How to Replace the Wiring Harness in an Engine

by Jack Hathcoat

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The average shade-tree mechanic made quite a to-do back in the 1980s, when electronic fuel injection debuted and cars started growing miles and miles of new insulated copper veins. More than a few old-schoolers swore off working on these newfangled contraptions altogether -- probably retiring to shoe horses or fix buggy bearings. But rewiring a new car and installing a new harness requires no more talent or advanced training than building, say, a medium-sized nuclear reactor. For the most part, it's just a matter of taking things step by step, inch by inch, and refusing to step back and look at the rat's nest you've created until it's done.

Step 1

Photograph the current wiring with extensive and detailed pictures of wire connections and routing. Use masking tape and label each wire connection for future reference. Use a shop manual and study the engine wiring diagram and note the color codes of the wires. Over time wire colors fade, and the wire colors with added stripes or hash marks become illegible.

Step 2

Disconnect and remove the battery from the vehicle. Use a socket wrench to remove the terminal clamps -- negative first, then positive -- and hold-down brackets that secure the battery. Avoid any stray sparks by setting the battery away from the car.

Step 3

Remove the old harness by systematically working through all the individual engine connections. Remove the main, bulkhead connection located on the firewall where the engine harness goes into the car to connect to the main computer. Most of the engine connections are snap-in plugs, but use a small screwdriver and socket to remove the assorted clips and retainers that hold the harness in place and to unbolt the alternator and starter wiring. Hidden sensors, such as the crank sensor and knock sensor, are sometimes hard to reach and easily overlooked. Work methodically and be thorough.

Step 4

Jack up the car and place jack stands securely under the frame. Lower the car onto the jack stands. Remove all the wire connections under the engine. Often, separate harnesses intertwine with the engine harness, such as the lights and engine fan harness. Isolate and separate these, and do not needlessly remove them.

Step 5

Label all the new harness connections with masking tape based on the photographic documentation and labels on the old harness. This saves a lot of time and confusion. Plan the installation by sorting the tangled wires, and learn exactly how they are routed and installed before proceeding. Once completed, install the new wires.

Step 6

Install the new wiring, and get all of the plugs connected before tucking the wires back into their respective conduit, or securing them to the chassis or engine. Wiring harnesses can be deceptive insofar as wire lengths and required split points. If finalize the routing before connecting anything, you may find yourself with too much or too little wire to snake around obstacles and make connections.

Step 7

Finalize your routing, and tuck the wires into their conduits. Lower the vehicle and reinstall the battery. Start the car and inspect the installation. Make sure the new wires are routed away from moving engine parts or hot exhaust pipes.

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