The GMC K25 Suburban front and rear turn signal bulb sizes include 198, 1156, and 199 configurations. The replacement chart covers vehicle production years spanning 1967 to 1978.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The GMC K25 Suburban uses three distinct bulb types across its front and rear turn signal positions for the 1967-1978 model years: the 198, 1156, and 1034, with the 199 appearing as a transitional fitment in a single model year for each position.
The 198 is a T5 miniature wedge bulb rated at 12V and 2W, making it significantly lower in wattage than the other bulb types specified for this vehicle.
The 1156 is a single-filament bulb with a BA15s bayonet base, rated at 12V and 21W, and is an ANSI-standardized type used across both the US and Canadian markets.
The 1034 is a dual-filament bulb with a BAY15d bayonet base, rated at 12V with a 21/5W output, and is classified as a variant of the 1157. Its dual-filament design and BAY15d base make it physically and electrically incompatible with the single-filament BA15s socket used by the 1156 and 199.
The 199 is a single-filament bulb with a BA15s bayonet base and shares the same 12V, 21W rating as the 1156, functioning as a variant of that type. The shared base and wattage indicate a degree of interchangeability between the 199 and 1156 in compatible sockets.
For the GMC K25 Suburban, the 198 wedge bulb is the most frequently specified type, covering the front and rear turn signal positions across the 1973-1978 model years.
The front and rear turn signal positions do not always share the same bulb type within a given model year. In 1972, the front position uses a 1156 while the rear uses a 199, and in 1971 the front uses a 199 while the rear uses a 1156.
The 1034 with its BAY15d base has an offset pin configuration that prevents it from being seated into a BA15s socket, meaning it cannot serve as a substitute for the 1156 or 199 regardless of wattage considerations.
The 198 wedge bulb uses a pinless wedge base that is physically distinct from the bayonet bases of the 1156, 199, and 1034, making cross-substitution between the 198 and any bayonet-based type impossible without socket modification.
The 1967 model year is the only year in the specifications where both the front and rear turn signal positions share the 1034 dual-filament bulb.