2020 Kenworth T680 Bulb Size Guide: Headlight, Tail, Fog & More

As an ASE-certified master technician, I’ve serviced countless heavy-duty trucks, and the 2020 Kenworth T680 is a regular in the bay. When you're managing a fleet or driving cross-country, maintaining proper visibility is critical for safety and DOT compliance. The 2020 T680 comes with a mix of lighting configurations depending on how the truck was spec'd from the factory. Many feature traditional halogen projector setups, while others are equipped with full factory LED assemblies that offer superior longevity and nighttime visibility.

Below is a comprehensive guide to the bulb sizes for the 2020 Kenworth T680. Whether you are replacing a burnt-out halogen or upgrading your rig to aftermarket LEDs, this guide will help you find the exact bulb you need.

Quick Reference: 2020 Kenworth T680 Bulb Sizes

Note: Many 2020 T680 models feature sealed LED assemblies from the factory (especially for tail, brake, and marker lights). If your truck has factory LEDs, the entire assembly must be replaced if a diode fails.

LED vs Halogen Upgrade Considerations

Upgrading your Kenworth T680 from standard halogen bulbs to LED replacements is a popular modification for owner-operators looking for better highway illumination. However, modern trucks utilize sophisticated multiplex wiring systems. Here is what you need to know:

• CANBUS Errors: The T680's computer monitors electrical draw. Because LEDs pull significantly less power than halogens, the truck may think the bulb is burnt out, triggering a dash warning or causing the lights to flicker. You will likely need CANBUS-ready LED bulbs or in-line load resistors.
• Housing Space: LED headlight bulbs often have built-in cooling fans or large heat sinks. The T680 headlight housings have dust covers that may be difficult to reinstall with bulky aftermarket LED bulbs. Look for low-profile fanless designs or use extended dust caps.
• Projector Housings: If your T680 has halogen projector lenses (common for the low beams), make sure you purchase LED bulbs designed specifically for projectors so the beam pattern remains focused and doesn't blind oncoming traffic.

How to Replace Each Bulb

Replacing bulbs on a Class 8 truck is generally straightforward, but accessibility can vary. Here are a few quick tips:

• Headlights (Low/High Beam): Open the hood and locate the rear of the headlight assembly. Twist off the circular dust cover, unclip the wiring harness, and twist the H11/H9 bulb counterclockwise to remove it. Avoid touching the glass of the new halogen bulb with bare fingers.
• Fog Lights: These are accessible from underneath the front bumper. You may need to remove a small splash shield depending on your aerodynamic package. It’s a simple twist-and-pull mechanism.
• Front Turn Signals: Accessible from behind the headlight housing with the hood tilted. Twist the socket out to pull the 3157 bulb.
• Rear Lighting (Tail/Brake/Turn/Reverse): Most modern Kenworths use standardized 4-inch round sealed LED grommet lights at the rear. If one fails, you simply pry the light out of the rubber grommet with a flathead screwdriver, unplug the 3-prong pigtail, and push a new sealed unit in.
• Cab Marker/Side Markers: Usually held in place by two Phillips screws or seated in a rubber grommet.
• Interior Dome Lights: Gently pry the plastic lens covers off with a plastic trim tool to access the 194 or 921 wedge bulbs.

Common Lighting Issues for this Generation

The 2020 Kenworth T680 is a highly reliable rig, but a few lighting quirks are common:

• Moisture in the Headlight Housings: The seals around the headlight lenses can sometimes fail, leading to condensation build-up inside the projector housing. This can blow halogen bulbs prematurely or corrode wiring.
• Chafed Wiring Harnesses: Due to the vibrations of heavy-duty operation, the wiring harness leading to the fog lights or rear tail lights can chafe against the frame rails, causing intermittent shorts.
• Burnt Pigtails: At the rear tail lights, corrosion from road salt and grime can ruin the 3-prong pigtail connectors. Always apply a dab of dielectric grease when installing new rear lights.

Electrical diagnostics on heavy-duty trucks can get complicated quickly. If you're dealing with persistent shorts, CANBUS issues, or damaged harnesses, it's best to leave it to a professional. Find a trusted shop on Repairius.