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LED vs Incandescent Lamps

LED lamps last longer and draw less current than incandescent lamps, but swapping them in can cause flasher, warning-light and compliance problems. This sheet lists the trade-offs and what to check before a retrofit.

Ref FA-0016FMVSS 108

Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.

What it is

Incandescent lamps make light by heating a filament. LED lamps use solid-state diodes and a small circuit, usually sealed in the lamp body. On trucks and trailers, most new marker, clearance, stop, turn and tail lamps are LED. Both types must meet FMVSS 108 for the function they serve. LED lamps for vehicles are usually sold as complete sealed units, not bulbs.

How to identify

Incandescent lamps have a replaceable bulb, often a numbered miniature bulb, behind a removable lens. LED lamps have a pattern of small diodes visible through the lens, and the housing usually cannot be opened. Most LED lamps carry a DOT mark and SAE function codes molded into the lens, the same as incandescent lamps.

Trade-offs

Factor	Incandescent	LED
Service life	Shorter; filaments fail from vibration and age	Much longer in most fleet use
Current draw	Higher	Much lower
Light-up time	Slower	Near instant, which can give following drivers more warning at stop
Repair	Replace the bulb	Replace the whole lamp
Heat on the lens	Warm lens helps shed snow and ice	Cool lens can build up snow and ice
Flashers and monitors	Work as designed	Low current can cause fast flashing or bulb-out warnings
Up-front cost	Lower	Higher

Key points

Thermal flashers depend on the current through incandescent bulbs. With LEDs they may flash fast or not at all. Use an electronic flasher rated for LED loads, or follow the vehicle maker's fix.
Bulb-out and lamp-monitoring systems may flag LEDs as failed. Some vehicles need a reprogram or a load resistor. Load resistors get hot. Mount them on metal, away from wiring.
LED "bulbs" that drop into an incandescent housing may not meet FMVSS 108 for that lamp, because the light pattern depends on the housing. A complete certified lamp is the safer replacement.
Low leakage current from some control modules can make LEDs glow faintly when off.
Good sealed connectors and dielectric grease where the lamp maker calls for it reduce corrosion at the pigtail, which is the most common LED failure point.

Common mistakes

Converting turn signals to LED without changing the flasher.
Installing a load resistor against plastic or a wire bundle.
Selling a mixed set, LED on one side and incandescent on the other, so the turn signals flash at different rates.
Assuming every LED lamp is DOT compliant. Check the lens markings.

Governing standards: FMVSS 108 (49 CFR 571.108). Vehicle and lamp manufacturer instructions for retrofits.

Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.

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