The Isuzu I-Mark front turn signal requires a 1157, 1073, or P21W bulb size, while the rear turn signal uses a P21W or 1073 bulb size. The replacement chart covers vehicle production spanning 1981 to 1989 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 P21W bulb is the most frequently specified type across the Isuzu I-Mark's turn signal positions, appearing in both front and rear applications across multiple model years.
P21W is an ECE-standard bulb used widely in European and international markets. It operates at 12V with a 21W single filament and uses a BA15s bayonet base.
The 1157 bulb follows the ANSI standard used in the United States and Canada. It is a dual-filament bulb rated at 12V, with filaments drawing 21W and 5W respectively, and it mounts via a BAY15d bayonet base.
The dual-filament design of the 1157 means it serves two electrical functions from a single housing, which distinguishes it from the single-filament bulbs also specified for the I-Mark.
The 1073 is an ANSI-standard single-filament bulb rated at 12V and 13W, using a BA15s bayonet base. It is described as a lower wattage variant of the 1156 bulb.
The P21W and 1073 share the same BA15s bayonet base type, but their wattage ratings differ significantly at 21W and 13W respectively, which affects their interchangeability despite the physical base compatibility.
The 1157 uses a BAY15d base, which is physically distinct from the BA15s base shared by the 1073 and P21W. The offset pins on a BAY15d base prevent it from being seated in a BA15s socket, making cross-substitution between the 1157 and the other two types mechanically incompatible.
For the Isuzu I-Mark, the front turn signal position uses all three bulb types across the 1981-1989 model range, while the rear turn signal position is specified with only the P21W and 1073 types.
The 1987-1989 front turn signal specification calls for the 1157, the only dual-filament application in the I-Mark's turn signal data, whereas all other specified positions use single-filament bulbs.
The 1982 and 1986 front turn signal positions share the 1073 specification, while the 1985 rear turn signal position is the sole instance of the 1073 being specified for a rear application on the I-Mark.