GMC 100 Dome Light Bulb Size

GMC 100 dome light bulb utilizes a 1156 or 94 size bulb for interior illumination. Replacement chart data spans 1956 to 1959 production years with corresponding bulb size specifications.

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
1958-1959	1156
1956-1957	94

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. Variant of 1157. 12V, 21/5W.	94

Other Locations

Additional Notes

The GMC 100 dome light draws from two distinct ANSI-standard bulb types across its 1956-1959 model years: the 1156 and the 94, both rated at 12 volts and recognized under US and Canadian standards.
The 1156 bulb, used in the 1958-1959 GMC 100, is a single-filament type mounted on a BA15s bayonet base, rated at 21 watts.
A BA15s base accepts only single-contact bayonet fittings, meaning the base has one locating pin and a single electrical contact at its tip.
The 94 bulb, fitted to the 1956-1957 GMC 100, uses a BAY15d bayonet base, which is an offset dual-contact design that physically prevents incorrect insertion of incompatible bulbs.
The 94 is a dual-filament bulb rated at 21 watts on its primary filament and 5 watts on its secondary filament, reflecting its classification as a variant of the 1157.
Because the 1156 uses a BA15s base and the 94 uses a BAY15d base, the two bulb types are not interchangeable despite sharing the same primary wattage of 21 watts.
The offset pin arrangement of the BAY15d base on the 94 bulb is a deliberate anti-reversal feature, as reversing polarity on a dual-filament bulb would route current through the wrong filament.
The transition from the dual-filament 94 to the single-filament 1156 between the 1957 and 1958 model years indicates a change in the dome light circuit design, as the single-filament 1156 serves only one switching function rather than two.