Stop, Turn & Tail Lamps

What it is

Tail lamps are steady red lamps that show the rear of the vehicle at night (SAE J585). Stop lamps are brighter red lamps that light when the brakes are applied (SAE J586). Turn signal lamps flash to show a turn and also serve as hazard warning lamps (SAE J588). FMVSS 108 sets how many are required, where they go and their color. On heavy vehicles, the most common replacement is a combination stop/turn/tail (STT) lamp.

How to identify

A combination STT lamp has one housing that runs dim for tail and bright for stop and turn. A separate turn lamp may be red or amber at the rear. Backup lamps are white and often share a cluster with the others. Common sizes and shapes on trucks and trailers are 4 in round, 2 in round, oval and rectangular, in grommet, flange or surface mount. The lens shows "DOT" and SAE function codes for each function the lamp serves.

Functions at a glance

Key points

• On many trucks and trailers, stop and turn share the same circuit and lamp. The trailer connector wiring determines how the signals reach the lamp. See the trailer connector sheets.
• A replacement lamp must carry every function the original did. A tail-only lamp in a stop/turn/tail position is wrong.
• Rear turn lamps may be amber or red. Keep the color the same side to side.
• Mounting style and connector matter as much as the lamp. Grommets, flanges and plugs are often separate items.
• If the vehicle has LED lamps on one side, use LED on the other so flash rate and brightness match.

Common mistakes

• Replacing a three-function STT lamp with a two-function stop/tail lamp.
• Mixing amber and red rear turn signals on the same vehicle.
• Forgetting the grommet and plug on a grommet-mount lamp order.
• Using a lamp whose lens has no DOT mark.

Governing standards: FMVSS 108 (49 CFR 571.108); SAE J585 (tail lamps); SAE J586 (stop lamps); SAE J588 (turn signal lamps); 49 CFR 393.11.

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.