Lights Out! Nine Wiring Hazards and How to Avoid Them

You don’t have to behind the wheel to long to realize that truck wiring is one of the most critical items on your “must maintain list”.
When working on any electrical problem, I start with the affected circuit. Let’s say you have a marker lamp out; all the rest of them work…

You don’t have to behind the wheel to long to realize that truck wiring is one of the most critical items on your “must maintain list”

When working on any electrical problem, I start with the affected circuit. Let’s say you have a marker lamp out; all the rest of them work – it’s just one. So the first thing is to check the affected lamp, swap it for a like lamp or a new spare that you carry. Now what, it still doesn’t work, we know the new one is good because it works in another location. We must now move on to the wiring harness. All circuits must have a positive (+, or battery) and a negative (-, or ground), the battery will always be through a wire of some type. The ground may have a wire but may also be through the body of the lamp or the final user of the power (such as a blower motor).

Tools for wiring repairs vary but basic tools include wiring pliers (designed to strip insulation and crimp connectors), an assortment of connectors, a Phillips screwdriver, and a flat blade screwdriver. Heat shrink tubing and dielectric grease are recommended as well. A test light is handy but each probe hole must be sealed after testing.

When looking at wiring harnesses there are nine common causes of failure, according to Grote, a leading manufacturer of lighting products. We will discuss each one’s cause and repair.

1) Moisture and corrosion – This is the most common problem I have found. Anywhere there is moisture and air there will eventually be corrosion. Corrosion is the chemical reaction between moisture, air and the wire, amplified by the conduction of electricity. This, in effect, causes the wire to eat itself. Corrosion can start through unsealed crimp connectors, cracks or punctures in wiring insulation.

The repair – Corrosion in a wire is normally green because they use copper wire and it is green when it’s corroded. This is sometimes hard to find because it can hide within the insulation, but the wire will be swollen. Connectors that are corroded may have simply disconnected themselves (fallen apart) or you might gently tug on the wire and it will pull apart easily. You may have to follow the wire backwards to the next lamp that is working, checking each connection and inspecting the wire for swelling or breaks along the way. After you find the corrosion you must remove ALL the affected area. This could mean replacing several feet of wire. Always use the same color and size wire you are replacing. Use heat shrink tubing on connectors to seal the joint and prevent the same thing from happening to your new connection. Heat shrink tubing is as its name states: tubing that shrinks when heated. Use caution when heating with an open flame: you don’t want to set your truck on fire. I use a heat gun, which has no open flame or fire. The other type of corrosion is where the ground meets the frame: rust. The only cure I know of is to remove the attaching hardware, bolt or screw, sand, file, or wire brush the affected area, and reinstall the hardware. New hardware is the best idea, and also use a dielectric grease on the contact surfaces. Dielectric grease is available at auto parts stores in small tubes – it goes a long way.

2) Chemical damage – Oil, grease, fuel and road de-icing fluids, to name a few, cause the insulation to soften or harden and crack. Either way the wiring has been compromised and must be repaired so a failure doesn’t happen.

The repair – replacement is the only option. Again, match the wire color and size, and use heat shrink tubing. In some cases, like under the hood, you can buy a ready-made harness from the truck OEM. Just unplug damaged harness, plug in the new one and you’re on your way. One more thing: find out what caused the damage (oil leak? fuel leak?) and fix it also or you will have the same problem all too soon.

3) Abrasion – Rub two things together and sooner or later one will fail. No different with wiring, only in most cases the wire will fail first. Wires rubbing on frame rails, cross members, air lines, or even other wires can cause abrasion damage.

The repair – this can be VERY difficult to find because it is often hidden inside the frame rail, or the damaged spot will be going through a cross member. Just be patient and follow the wire from the failed lamp towards the next working lamp, gently tugging and twisting the wire so you can see 360 degrees around the wire. Look closely anywhere the wire is loose and could jiggle and rub. The damage could be as long as several inches or just the thickness of the rubbed object. Repair as above, matching wire size and color, heat-shrink the connectors, and then tie the wire solid. It should not be able to move unless it needs to, at a hinge for example. Use plastic wire ties also called zip ties or cable ties. I keep 8” ties to do anything I need. You can put two together if longer ones are needed. Zip Ties are available at part stores, hardware stores, and any store with an automotive department.

4) Impact – This will happen anywhere wiring is exposed to where rocks or other road debris could come in contact with said wiring. This is most common at the rear of trucks and trailers near the taillights but is not limited to this area.

The repair – once the affected area has been found, more often than not, the wiring harness will have to be replaced or rebuilt, as a lot of harnesses in this area are custom made to fit each truck. This is easier to do than it sounds, just replace each wire with, as above, same color, and same size, use heat shrink on connections and in addition use wire loom. This is a roll of plastic tubing split to allow you to build your harness then insert into the loom. Loom will help protect wiring from impact damage but you should find a way to use metal for a better protection. If metal is not an option for you, the loom, surrounded by heater hose split to allow easy installation over loom will help. Be sure to tie this up well to prevent abrasion and tension (we will talk about this later).

5) Vibration – This is the #1 cause of bulb failure. The filaments in standard bulbs when heated (this is how they create light) are very fragile and excessive vibration is a death sentence for them. Most lamp assemblies are designed to handle the normal or expected vibration for its intended location. Causes of excessive vibration:
where do I start? Bad shocks, tires out of round or out of balance, rough roads…. well you get the picture.

The repair – we will address the bulb. There are heavy-duty bulbs on the market, sometimes called long life bulbs. These are better than standard duty and worth the price if you have a constant issue with this. I also recommend using LED’s. These are lamps built to last hundreds of thousands of hours, withstand harsh vibration, and be maintenance free for years. Downside: the good ones are expensive, but worth it. You may also need to do some modifications to replace incandescent lamps with LED’s, requiring a welder.

6) Grit and sand – Goes along with the abrasion problem: just add grit or sand. This also attracts moisture.

The repair – keep it clean. I’ve often said, “A clean truck is a happy truck.” This includes the underside. A good rinse does wonders to clear grit and sand from around wiring harnesses and also suspension and other close fitting parts. The damage caused is similar to abrasion, the ultimate fix the same.

7) Extreme temperatures – Nothing likes to be overheated, and electrical systems are not excluded. Leaving the lights on while backed into a dock could overheat the lamps and the wiring; I even saw a warning at a shipper with a photo of a trailer that had been backed into the dock with the lights on and the resulting fire. The lamps do get hot enough to start a fire if there isn’t enough air to circulate to cool them.

The repair – any lamp that has been overheated needs to be replaced. There are special lamps for high temperature conditions – an area that is exposed to heat often, around or near the exhaust, or just doesn’t get good air circulation for cooling. Again, using LED’s will lessen the temperature problem as they don’t get hot, and may be better in the high temperature areas as well.

8) Tensile loads – I mentioned this before, this is the affect of pulling on the wiring. In some cases the wiring is in a place that catches mud or ice the weight of the mud/ice will pull the wires out of connectors causing failure.

The repair – tie up the wires and protect them from mud/ice. When under your truck, don’t grab the wiring to help you pull yourself around, and watch for wiring that needs to have slack that it doesn’t hang on anything (like the pigtail between tractor and trailer hanging on the deck plate). Tensile damage is usually at a connector, replace connectors as needed, find cause and reroute wiring as needed.

9) Flexing – This is where a wire needs to move: at a hinge, between the truck and trailer, at the hood hinge. Installing the wire with too tight a radius will cause the wiring to kink and eventually break; too large a radius and the wiring could hang on something.

The repair – allow the wiring plenty of room to move, to open and close the hood, door, whatever needs to move. Watch for too much slack, and adjust as needed. If flexing damage has occurred, find the break, repair, and adjust the harness as needed to prevent binding or kinking.

I hope this helps! Until next time, keep ‘em lit!!!