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Keepa U Hands Off!

by Jim Park

Wiring problems are about as much fun as a root canal. No one is immune to them, they can cost a fortune to fix - or even worse, a fortune to re-fix, and once they start, it seems they’re with the truck for life. Veteran mechanic Hal Trueman, now shop foreman at MacKay's Truck & Trailer Center in Truro, N.S., says he'd rather tackle a big dirty job like changing a clutch than fix a tail light.

"Wiring is a nightmare," he says bluntly. "You can start tracing wiring through the harness, tear the harness down, and you still won't find the problem. I learned early though, go to the easiest and most likely cause of the problem first: the power source, grounds, and connectors. But that won’t always help you either."

Corrosion is usually the culprit, along with loose connectors, but the root of the problem isn't always obvious. The location of the corroded wire, or the loose connection may not be where you'd expect to find it, and the indicators can often be misleading. Fault codes displayed by various electronic components may suggest a particular sensor has gone bad, when in fact, another sensor somewhere in the system may be getting an improper voltage, and sending a bad signal to the original sensor. That leaves even the best mechanics flummoxed, and you, with a very expensive repair bill.

Thus, the temptation to tackle some electrical issues on your own can be overwhelming. The backyard mechanic's favorite tool is actually his worst enemy: the old pointed circuit-probe.

"When you stick that probe into a wire, piercing the insulation, you’ve just opened up a hole for moisture to seep in," notes Trueman. The wire you're checking might be perfectly good, but you’ve just killed it. Moisture will get in and the wire will corrode at that point sometime in the future."

Insulated wires have a peculiar trait: moisture that gains entry to the copper core through or under the insulation, will seep – or wick – along the wire, damaging a larger portion than you might expect.

What's worse, today's road de-icing compound-de-jour, magnesium chloride, tends to wick through the wire at a faster rate than road-salted water. It's said to be less corrosive over-all, but when corrosion sets in, you might find a larger length of the wire is damaged.

Corroded copper wire is greenish in color, and as the corrosion advances, the copper wire turns to a green powder. So if you're cutting into a wire to repair it, make sure you cut far enough back that you've taken out all the corroded material. If not, the problem will reoccur.

From Bad to Worse
Messing with the wiring on a recent vintage truck is no longer something safe to try on your own. The level of complexity is astounding, and the interconnectivity of seemingly unrelated components means that crossing a wire, or shorting out two live contacts, could mean the end of a $3,000 engine control module (ECM).
Trueman says on some trucks, you’ll find five sensors connected to a single power source. A failure anywhere could show a fault code that’s not even close to the source of the problem.

"We don't see many ECM failures. Mostly it's sensors and wiring," he says. “We see a lot of two-minute fixes that take three hours to find. A technician that's good with wiring issues will know where to look for problems. They're good at narrowing the problem down, but just getting in there and poking around is no way to try to fix an electrical problem anymore."

It's hard to swallow a $500 repair bill for a rotten wire, but that's where we are today. Brad Anderson, shop foreman at Tubby's Truck and Trailer Service in London, Ont. says many of the electrical problems his mechanics see at roadside stem from bad battery connections or shorts on the power cables leading from the batteries and/or to the ECM.

"That cable is hard to get to when inspecting the truck, so cable damage can be difficult to detect," he says. "Look carefully at all the wiring around the battery box, and at the cable tucked up near the spring shackles on the left side of the engine. That’s where we find a lot of trouble."

On trailer jobs, he always looks first to the spot where the last repair was made, and to junction boxes where water can seep in and rot the wires.
So there’s a couple of bits of sound advice: don't touch what you're not absolutely sure of, and if you’re going to try a repair on your own, stick to the basics; power sources, grounds, and connectors. Beyond that, keep your hands off.