73 87 Chevy Truck Wiring Diagram

Reading the Factory Color Code System

The electrical systems in 1973 through 1987 Chevrolet C/K trucks share the same GM color-coding standard that everyone complains about but nobody really learns until they are elbow-deep in a harness. The colors themselves are simple enough: the first color is the base wire, and the second color is a stripe running along it. So a white wire with a blue stripe reads as white/blue. The gauges and instrument cluster use that exact same logic. Under the dash, near the steering column, you will find most of the primary junction points for 73 87 Chevy Truck Wiring Diagram purposes, and that is where you should start every time. Before you pull anything apart, I need to address something most people get wrong about these diagrams. They assume the truck is stock and that none of the colors have faded or gotten dirty over fifty years. That assumption will cost you time. I spent an entire afternoon tracing a phantom short on a 1978 K5 Blazer because someone had repaired a broken tan wire with black tape three decades earlier, and the diagram showed tan where there was nothing but black insulation. You cannot trust the diagram blindly. You have to verify. Here is the practical workflow I use. First, identify the component you are working on. Then look up its wire color in the diagram for your specific year and engine. After that, take a multimeter and confirm the color at the connector yourself, not just by looking at what you think the wire should be. Faded original harnesses often show the stripe color more clearly than the base color. A dark green wire with a white stripe can look almost entirely green after sun exposure. Flip the wire over and check the opposite side.

The main power feed on every model in this range runs from the starter solenoid through the engine compartment fuse block. The big terminal on the solenoid takes the battery cable, and the smaller stud feeds the ignition switch circuit through a pink wire on most years. By 1981 and later, GM switched to a fusible link system in the engine compartment rather than the older inline fuses, which means a melted fusible link inside the insulation can look fine from the outside. Cut the link out and measure it for continuity before you assume the main feed is live. The instrument cluster on these trucks uses a shared ground that runs through the dash metal to the firewall. If your gauges are reading erratically or sitting at zero, check that ground connection first. It is a silver colored wire on most diagrams, but on many trucks it has corroded into nothing but green powder at the chassis point. Scrape the paint off the firewall mounting bolt and run a fresh ground strap if the original path is compromised. I fixed a 1984 pickup with a dead temperature gauge by cleaning a single grounding point behind the dash that had never been touched since the factory. The charging system changed noticeably during this generation. Early models use the internal regulator setup mounted on the alternator itself. Late models, especially 1980 through 1987, move to an external voltage regulator and add a warning light circuit that runs through the ignition switch. If you are swapping a later alternator into an earlier truck, the plug types differ and the wire assignments do not match one to one. The delta system on later alternators requires a specific field excitation path that the older wiring does not provide without modification.

Headlight circuits are another common failure point. The dimmer switch on the column routes power through a yellow wire for high beams and a yellow with black stripe for low beams, but the connectors under the dash tend to melt from heat and vibration over time. I have removed more than a few blackened plastic connector bodies from 1976 and 1977 trucks where the high current load had degraded the terminals. When you find melted connectors, replacing just the bulb will not fix the problem. You need to splice in new housing or upgrade to a relay-based headlight circuit that bypasses the column switch entirely. The tail light and turn signal system shares a common ground at the frame rail on the driver side rear. The wire is usually orange in the diagram, but on trucks that have seen road salt, that orange wire is frequently broken inside the insulation where it passes through a grommet. Probe the wire at both ends with a test light while someone operates the turn signals. If one side lights and the other does not, you have a break in the run between the connectors. If you are looking for a complete 73 87 Chevy Truck Wiring Diagram reference, the factory shop manuals from Bentley Publishers remain the most accurate source available. The Chilton and Haynes books contain simplified versions that leave out certain year-specific changes, particularly around the emission control wiring added between 1975 and 1980. General Motors published official service manuals for each model year, and those are the ones I rely on when the aftermarket sources disagree. They are not free, but the ones you find on various truck forums and enthusiast sites often have errors in the later model years.

One thing worth noting about the 1985 and 1986 models is the introduction of the GM Computer Control system for certain engine packages. The wiring harness for those trucks includes additional sensors and control modules that do not appear on earlier diagrams. If you are pulling a harness from a donor truck, make sure it matches your engine and year exactly. The pin layouts on the main connectors differ between carbureted and fuel injected versions, and forcing a mismatched plug will bend terminals inside the connector housing. The most useful trick I have picked up over the years is keeping a small notebook of actual wire colors you find in each truck rather than relying solely on the printed diagram. Every truck has been modified at some point. The previous owner may have repaired a broken wire with something completely wrong, or moved a connector to a different location. The diagram tells you what the truck should have. Your multimeter and your own eyes tell you what it actually has. Both matter. When sourcing replacement harnesses, the reproduction options from companies like Standard Harness and Circuit City are decent for restore-quality work, but they do not always include the year-specific variations in the instrument cluster or emission control sections. Order early and compare the included documentation against your factory manual before you begin installation. A harness that looks correct at first glance may be missing a connector tap for a secondary air injection valve that your particular engine requires.

Ground points on these trucks are widespread and generally overlooked. The chassis to engine ground strap is a flat braided copper strip on most models, and it corrodes from the inside out where it contacts the mounting bolts. Remove it, clean both surfaces with a wire brush, and reinstall with star washers. A poor engine ground will cause every electrical symptom you can imagine, from erratic gauge behavior to no-start conditions that mimic a bad starter. I spent two days chasing a crank-no-start on a 1975 C10 before I pulled the ground strap and found it separated internally. The outside looked fine. The turn signal flasher relay is mounted under the dash on the driver side corner, and the socket behind it corrodes frequently. Remove the relay, inspect the terminals, and apply dielectric grease before reinstalling. If the flasher clicks rapidly on one direction but not the other, you likely have a burnt out bulb on that side, but if both bulbs are good and the behavior persists, replace the socket assembly rather than trying to clean and reuse it. The plastic degrades and the terminals lose tension. Windshield wiper circuits run through the switch on the column, and the connector at the switch end tends to develop resistance from moisture ingress. The wiper motor itself draws significant current, and voltage drop across a corroded connector will make the wipers slow or intermittent, especially on low speed. Test for voltage at the motor terminals while the switch is engaged. If you see a drop of more than half a volt compared to battery voltage at the input, the problem is in the wiring or connectors between the switch and the motor, not the motor itself.

Brake warning lights and the combination switch on the dashboard share circuit paths that cross between the braking system and the lighting system. The residual pressure valve in the brake system keeps light pressure in the lines to prevent air ingestion, and the brake light switch activates the center high mount stop lamp on 1982 and later models. If your brake lights work but the dash warning light stays on after release, check the switch adjustment at the pedal bracket. It is a simple fix that is easily missed when you are focused on the wiring instead of the mechanical component. Air conditioning wiring on these trucks varies significantly depending on whether the unit is factory installed or an aftermarket addition from the original owner. Factory A/C systems use a dedicated relay and control module that plugs into a specific connector under the dash. Aftermarket installs typically splice into existing circuits without regard for the original diagram, which makes troubleshooting difficult if you do not know which wires are original and which are additions. Trace every wire back to its source before making assumptions about what circuit it belongs to. The fuel sender unit on these trucks changes resistance as the level drops, and the sending unit wire runs from the tank through the frame rail to the dashboard gauge. Frame rail corrosion on rusty trucks commonly breaks this circuit or introduces high resistance that causes the gauge to read empty when the tank is half full. Measure the resistance at the sender connector with the tank level known, then compare it to the specifications for your year. Deviations of more than twenty percent usually indicate a bad sender or a high resistance path in the wire run.

horn circuits are straightforward but the ground path through the steering column bearings is a known weak point on these trucks. If the horn works intermittently or not at all, the issue is often a broken ground wire inside the column loom rather than the horn itself. Remove the column covers and inspect the wires that pass through the flex point near the tilt mechanism. I have replaced more horn grounds than I can count, and nearly all of them were broken wires that looked intact from the outside.