GMC K3500 Back Up Light Bulb Size

Additional Notes

• The GMC K3500 back up light specifications cover model years 1979 through 2000 and reference three distinct bulb sizes: 1156, 2356, and 2456.
• The 1156 is the most frequently specified bulb across this range, appearing in the GMC K3500 for 1979-1989, 1993-1997, and 1982-1986. It is a single-filament bulb rated at 12V and 21W, fitted with a BA15s bayonet base, and recognized under ANSI standards in both the US and Canada.
• The 2356 is a dual-filament bulb operating at 12V and uses a BAY15d bayonet base. Although it shares a similar bayonet-style mounting with the 1156, the BAY15d base has offset pins, which physically prevents it from being interchanged with the BA15s socket used by the 1156.
• The 2456 is a festoon-style bulb rated at 12V and is designated for specialty applications. Its form factor differs entirely from both the 1156 and 2356, as festoon bulbs seat by contact pressure at each end rather than through a bayonet base.
• Because the 1156, 2356, and 2456 each use a different base type, none of the three bulbs are directly interchangeable with one another, even though all three operate at 12V.
• The 2456 festoon bulb is specified for the GMC K3500 in 1990, 1991, 1998, and 1999, representing a departure from the bayonet-base bulbs used in surrounding model years.
• The 2356 dual-filament bulb is specified for the 1992 and 2000 model years. Its dual-filament construction means it contains two separate filaments within a single envelope, though in a back up light application only one filament circuit is typically engaged.
• All three bulb sizes listed in these specifications conform to ANSI standards recognized in both the United States and Canada, indicating standardized electrical and physical tolerances across manufacturers.