GMC P4500 front and rear side marker bulbs utilize 194, 1157, 168, and 1895 bulb sizes depending on location and model variation. The replacement chart covers 1979 to 1990 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.
The GMC P4500 front and rear side marker positions draw from four distinct ANSI-standard bulb types across the 1979-1990 model range: the 194, 168, 1157, and 1895.
The 194 is a T10 miniature wedge bulb rated at 12V and 3.78W, and it is the most frequently specified bulb across both the front and rear side marker positions on the P4500.
The 1157 is a dual-filament bulb with a BAY15d bayonet base rated at 12V with a 21W/5W filament configuration. Its dual-filament design distinguishes it electrically from the single-filament wedge bulbs also used on this vehicle.
The 168 is a T10 miniature wedge bulb rated at 12V and 4.9W. Although it shares the same T10 wedge form factor as the 194, its wattage rating is higher, and the two are not the same part number.
The 1895 is a T5 miniature wedge bulb rated at 12V and 0.9W, making it the lowest-wattage bulb in these specifications. Its T5 form factor is physically smaller than the T10 used by the 194 and 168.
On the GMC P4500, the front side marker position for vehicles with two rectangular headlights consistently uses the 194 bulb from 1979-1988, while the standard Medium Duty configuration without that headlight distinction uses the 1157 across most of those same years.
The headlight configuration of the P4500 is a documented fitment condition, as the two rectangular headlight variant and the standard Medium Duty variant require different front side marker bulbs in overlapping model years.
The rear side marker position transitions from the 1895 in 1979-1980 to the 194 in 1981-1985, then to the 1157 in 1986-1987, and back to the 194 in 1988-1990, reflecting multiple bulb changes across the production span.
The 1157 bayonet-base bulb is not interchangeable with the 194, 168, or 1895 wedge-base bulbs, as the bayonet and wedge base types require different socket designs.
The 1895 T5 wedge bulb used in the 1979-1980 rear side marker position is not interchangeable with the T10-based 194 or 168, as the two wedge sizes are physically incompatible despite sharing the same base style.