Volvo 780 front turn signal bulb size measures P21/5W while rear turn signal bulb size measures P21W for the specified vehicle. Replacement chart data covers 1987 to 1991 production years with corresponding bulb specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The Volvo 780 uses two distinct bulb types across its turn signal system: P21/5W at the front and P21W at the rear, a split that holds across the full 1987-1991 model range.
Both bulb types conform to ECE standards, a regulatory framework used in Europe and internationally, meaning they are governed by uniform performance and construction requirements.
The P21/5W is a dual-filament bulb, operating at 12V with a 21W primary filament and a 5W secondary filament. In automotive applications, this dual-filament design allows a single bulb to serve more than one lighting function simultaneously.
The P21W is a single-filament bulb, also rated at 12V but drawing a fixed 21W. It serves a dedicated signalling role without a secondary filament.
The P21/5W uses a BAY15d bayonet base. The offset pins on a BAY15d base are positioned at different heights, a physical feature that prevents incorrect filament orientation during installation.
The P21W uses a BA15s bayonet base. Unlike the BAY15d, the BA15s is a single-contact base with symmetrically placed pins, reflecting the bulb's single-filament construction.
Because the P21/5W and P21W share a similar physical profile but carry different base types, BAY15d and BA15s are not interchangeable. Installing the wrong base type in either the front or rear housing of the Volvo 780 is not mechanically possible without modification.
The wattage ratings differ functionally between the two positions. The front position draws either 21W or 5W depending on which filament is active, while the rear position draws a constant 21W during signalling operation.