How to Perform Truck Electrical Repairs


Electricity is one aspect of truck repair where a little knowledge can do more harm than good. Electrical problems don’t always present themselves in a way that leads intuitively to the root of the problem. Due to the complex nature of modern electrical and electronic systems, unskilled, lazy, or even overconfident technicians can waste tons of time and money chasing ghosts or replacing flawed parts.

Some of Canada’s largest fleets have learned from experience the true cost of improper diagnostics and repairs and have rigorous procedures and standards in place for electrical repairs. These provide clearly defined diagnostic steps to determine a problem and outline the correct repair procedures. If nothing else, it brings some consistency to the process.

Electrical problems don’t always present themselves in a way that leads intuitively to the root of the problem. Semi-skilled technicians can do more harm than good with improper electrical repairs. (Photo: Jim Park)

Many experienced technicians develop their own way over the years, of course. Bison Transport Technical Training and Product Development Manager Tony DeGroot calls it technician discretion. It’s not necessarily bad, he says, but sometimes a problem isn’t what it seems.

“Most good technicians understand the troubleshooting and repair processes, but they don’t always follow them,” says DeGroot. “They can fall into the trap of saying, ‘I’ve seen this problem before, I know how to fix it,’ but without doing proper diagnostics, things can be overlooked.”

Even if a technician has performed a certain repair a dozen times, there is always a chance that the problem is not what it seems. This is where the process becomes important. DeGroot advises his technicians to start with the basic elements of any circuit: the source, a switch or control, the load, and a ground.

“Start by determining if you have no voltage at the light, for example, or a failing light,” he recommends. “Then try to isolate the problem by breaking the circuit into smaller pieces.”

If the light is good, but there is no power, check the fuse or power source to make sure there is voltage. If good, check a connector at a midpoint in the circuit. If there is power, you know the fault is between that and the load. If there is no power, you know the problem is between the source and that connector.

Most good technicians understand the troubleshooting and repair processes, but they don’t always follow them.

– Toy DeGroot, Bison Transport

Experience has taught DeGroot that wiring problems are more likely to be chafing than corroded connectors. He says the tie straps used to secure the harnesses to the frame are usually too tight, or it could just be the vibrations, but there are usually telltale signs to look for when visually inspecting the harness. .

“Just because the harness is still held in place, or it may seem barely rubbed, the wire insulation may be broken,” he says. “If it’s rubbed, you now have an access point for moisture and salt, and that’s how corrosion starts.”

Tie-down points are likely locations for wiring faults, but wires have been known to break inside the sheathing due to movement and vibration. Visually, you can inspect long lengths of unsupported cables or cables attached to moving components such as axles or wheel ends. But before you start separating the harness and wire harness, narrow down the search by opening a connector, if possible, checking for power at that point in the circuit, as described above.

broken electrical contacts
Loose or broken contacts in a connector can be difficult to diagnose. Pins can easily be damaged when using the wrong tools. (Photo: Jim Park)

Pay attention to the connectors

The connectors are designed to be opened, but take it easy. Do not separate them by pulling on the wire as this may damage the integrity of the connection. Avoid damaging the seal and always pour a little dielectric grease into the connector when reassembling. Bison uses Krown T-40 rust remover on all electrical connections.

According to an electrical maintenance and repair expert at Erb International who was hesitant to have his name mentioned in this story, the biggest risk to the connector is to be damaged by probing the terminals with the wrong tool.

“Technicians sometimes force a multimeter probe or even the end of a bare wire into the connector, pulling the terminal apart to the point where they come loose,” he warns. “A loose connection can create an intermittent point of high resistance, which creates heat. This will amplify the problem as the hot terminal will grow and [further compromise] the contact.”

He recommends doing a pin rub test after reassembling the connector to make sure it’s tight.

“Feel the drag as you reconnect both ends of the connector,” he suggests. “If no drag is felt, the connector terminal is probably pulled apart. The drag test can be performed while measuring the voltages at the connectors for other reasons.”

The Wiggle test is useful for identifying loose connector pins and wiring connections in general. After attaching a voltage meter, gently wiggle or shake the connector while watching the display. If the voltage doesn’t stay constant while the connector is manually manipulated, there’s probably a loose pin in there,” Erb’s expert explains. “It can help reduce an individual section of the harness [that may be faulty].”

Correctly crimped connector
Here is an example of a properly crimped butt connector. (Photo: Tony DeGroot)

Appropriate wiring splices

When it comes to wiring repairs, proper splicing technique is the key to a lasting repair. Erb International’s corporate fleet manager, Jim Pinder, says poor wiring repairs are a constant source of frustration.

“An inadequate previous repair will fail again, but what makes it worse is that the technology fixing the problem first figured out what the previous technology did with the repair,” he says. “Since the original repair did not fix the problem, you now have two problems.”

Bison’s DeGroot has shared with us a sort of handbook that he hands out to all Bison technicians involved in electrical repairs. It describes exactly how the company wants to repair the wiring, what tools to use, and how to perform the repair correctly. He allowed us to use a few paraphrased excerpts here:

  • Using the appropriate cutting tool (cable cutter, side cutter, or lineman’s pliers), cut the wire at a 90 degree angle. And using a suitable stripping tool, not pliers or your teeth, strip just enough insulation from the wire to fit the butt connector, about 3/16 to 3/8 inch. Avoid nesting or cutting a wire strand during stripping.
  • Before crimping a butt connector, make sure the copper is clean and shiny. If the copper wire is oxidized (black), keep backing up until you find clean, shiny copper.
  • Always use the correct gauge wire for the repair and try to use wire the same color as the original.
  • When splicing a group of wires in one area, make sure the joints are staggered.
Use terminal crimping tools designed for this purpose. Not just any old tool will do. (Illustration: Tony DeGroot)

Completing the repair by crimping the connection is critical to its durability and resistance to moisture ingress (corrosion). Bison Transport only uses clear heat shrink tubing lined with adhesive for a better seal and so the connection can be checked for signs of corrosion in the future.

  • Slip a piece of heat shrink over the wire before attaching the terminal. Insert the wire into the butt connector, but leave a small gap between the butt connector and the wire insulation.
  • Crimp the connector using an approved tool, then perform a pull test to ensure a tight connection.
  • Choose the correct size heat shrink for the size wire/butt connector you are using. Too big a diameter and it won’t shrink enough. If the diameter is too small, it will be difficult to install and may crack when heated.
  • Cut the heat shrink to the correct size. there should be a half inch overlap on the wire insulation. For example, if the splice length is half an inch, the total length of the heat shrink should be at least 1.5 inches.
  • Use the appropriate tool to heat up the heat shrink tubing. Do not overheat or melt the heat shrink, as this will cause problems later. Method 1: Start at one end and slowly work your way to the other end while rotating the wire or tool to ensure the heat is evenly distributed. Method 2: Start in the middle and work your way slowly to one end while rotating the wire or tool to ensure the heat is evenly distributed, then come back to the middle and do the same for the opposite end.
  • Make sure the adhesive is out at each end.
  • Secure the harness with Tesa tape with a 50% overlap.
Badly stripped wires
Examples of improperly stripped wires. (Illustration: Tony DeGroot)

A final word of caution

Modern truck wiring carries more than volts and amps to various components. Many of these wires also carry multiplex signals with messages to and from various controllers on the truck. Multiplex communications and the sensors that produce them can be very sensitive to voltage and resistance.

Think of the wiring harness that goes all the way to the driver’s door. It can hold a dozen wires and it sometimes opens and closes dozens of times a day. With all this movement, it’s easy to see why something in this batch could fail, warns Justin Horton, shop manager at Gateway Trailer Repairs in Edmonton.

“Multiplexing is based on voltages. It is no longer just this wire that performs this function. It responds to different tensions,” he says. “When you create resistance in a wire because it’s corroded or partially broken, weird things start to happen.”

Erb’s expert says that using load test equipment to test the condition of a wire using voltage at the battery level can damage sensitive electronics.

“The technician can simulate an electrical load on both the source and the circuit ground, which is an excellent way to test power sources connected to battery voltage. However, care should be taken when for use on voltage supplied by control modules, and under no circumstances should it be used on data link wiring.

It’s just one more thing to worry about, and one more reason to make sure your electrical technicians are properly trained.


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