The GMC W7500 Forward uses 194 and 1157 bulb sizes for front side marker applications, with 194 bulbs specified for rear side marker positions. The replacement chart covers vehicle production spanning 1984 to 1996.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 194 bulb is a T10 miniature wedge type operating at 12V with a power draw of 3.78W. Its wedge base design requires no locking mechanism, as the bulb is held in place by tension from the socket housing itself.
The 1157 bulb is a dual-filament type with a BAY15d bayonet base, rated at 12V and drawing either 21W or 5W depending on which filament is active. The BAY15d base features two pins offset at different heights to prevent incorrect insertion.
Both the 194 and 1157 are ANSI-standardized bulbs recognized in both the United States and Canada, meaning their physical and electrical specifications conform to a defined industry standard rather than a proprietary format.
The GMC W7500 Forward front side marker uses the 194 bulb across the Tiltmaster variant from 1984-1996, while the Medium Duty variant of the same vehicle uses the 1157 bulb during the years 1984, 1988, and 1990-1992.
The 194 and 1157 bulbs are not interchangeable. They differ in base type, wattage, filament count, and socket design, meaning a housing built for one will not accept the other without modification.
The front side marker application on the W7500 Forward is the only position in these specifications where the 1157 bulb is listed, as all rear side marker entries reference the 194 exclusively.
The rear side marker on the W7500 Forward uses the 194 bulb across all listed years from 1984-1995, with no variant-specific split in bulb size for that position.
In 1992, the front side marker position lists two separate bulb sizes simultaneously: the 194 for the Tiltmaster and the 1157 for the Medium Duty. This reflects a sub-model distinction rather than a model year change, and selecting the wrong bulb for the specific sub-model would result in an incompatible fit.