Pontiac Chieftain Back Up Light Bulb Size

The Pontiac Chieftain back up light utilizes bulb sizes P21W, 1073, and 1129 depending on the specific version of the vehicle. Available data for the lighting components covers the range from 1950 to 1958.

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
1958	P21W
1955-1957	1073
1950-1954	1129

Bulb Specifications

Description	Bulb Size
ECE standard (Europe & worldwide). Single-filament signalling bulb with BA15s bayonet base. 12V, 21W.	P21W
ANSI standard (US & Canada). Single-filament bulb with BA15s bayonet base. Lower wattage variant of 1156. 12V, 13W.	1073
ANSI standard (US & Canada). Dual-filament bulb with BAY15d bayonet base. Variant of 1157. 12V, 21/5W.	1129

Other Locations

Additional Notes

The 1073 bulb follows the ANSI standard used in the US and Canada, operates at 12V and 13W, and uses a single filament with a BA15s bayonet base. It is a lower wattage variant of the more common 1156 bulb.
The 1129 bulb also follows the ANSI standard, operates at 12V with a dual-filament configuration rated at 21W and 5W, and uses a BAY15d bayonet base. It is a variant of the 1157 bulb.
The P21W bulb follows the ECE standard used in Europe and worldwide, operates at 12V and 21W, and uses a single filament with a BA15s bayonet base.
The Pontiac Chieftain used the 1129 bulb for the back up light across the 1950-1954 model years.
The 1073 and 1129 bulbs both use bayonet-style bases, but they are not interchangeable. The 1073 uses a BA15s single-contact base, while the 1129 uses a BAY15d dual-contact base with offset pins.
The Chieftain transitioned from the dual-filament 1129 to the single-filament 1073 between the 1954 and 1955 model years.
The 1073 bulb was specified for the 1955-1957 model years, making it the most frequently listed bulb across the Chieftain back up light specifications.
The 1958 Pontiac Chieftain shifted to the P21W bulb, which shares the BA15s bayonet base with the 1073 but operates at a higher wattage of 21W compared to the 1073's 13W.
Although the P21W and 1073 share the same BA15s base type, substituting one for the other would result in a wattage difference that may affect circuit load and light output.