High Wiring Act

Jeff Boesch, North American Institute service training instructor, uses a digital voltmeter to get a precise voltage reading.

One of a trucker’s worst nightmares is suddenly finding something is wrong with his rig’s wiring.

An intermittent shutdown of one of a truck’s lights or the heater fan on a cold night can be a major nightmare. Here’s how to keep the problem away and, if it does occur, find and repair it permanently.

The truck OEMs have arm-wrestled with large numbers of expensive warranty claims to fix truck electrical problems over the years. The result is that things have improved. “Harnesses are routed with great care to reduce the potential for damage, and rigid standards apply during manufacture,” says Craig Brewster, engineering manager at Peterbilt Motors. “Deterioration should not be a major maintenance issue.”

A common scenario leading to electrical trouble is someone making a repair or changing something, then failing to put back tie straps, which allows “rubbing.” And rubbing soon means deteriorated insulation – and a ground.

“You should be diligent in looking at routing and inspecting tie straps,” Brewster says. “Whenever you work on the truck and disturb any, put them back the way they were.” Also, make sure you don’t tie the straps too tight where the engine or cab flexes.

Mack’s Curtis Dorwart, senior staff engineer of electrical and electronic engineering, says, “Look for harnesses that are in contact with parts, especially parts that are likely to move and rub the harness through. If you find a rub mark, failure is only a matter of time. Secure the harness with clamp-like heavy-duty, ultra-violet and heat-stabilized tie wraps, also called band clamps. If the conduit or wire insulation has been breached, repair the wire by replacing the rubbed section and use a heat shrink tubing that contains a sealant. Keep your harnesses away from hot things like exhaust pipes, exhaust manifolds, EGR coolers and the like.”

To minimize the risk of rubbing damage, pay attention to engine mounts, cab mounts and shock absorbers that keep motion among parts of the truck under control, Dorwart says.

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Al Hertzog, director of dealer and customer training at the North American Institute operated by Volvo and Mack, says rubbing damage can sometimes be stopped by splitting a section of rubber hose and installing it over the harness. When replacing ties, he recommends using something designed to survive both heat and cold.

Keeping your wire insulation perfectly intact is also of the utmost importance, says Joe Bell, engineer in the electrical test group at International Truck and Engine Corp. “If you get a tiny leak, moisture will wick in like pulling soda through a straw,” he says. “The effect will pull water several feet up the wiring, which will then corrode copper wires, terminals and even modules.” Even poking the sharp end of a test light through a wire’s insulation can cause this problem.

You might want to test for voltage either at a terminal or with an inductive ammeter that doesn’t cut through insulation.

After improper repair work, the second big deterioration issue is water. “Salt and other corrosives come up off the road,” says Hertzog. “Also, the trucker himself can cause damage when using a pressure washer with strong detergents. The steam or high-pressure water can inject corrosive chemicals right into the connections. Whatever you do, when using a pressure washer, don’t hold the wand right near the wiring. The best way to wash is to spray on a mild detergent and use that to clean the truck. Then, use the pressure washer without detergent to flush it off.” It also helps to turn down the pressure on the pressure washer, he says. If you suspect you’ve been injecting washing fluids into your connections, Brewster suggests pulling them apart and cleaning them with terminal cleaner spray.

When moisture and salt corrode the electrical connections, resistance in the circuit increases and current flow decreases, says Carl Garrod, service training manager at Kenworth. This results in a poorly operating circuit. Apply a light coat of dielectric grease to the inside of the connectors for protection, but make sure you choose a grease that is designed for electric connectors and that is “compatible with the silicon terminal seals and plastic connectors on the truck,” says International’s Bell. “It should be recommended by the dealer or state that it ‘meets manufacturers’ requirements.’ The wrong grease could cause the plastic connectors and the seals to deteriorate.”

Another important preventive measure is washing off road chemicals as soon as possible. And remember that engines and cabs flex in relation to the frame 1/4 to 1/2 inches. Have play in the wire – don’t make it too tight when making a repair or adding new wire.

Also, make sure you don’t forget about the ground side of the circuits. “Everybody thinks of the positive side of circuits because they carry voltage. But the ground side is just as critical,” Hertzog says. “So make sure your cab and chassis grounds are clean. If you see corrosion, disconnect the ground strap and clean it. When you have corroded grounds, the engine electronic module often goes crazy.” Bell recommends a coating of heavy grease afterward to keep moisture and salt away.

Brian Marshall, manager of the electrical test group at International, recommends occasionally applying a little white lithium grease (not dielectric grease) to the insides of your 7-way trailer sockets at both ends of the cable. “Corrosion gets trapped in there because those parts are exposed to salt water and chlorides from the road,” he says. “The grease keeps corrosive water and dirt out and keeps the parts from corroding together. I have seen truckers using prybars trying to get corroded plugs apart.”

This wiring diagram provided by Mack shows the protection for the circuits that service the cab. Once you learn the symbols, which are usually revealed in a key, such diagrams become easy to read.

The key to maintenance is attention to detail. “As you inspect the truck periodically, get a feel for how the wiring looks so that you will notice when something has slipped,” Brewster says. “It doesn’t seem like a big thing when a harness is drooping a bit, but if it gets in the way of something that’s moving and gets damaged, it can quickly become a big deal.”

Finding the trouble
Most truckers know that if you lose power to part of your electrical system, it’s commonly related to a blown fuse. But it’s important to realize that the idea of a fuse that blows or fails because the fuse itself is defective is almost a myth. “Fuses and links almost always fail because of a larger problem,” Brewster says. Bell says failure of smaller fuses after a long time due to “motion and fatigue” does happen, but it’s “not very common.”

There is normally a fuse box in the cab for cab electrical accessories and another under the engine compartment for powertrain electricity. Dorwart recommends first cleaning dust and grease off the lid or cover. He says you can normally see the broken filament in a fuse that has blown. It’s probably OK to try a new fuse just in case the old one failed, but, says Bell, “Use only the recommended size because the greater the amount of current, the greater the heat of the wiring. Fuses are not there to protect the device but to protect wiring itself from overheating and possibly burning.” Brewster says, “Even if the fuse is 25 percent bigger than it should be, you could create an expensive repair for yourself.” Garrod says this often happens because the wires actually melt. Fuse amperages are color-coded in a standard way on all trucks (see chart below) to make them easy to identify.

Fuses that have opened up because of an overload look different depending upon just how much overload there was. “When a fuse blows because of a mild overload due to too many electrical accessories on that circuit or when a load draws a few too many amps because it is just starting to go bad, the fuse will simply melt and leave the window clear,” Garrod says.

In the case of a short to ground, bare wiring touches the frame or another ground, and power flows directly back to the battery rather than going through a motor, light bulb or electronic accessory. In a short circuit too much current flows because there’s no load. It’s like putting the truck in neutral, flooring the throttle and having a diesel with no governor to limit rpm. “If the fuse blows because of a short to ground, this causes rapid burning of the fuse, resulting in a smoke stain inside the glass or plastic,” Garrod says. “If a driver finds any smoke stain inside a fuse, they should closely inspect that circuit for a wire between the battery and the load that has insulation worn through and is touching bare metal.”

International’s Bell says the gap where the fuse burned out will be small with a mild overload, while “if the fuse element is completely destroyed and appears burned, the fuse has been forced to carry a very large amount of current.”

If it’s not obvious whether or not a fuse has blown, you may be able to use a voltmeter or, on a circuit that feeds only lights and/or motors, a test light to connect probes on either side with ground. Make sure not only the ignition switch but the switch for the involved accessory (like a turn signal) is turned on, then check for 12-volts on both sides. If it’s there on only one side, the fuse is blown.

Starting with the fuse box will identify the circuit with trouble if a fuse has blown. But going further gets more complicated. The best approach is more preventive education than you can get from the owner’s manual. Brewster suggests getting to a dealer to study the service manual. “Look at the schematics. You must understand how the system should work,” Brewster says. “If you do, simple clues will mean a great deal to you. Also, it will help to be aware of the labels and color-coding on the wires.”

Carl Garrod suggests asking your technician to help you understand routing the next time you are in for service.

This fuse took 30 seconds to fail as it was a 20-amp size overloaded only to 25 amps.

Bell also suggests you use a circuit diagram. “Troubleshooting without one is like driving without a map, even when you know the truck,” he says. “You need to be familiar with the symbols on the schematics and become familiar with the basic philosophy of how the system is laid out.”

To aid in service, International marks each circuit with a circuit number and a “system” color code. For example, the headlamp circuits are yellow with high beam marked 52 and low beam marked 53. All the manufacturers have similar means of defining circuits. The manufacturer’s wiring diagrams include details on how the circuits are identified. The wiring diagrams for the truck are generally only available at the manufacturer’s dealership.

Al Hertzog recommends purchasing the OEM electrical manual. “It usually shows you schematics, how to properly repair connectors and how to use test meters,” he says.

If you don’t already know something about electricity, you could take a basic course at a vocational-technical school.

Before you try to diagnose a circuit problem, Brewster says, you should figure out if it is electrical or electronic. “Understanding the architecture of the system will show you where to draw the line on troubleshooting.”

If you use the right tools and educate yourself in the inner workings of your system and the way electricity works, you should be able to head off electrical trouble and even make safe minor repairs to your electrical system.


Ampere Rating Color
3 Violet
4 Pink
5 Tan
7-1/2 Brown
10 Red
15 Light Blue
20 Yellow
25 Natural (White)
30 Light Green

Ampere Rating Color
20 Yellow
30 Green
40 Amber
50 Red
60 Blue
70 Tan
80 Natural
100 Violet
120 Pink

For Further Information Contact:

Freightliner Corp.
(503) 745-8000

Kenworth Truck Co.
(425) 828-5000

Mack Trucks, Inc.
(610) 709-3011

Peterbilt Motors Co.
(940) 591-4000

International Truck
and Engine Corp.
(800) 448-7825

Volvo Trucks of North America
(336) 393-2000

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