Fleet & Auto Reference Library · Technical Reference

Lighting Overview — FMVSS 108 Lamp Functions

FMVSS 108 sets which lamps, reflectors and conspicuity markings a truck or trailer must have, and what color each must be. This sheet lists the common lamp functions so a counter can match a part to the job it does on the vehicle.

Ref FA-0015FMVSS 10849 CFR 393.11

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

What it is

FMVSS 108 (49 CFR 571.108) is the federal standard for vehicle lamps, reflective devices and associated equipment. Vehicle manufacturers must meet it when the vehicle is built. Lamp makers certify their products to it, usually marked "DOT" on the lens. For vehicles in interstate commerce, FMCSA's 49 CFR 393.11 requires that the lighting be kept in place and working. Its table lists required devices by vehicle type.

How to use it

Find the function the customer is replacing, then match color, mounting location and the SAE function code on the old lens. A lamp that fits the hole but serves a different function, or the wrong color, is not a correct replacement.

Common lamp functions on trucks and trailers

Function	Color	Where it is used
Headlamps	White	Front of power units
Tail lamps	Red	Rear
Stop lamps	Red	Rear
Turn signal lamps	Amber front; red or amber rear	Front and rear; hazard warning uses the same lamps
Backup lamp	White	Rear of power units
License plate lamp	White	Over or beside the rear plate
Side marker lamps	Amber front; red rear	Sides, near front and rear
Intermediate side marker lamps	Amber	Sides of longer vehicles, near the middle
Clearance lamps	Amber front; red rear	Upper corners of vehicles 80 in or more wide
Identification lamps	Amber front; red rear	Group of three on the center line of vehicles 80 in or more wide
Reflex reflectors	Amber front and side; red rear and rear side	Sides and rear
Conspicuity treatment	Red and white rear and sides; white upper rear corners	Trailers and tractors as required by FMVSS 108

Key points

Clearance and identification lamps are only required on vehicles 80 in (2032 mm) or more in overall width. Most heavy trucks and trailers fall in this group.
Front-facing lamps other than headlamps are generally amber, and rear-facing lamps are red, except backup and license lamps, which are white.
Exact mounting heights, spacing and photometric output are in the FMVSS 108 tables. Use them, or the vehicle builder's data, for new installations.
Retroreflective conspicuity sheeting is marked DOT-C2, DOT-C3 or DOT-C4. Replace it with marked sheeting, not generic reflective tape.
A lamp marked "DOT" and with the right SAE function code is the safe match for a replacement.

Common mistakes

Replacing a red rear clearance lamp with an amber one because it was the same size.
Using an unmarked lamp or tape that is not certified.
Treating the three identification lamps as optional decoration.
Selling a combination lamp without checking that every function it replaces is included.

Governing standards: FMVSS 108 (49 CFR 571.108); 49 CFR 393.11 and related sections of Part 393.

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.

More in Technical Reference

← All Fleet & Auto sheets