chassis ground near the brake pedal support bracket–a single 1/4″ ring terminal bolted to the frame. Remove the nut, clean the contact surfaces with a wire brush, and reattach, ensuring no paint or rust remains. A poor ground causes intermittent operation, mimicking wire failure.
For splices or damaged sections, strip back the insulation and splice using solder and heat-shrink tubing–twist-and-tape methods degrade over time. Route repaired wires along original paths, securing them with plastic loom or zip ties to prevent chafing against sharp edges. Reconnect the battery, activate each indicator sequence, and verify consistent operation before reassembling the dash.
Decoding Wire Colors for Early Lighting Circuits on a C/K-Series Pickup
Begin by locating the fuse block beneath the dashboard–early models use a two-prong flasher relay (white plastic casing). Pinch the relay between thumb and forefinger; if resistance is absent, the bimetallic strip has degraded. Replace immediately using a TSM-36 part or equivalent to ensure proper load balance across left and right switched outputs.
| Function | Factory Color Code | Alternative Identifier |
|---|---|---|
| Flasher input (hot in ignition) | Pink | 0.8 mm² (18 AWG) |
| Left switched output | Dark green with white stripe | Trailer harness tee near frame rail |
| Right switched output | Dark brown with white stripe | Engine bay harness grommet |
| Brake switch feed | Light green | Pedal bracket connector |
| Ground return | Black | Steering column support bolt |
Trace each conductor from the column-mounted switch to the bullet connectors behind the grille; oxidation often mimics filament failure. Use a multimeter in continuity mode–probe socket shell to wire core; readings above 0.5 ohms indicate corrosion requiring solder splice and heat-shrink tubing. Avoid twist-and-tape joints; these degrade under chassis flex.
Check the harness at three critical junctures: underneath the driver-side kick panel, at the firewall grommet (2″ downward from master cylinder), and where the loom splits near the rear crossmember. Remove protective loom tape only where necessary; otherwise, fibrous debris contaminates the splice. Label severed ends with masking tape and Sharpie before disconnecting battery ground terminal–minimum 15-minute wait prevents capacitor discharge injuries.
Verify the hazard warning module (located behind rear bench seat on crew cab models). Its white lead should pulse simultaneously with front and rear lamps; static light confirms internal relay failure. Replace the module box-rivet style; reuse the original phenolic connector block unless melted–then swap for a Delphi PN 1355396 single-pin connector block to restore full circuit integrity.
Testing and Troubleshooting Flasher Mechanism Contacts
Begin by disconnecting the steering column harness plug from the stalk assembly. Use a multimeter set to continuity mode to probe pins A (brown wire) and D (light green wire) on the female connector while activating the left indicator lever. Consistent beeping confirms circuit integrity; silence or intermittent readings indicate corrosion in the contact plate or a fractured trace inside the housing. For the right side, test pins A (brown) and C (dark blue) under identical conditions. Corrosion on the contact ring often mimics a failed switch–clean oxidation with 600-grit emery cloth and dielectric grease before reassembly.
If continuity tests pass but lamps remain inoperative, inspect the hazard relay socket (located behind the instrument cluster on early models) for melted solder joints on terminals 30 and 87. Apply a 12V test light to terminal 85 with ignition on; a dim glow suggests a weak ground at the frame rail (G100, near the brake master cylinder). Replace the flasher unit if relay functions correctly but bulbs flash at double frequency–this indicates an internal short bypassing the thermal element.