Pontiac LeMans Back Up Light Bulb Size

Additional Notes

• The 1156 bulb is a single-filament lamp rated at 12 volts and 21 watts, fitted with a BA15s bayonet base. It is standardized under ANSI specifications for use in the United States and Canada.
• The 1157 bulb is a dual-filament lamp operating at 12 volts, with a primary filament rated at 21 watts and a secondary filament rated at 5 watts. It uses a BAY15d bayonet base and is also an ANSI-standardized type for the US and Canadian markets.
• The BA15s base of the 1156 and the BAY15d base of the 1157 are physically distinct and not interchangeable. The BAY15d base features offset pins to prevent incorrect installation, while the BA15s base has a single-contact, straight-pin design.
• The Pontiac LeMans used the 1156 bulb for its back up light across the majority of model years covered in these specifications, specifically 1963-1970, 1972, and 1974-1981, with the exception of 1971, 1973, and 1980.
• The 1157 bulb was specified for the Pontiac LeMans back up light in 1971, 1973, and 1980. These three years require the dual-filament BAY15d-based lamp rather than the single-filament BA15s type used in surrounding model years.
• Because the 1156 and 1157 share the same 21-watt primary filament rating, their light output in back up light applications is equivalent. The functional difference lies in the additional 5-watt filament present in the 1157, which serves a secondary circuit not relevant to a single-function back up light socket.
• Substituting a 1156 for a 1157 in a socket wired for dual-filament operation carries a risk of incomplete circuit function, as the secondary filament contact would be absent. The reverse substitution, placing a 1157 into a 1156 socket, is prevented by the differing bayonet base designs.