GMC W4500 Forward Back Up Light Bulb Size

The GMC W4500 Forward back up light uses 1156 and 1073 bulb sizes for the rear illumination components. Data for the specified vehicle covers the range from 1992 through 2001.

The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.

Bulb Size by Year Range

Years	Bulb Size
1997-2001	1156 (Tiltmaster)
1996	1073 (Tiltmaster)
1995	1156 (Tiltmaster)
1994	1073 (Tiltmaster)
1993	1156 (Tiltmaster)
1992	1156

Bulb Specifications

Description	Bulb Size
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. 12V, 21W.	1156
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Lower wattage variant of 1156. 12V, 13W.	1073

Other Locations

Additional Notes

The back up light bulbs specified for the GMC W4500 Forward across its listed model years are the 1156 and 1073, both of which are ANSI-standard single-filament bulbs rated at 12 volts and fitted with a BA15s bayonet base.
The 1156 bulb is rated at 21W, while the 1073 is rated at 13W, making the two bulbs electrically distinct despite sharing the same BA15s base type.
Because the 1156 and 1073 share the same BA15s bayonet base, one can physically seat in a socket intended for the other, which presents a risk of incorrect substitution if wattage ratings are not verified before installation.
The 1156 bulb is specified for the GMC W4500 Forward (Tiltmaster) for the 1993, 1995, and 1997-2001 model years.
The 1073 bulb is specified for the 1994 and 1996 Tiltmaster model years, representing a lower-wattage fitment relative to the surrounding years.
The 1992 model year carries a 1156 specification and is the only entry in the data without the Tiltmaster sub-model designation.
The BA15s bayonet base uses a single contact pin offset to one side, a design that allows only one insertion orientation and distinguishes it from the dual-pin BA15d base used on two-filament bulbs.