The Peterbilt 378 front and rear turn signal bulbs utilize a 2057 or 198 size bulb configuration. The replacement chart covers vehicle production spanning 1988 to 2007 with corresponding bulb size specifications.
The year-by-year compatibility chart below provides a quick reference to verify the correct bulb size for your vehicle.
The 2057 bulb is the dominant turn signal bulb across the Peterbilt 378 specifications, appearing in both front and rear positions across the full model range covered in the data.
The 2057 is a dual-filament bulb with a BAY15d bayonet base, operating at 12V with filament ratings of 26.9W and 8W, making it suitable for combined stop and turn signal functions.
The 198 is a single-filament miniature wedge bulb of the T5 type, rated at 12V and 2W, representing a significantly lower wattage output than the 2057.
Both the 2057 and 198 are ANSI-standardized bulbs recognized in the United States and Canada.
On the Peterbilt 378, the correct rear turn signal bulb is tied to the headlamp configuration of the vehicle, with certain years requiring the 198 under specific headlamp setups and the 2057 under others.
The 1996 front turn signal position lists the 198 bulb exclusively for trucks equipped with 4 rectangular headlamps, while all other 1996 front configurations use the 2057.
The 1988 and 1989 front turn signal positions each carry two bulb entries, with the 198 specified for vehicles fitted with halogen capsule headlamps or 4 round headlamps respectively, and the 2057 for remaining configurations.
Because the 2057 is described as a brighter variant of the 1157, the two share a related design lineage, though the specifications list only the 2057 for this application and the two are not interchangeable without verification.
The rear turn signal on the Peterbilt 378 shows the most variation in bulb requirements across model years, with headlamp type serving as the determining factor in several years between 1988 and 1997.
Substituting the low-wattage 198 wedge bulb for the 2057 in positions that call for the latter would result in a substantial reduction in output, as the 2W rating of the 198 is far below the 26.9W high-filament rating of the 2057.