GMC R3500 Back Up Light Bulb Size

The GMC R3500 back up light requires 3156, 1156, or P21W bulb sizes depending on the specific version of the component. Data for the bulb sizes covers the range from 1987 to 1991.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1990-1991	3156
1987-1989	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament wedge bulb (T25). 12V, 27W.	3156
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156

Other Locations

Additional Notes

The GMC R3500 uses two distinct bulb types across its back up light specifications: the 3156 and the 1156.
Both the 3156 and 1156 are single-filament bulbs operating at 12 volts, conforming to ANSI standards recognized in the United States and Canada.
The 3156 is a T25 wedge-style bulb rated at 27 watts, used in the GMC R3500 for the 1990-1991 model years.
The 1156 uses a BA15s bayonet base and is rated at 21 watts, covering the 1987-1989 model years.
The BA15s designation on the 1156 refers to a single-contact bayonet base with a 15mm collar, a base type that is not interchangeable with the wedge socket used by the 3156.
The 3156 and 1156 differ in both base type and wattage, making them physically and electrically distinct despite both being single-filament 12-volt bulbs.
The shift from the 1156 to the 3156 occurred between the 1989 and 1990 model years of the R3500, representing a change in both socket format and power draw.
The higher wattage of the 3156 at 27 watts produces greater light output compared to the 21-watt 1156, a relevant factor in back up light performance.