Contemporary commercial trucks rely on their electrical systems to control a wide swath of automated functions. A battery powers the engine control unit (ECU) and its sensors; the ECU runs the starter relay, the fuel injectors, even emissions control systems—everything a truck uses to get going and stay safe when on the move.
As a result, even small electrical problems can have a big, negative impact.
This guide aims to give you the power to catch the early warning signs of electrical system trouble, the insights to hone in on their source, and the resources to fix minor issues before they spiral into system-wide problems.
Signs Your HDV Has Electrical Issues
Dysfunctional and failing parts in your truck’s electrical system leave signs of impending trouble. Stay alert to these warnings.
Malfunctioning Lights, Wipers, or HVAC
When lights flicker or can’t brighten to their full luminosity, that’s a telltale sign.
Likewise, electrical issues can de-power sensors. In most modern trucks, sensors enable the systems to “read” changes in windshield moisture and cab temperature, and respond with appropriately regulated windshield wiping action and HVAC output, respectively.
Note erratic or sluggish responses as potential symptoms.
Dashboard Warning Lights
An HDV's dashboard is equipped with indicators: glowing icons to warn the driver of problems. The "battery warning," "ABS warning," and "diesel glow plug" icons can alert you to distinct electrical system issues.
Battery Indicator
To flag battery, alternator, and adjacent issues, most trucks’ systems will illuminate the battery warning light.
If this rectangular icon with +/- symbols inside glows, there’s a problem.
ABS Indicator
The ABS warning light (typically the letters “ABS” inside a circle) glows when the anti-lock brake system is impaired. A dysfunctional electrical system is often implicated.
The system is either insufficiently powering the ABS control, or it’s putting the ABS at risk of overload.
Diesel Glow Plug Indicator
Certain HDVs have a diesel glow plug indicator, a coil symbol on the dashboard.
Its light indicates the electrical glow plugs (devices that heat cold air in the engine prior to ignition) are still warming, and you shouldn’t start the engine yet.
If the indicator is flashing, or if it remains on for longer than a few seconds, heed that warning.
Burning Smell
If anything in an HDV smells like it’s burning, take that sign seriously. Short circuits and damaged electrical components can cause the system to overheat, melting wire coatings and insulation.
The melting components themselves can release an odor, and the exposed wires can start a smoky electrical fire.
Visible Sparks
When an electrical system loses control of electric currents, the electricity leaps out as sparks. Sparks pose an electrical fire risk, and they must be addressed swiftly.
Frequently Blown Fuses
A single blown fuse is a problem, but it usually only indicates a problem with the fuse itself. However, if fuses are blown more frequently, there’s likely an underlying issue with the truck’s electrical system to resolve.
Battery Failure
Batteries in HDVs face a higher rate of premature failure than those in smaller vehicles. When the battery fails, it’s a sign to investigate factors like the battery’s durability, the temperature of its environment, and its physical securement in the system.
Visible Wiring Damage
If you can see the damaged wires, there are often more problems that are less obvious at-a-glance. It’s important to both repair the visible damage and determine its underlying cause.
Top 10 Electrical Problems Afflicting Commercial Trucks
When most commercial HDVs and trucks experience electrical system dysfunction, the problems can be categorized into types. Understanding the ten most commonplace types makes solving all of them easier.
1. Corroded, Sulfated, Low-Voltage, or Dead Battery
The average battery sustains more damage in an HDV than it does in other vehicles.
The intense vibration and temperature extremes can crack the battery’s protective plating, quicken corrosion, and loosen or damage the battery’s neighboring electrical components—leading to acid stratification, sulfation (under-charging), and short-circuit-induced overload.
2. Worn or Damaged Alternator Components
The alternator is the vehicle’s electrical generator. It converts the rotating, mechanical energy of the engine’s crankshaft (powered by fuel combustion) into electricity, charging the HDV’s battery and powering the electrical system.
Several complex components work together to serve the alternator’s function, typically including a rectifier, a serpentine belt, a rotor, and multiple bearings, brushes, and diodes.
Identifying the malfunctioning component in an alternator can save the expense of replacing it altogether.
3. Poorly Connected, Loosely Harnessed, or Worn Starter Motor
The starter motor and its switch (a solenoid) utilize an electric current to start the ignition, getting the engine running. The solenoid closes a vital circuit, and a gap or imprecise closure can cause a short. It's held in place with a harness, connecting it to the electrical system safely.
4. Blown or Malfunctioning Fuses
Dysfunctional fuses are often a source of electrical problems in HDVs. They’re frequently responsible for low or erratic power to lights and wipers, a burning smell, and ignition trouble.
5. Corroded or Overheated Relays
Damaged relays can’t switch their circuits on and off as needed. Left on, the voltage may overload and overheat components. Left off, components don’t get the power they need consistently, and either don’t work or only work erratically.
Dysfunctional relays are commonly the root cause of contact welding, components and transmission gears melting, extreme heating, and electrical fires in trucks.
6. Loose, Damaged, or Exposed Wiring
Wiring problems are frequently the source of electrical trouble in a truck. Exposed and damaged wires, fraying, corrosion, and loose connections all impair wires’ ability to facilitate and direct the movement of electric currents.
7. Corroded or Loose Grounding Points
Grounding points are vital. They complete circuits, letting an electrical current return to the battery after doing its job. They also absorb electric discharge and dissipate static electricity.
When they’re damaged or loose, currents can flow off-course. Components don’t receive enough electric power to function well, while stray, ungrounded sparks can cause further damage.
8. Slipped Belt
The serpentine belt is key to the alternator working effectively. When it slips, it causes a chain reaction of dysfunction in the electrical system.
Slipped belts aren’t uncommon. They can happen if the belt is worn or damaged, if the pulleys are misaligned, or if the belt tension isn’t established and maintained correctly.
9. Short Circuits
A short circuit is an accidental pathway for an electrical current that’s shorter (and offers less resistance) than the intended pathway. A short circuit often bypasses important parts that need the current’s power, and its high voltage can cause overload.
Most short circuits are caused by poor wire routing, moisture intrusion, and worn out insulation.
10. Vehicle Circuit Overload
Vehicle circuit overload is the term for a problem wherein a circuit carries an electrical current that’s too powerful or excessive. The extra current exceeds the limits of the circuit’s design.
Exceeding the circuit’s electrical capacity can blow fuses, damage wires, and overheat the system. Overloads are typically caused by added, aftermarket parts drawing on the circuit in a way designers didn’t anticipate, the simultaneous use of many accessories, or the failure of regulating components.
How to Diagnose Electrical Problems (Step By Step)
Get to the root of your HDV’s electrical system issue with strategic diagnostics. This seven-step process reveals the core problem.