Solving Repeated Bulb Failures in Commercial Vehicle Lighting Systems

[MikePhua]

The Problem of Persistent Incandescent Bulb Blowouts
In commercial trucks and utility vehicles, lighting systems are critical for safety and compliance. Yet some operators face a frustrating issue: repeated failure of incandescent bulbs, often in the same location and without clear mechanical cause. One such case involved a 2015 Iveco Eurocargo truck retrofitted with LED rear lights and additional incandescent units wired in series to satisfy the vehicle’s current-draw requirements. Despite resolving fault codes, the truck began blowing bulbs—primarily on the driver's side rear assembly—at unpredictable intervals.
This type of failure is not uncommon in mixed lighting systems, especially when aftermarket modifications are introduced. Understanding the root causes requires examining both electrical and mechanical factors, as well as the interaction between vehicle electronics and lighting components.
Mechanical Versus Electrical Failure Modes
Incandescent bulbs fail for two primary reasons:

• Mechanical shock or vibration that breaks the filament
  
• Electrical anomalies such as voltage spikes or poor grounding
  

In this case, mechanical damage was ruled out. The bulbs were mounted on manually operated ramps, and failures occurred even when the ramps were stationary. The bulbs were high-quality Narva units, not low-grade imports, and failures were concentrated in one assembly.
Electrical causes became the focus. Voltage spikes can occur when inductive loads—such as motors or solenoids—shut off, releasing stored energy back into the system. While no auxiliary motors were present, factory systems like power windows and windshield washers could still contribute. However, no correlation was found between their use and bulb failures.
Grounding and Circuit Integrity
Grounding issues are a common culprit in lighting failures. A poor ground can cause voltage fluctuations, reverse current flow, or uneven load distribution. In this case, the lighting assemblies were grounded both through the harness and directly to the chassis. All grounding points, including the negative battery cable and harness earths, were cleaned and verified.
Despite these efforts, bulbs continued to fail sporadically. This suggests that the issue may lie deeper in the circuit—possibly in the harness routing, connector integrity, or interaction between the LED and incandescent systems.
Recommendations for grounding and circuit checks:

• Use a multimeter to measure voltage drop across ground points under load
  
• Inspect connectors for corrosion, looseness, or pin misalignment
  
• Verify that LED and incandescent circuits are isolated or properly balanced
  
• Consider adding surge protection devices or transient voltage suppressors
  

The Role of LED Integration and Load Simulation
Modern trucks often use electronic control modules (ECMs) to monitor lighting circuits. LEDs, with their low current draw, can trigger fault codes unless resistors or load simulators are added. In this case, the operator chose to wire additional incandescent bulbs in series rather than use resistors. While this resolved the fault codes, it may have introduced instability in the circuit.
LEDs and incandescent bulbs behave differently under load. Incandescents have a warm-up period and resistive load, while LEDs are instant-on and can reflect voltage spikes. Mixing the two without proper buffering can lead to unpredictable behavior.
Solutions for LED integration:

• Use computer-safe LEDs with built-in resistors or load equalizers
  
• Install dedicated LED-compatible modules if available
  
• Avoid series wiring that combines different bulb types
  
• Test circuits with an oscilloscope to detect transient spikes
  

Component Quality and Brand Variability
While Narva is a reputable brand, bulb quality can vary between production batches. In one fleet case, switching to a different brand resolved similar issues. Manufacturing inconsistencies, filament thickness, and base material can affect durability under vibration and voltage stress.
Operators should:

• Source bulbs from consistent, traceable suppliers
  
• Compare filament construction and base design between brands
  
• Test new bulbs in controlled conditions before full deployment
  

In this case, the operator replaced the Narva assemblies with take-off units from a Scania truck. After four trips with no failures, the issue appeared resolved. This suggests that the original assemblies may have had internal faults or were incompatible with the modified circuit.
Preventative Measures and Long-Term Strategy
To prevent future failures:

• Standardize lighting components across the fleet
  
• Use LED-only systems with proper ECM integration
  
• Install voltage regulators or spike suppressors in sensitive circuits
  
• Document all wiring modifications and maintain a schematic for troubleshooting
  

For vehicles with mixed lighting systems, consider transitioning fully to LED with certified retrofit kits. These kits often include resistors, compatible connectors, and ECM programming support.
Conclusion
Repeated incandescent bulb failures in commercial vehicles often stem from complex interactions between grounding, voltage regulation, and mixed lighting systems. While mechanical damage is easy to rule out, electrical anomalies require deeper investigation. By isolating circuits, verifying grounding, and selecting compatible components, operators can restore reliability and reduce maintenance costs. In the end, lighting is more than visibility—it’s a reflection of system integrity.