The GMC FC100 front and rear turn signal bulbs both utilize a 1129 size bulb for proper operation. The replacement chart covers vehicle production spanning 1950 to 1952.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 1129 bulb is the sole turn signal bulb type listed for the GMC FC100 across all documented model years, appearing in both front and rear turn signal positions for 1950-1952.
The 1129 is an ANSI-standardized bulb recognized in both the United States and Canada, meaning it conforms to a defined set of electrical and physical standards shared across those markets.
As a dual-filament bulb, the 1129 carries two separate filaments within a single envelope, allowing it to serve two distinct lighting functions from one unit, such as a brighter turn signal flash and a dimmer running light.
The 1129 uses a BAY15d bayonet base, which is an offset twin-pin mount. The asymmetric pin placement of the BAY15d base prevents incorrect rotational insertion, ensuring the two filaments align with their intended circuits.
The 1129 operates at 12 volts with a wattage rating of 21/5W, where 21W corresponds to the high-intensity filament and 5W to the low-intensity filament.
The 1129 is described as a variant of the 1157 bulb. While both share the dual-filament, BAY15d configuration and 12V/21/5W ratings, they are not confirmed as interchangeable within these specifications, and substitution without verification of exact fitment could result in improper function.