White/GMC WHM front side marker bulb uses 1157 size while rear side marker bulb uses 194 size. Replacement chart covers 1988 to 1991 production years with bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The White/GMC WHM uses the 1157 bulb for the front side marker position across the 1988-1991 model years covered in the specifications.
The 1157 is a dual-filament bulb with a BAY15d bayonet base, rated at 12V with a 21W/5W output, conforming to ANSI standards for the US and Canada.
The dual-filament design of the 1157 means it carries two separate tungsten filaments within a single envelope, allowing it to serve two distinct brightness levels from one housing.
The BAY15d bayonet base features two offset pins, a physical characteristic that prevents accidental substitution with single-filament bayonet bulbs sharing a similar base profile.
The rear side marker position on the WHM uses the 194 bulb across the same 1988-1991 model years.
The 194 is a single-filament miniature wedge bulb in the T10 form factor, rated at 12V and 3.78W, also conforming to ANSI standards for the US and Canada.
The T10 wedge base relies on the bulb's glass envelope itself to make contact within the socket, rather than a separate metal base, which distinguishes it from the bayonet-style 1157.
The 1157 and 194 are not interchangeable, as they differ in base type, wattage, filament count, and physical form factor.
The significant wattage difference between the 1157 at 21W/5W and the 194 at 3.78W reflects their respective roles, with the higher-output 1157 serving the more visible front marker position.