Corner Wrench: Don’t splice electrical car accessories without a plan

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Electrical repairs are never fun, so why do so many people think that DIY add-ons will be easy?

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A wiring harness is often the first component fitted to a vehicle’s bare chassis at the factory, so these tricky diagnoses and fixes often require DIY- or shop techs to remove plenty of tricky components to even access harnesses and connectors. Trickier still, those with called fingers will know that many key parts contain small, densely packed wiring pins in hard to reach places. Of course Canadian winters wreak a special havoc on these systems with corrosion, compounding how easily the low-voltage signals in many of these wires can be disrupted.

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Still, even worse wiring woes can tangle a car’s drivability when someone tries to connect an electrically powered accessory without full knowledge of the circuits they’re tapping into and the results of adding electrical load to such sensitive systems. Worse still, many such messes are created without any consideration for the protection and security of the newly ‘installed’ additional wiring — some folks genuinely think that as long as everything’s under the hood, it’s all okay!

On many modern fuel-injected passenger car and light truck engines, adding as little as a drop too much electrical solder can increase a circuit’s resistance enough to alter its signal. If an engine computer, for example, is expecting a signal of between 0.1 and 1.0 volts, any additional resistance can drop the signal out of range, causing a check-engine light to flash. Quick-splice wire connectors can do the same thing — not to mention leaving bare wire exposed to the elements.

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The best way to identify safe sources to tap for power and grounds is with the automaker’s wiring diagram. Few of these are available in printed form anymore, but a cooperative dealership parts or service department might get you access to the appropriate print-out pages. If reading wiring diagrams isn’t your forte, try asking nicely (with some fresh-baked bribes); a knowledgeable shop tech might realize a few minutes of advice could save them the unenviable slog of diagnosing and repairing your mess later on. Of course accessories from reputable makers should include instructions and guides to steer you in the right direction before the wire cutters and soldering irons ever reach the bench.

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If you’ve beaten the odds and successfully installed your add-on with no check-engine lights or other electrical problems, don’t forget to protect that new wiring. Electrical tape is no substitute for dielectric grease and waterproof heat-shrink to keep moisture out of connections.

If possible, use the factory wire-loom shielding to route your new wires. These are almost always split along the side, making it easy to tuck in your wires. Other than making for a neat job, these conduits can reduce the risk of wire damage from vibration or chafing against moving parts. If there are no protective loom covers in the area your new harnesses have to navigate, they’re readily available in various diameters from any auto parts store — and for a fraction of the cost of repairing damage down the road.