2021 Peterbilt 389 Fuse Box Diagram & Amperages

The 2021 Peterbilt 389 is an absolute icon of the open road, blending classic styling with modern multiplexed electrical architecture. Whether you are running a PACCAR MX-13 or a Cummins X15 under the hood, a solid understanding of your electrical system is crucial for keeping those wheels turning and avoiding costly downtime.

As an ASE-certified master technician, I have traced plenty of electrical gremlins on Class 8 trucks. Below is a comprehensive guide to the fuse box locations, relay centers, and fuse block diagrams for the 2021 Peterbilt 389 to help you diagnose electrical issues out on the road.

Note: Heavy-duty trucks are highly customizable. Your exact fuse configuration will vary based on engine choice, transmission, sleeper size, and factory-installed auxiliary accessories. Always verify the printed legend on the inside of your specific fuse box lid.

Quick answer: Where are the fuse boxes on a 2021 Peterbilt 389?

On the 2021 Peterbilt 389, you will typically find fuses and relays divided into a few key locations:

1. Main Cab Power Distribution Center (PDC): Located inside the cab, usually behind a trim panel on the driver's side dash/kick panel area (near the clutch/brake pedal assembly).
2. Under-Hood / Firewall Chassis PDC: Located on the driver's side firewall under the hood. This houses main power feeds, engine management fuses, and exterior lighting relays.
3. Sleeper Auxiliary Fuse Panel: (If equipped with a sleeper) Located under the sleeper bunk or behind the sleeper control panel, housing HVAC and accessory fuses for the rear cab.

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Driver's Side Dash / Kick Panel Fuse Box (Main PDC)

The Main PDC in the cab controls the bulk of your interior electronics, dashboard gauges, infotainment, and cab comfort features.

Fuse #	Amp	Color	Circuit / Component Protected
F1	15A	Blue	Radio / Infotainment System
F2	10A	Red	Instrument Cluster / Gauges
F3	20A	Yellow	Cigar Lighter / 12V Power Outlet
F4	15A	Blue	Diagnostic Link Connector (OBD/9-pin)
F5	30A	Green	Cab Blower Motor / HVAC
F6	10A	Red	Ignition Switch / Key ON Power
F7	15A	Blue	Cab Dome / Map Lights
F8	20A	Yellow	CB Radio Power Prep
F9	10A	Red	Airbag / SRS System
F10	15A	Blue	Heated Mirrors
F11	20A	Yellow	Power Windows (Driver/Passenger)
F12	10A	Red	Power Door Locks
F13	15A	Blue	A/C Compressor Clutch

Common Relays in this Box:

• Ignition Relay
• Accessory Relay
• Blower Motor Relay
• Horn Relay

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Under-Hood / Firewall Power Distribution Center

This fuse block is subjected to engine heat and road vibration. It handles high-amperage chassis circuits, engine management (ECM/PCM), and heavy-duty exterior lighting.

Fuse #	Amp	Color	Circuit / Component Protected
EF1	30A	Green	Engine Control Module (ECM) Main Power
EF2	20A	Yellow	Fuel Pump / Lift Pump
EF3	30A	Green	ABS / Bendix Air Disc System Power
EF4	20A	Yellow	Headlights (Low Beam - Left/Right)
EF5	20A	Yellow	Headlights (High Beam)
EF6	15A	Blue	Starter Relay Control / Solenoid
EF7	30A	Green	Trailer ABS Power
EF8	20A	Yellow	Trailer Brake / Marker Lights Feed
EF9	15A	Blue	Tail / Park / Clearance Lights
EF10	15A	Blue	Brake Lights / Stop Lamp Switch
EF11	10A	Red	Dosing Control Unit (DEF/Aftertreatment)
EF12	20A	Yellow	Air Dryer Heater

Common Relays in this Box:

• Starter Motor Relay
• ECM Power Relay
• Trailer Lighting Relays
• Headlamp Relays

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Sleeper Auxiliary Fuse Box (If Equipped)

If your 389 has a flat-top or stand-up sleeper, there is a dedicated fuse block for the sleeper amenities to prevent drawing down main cab circuits.

Fuse #	Amp	Color	Circuit / Component Protected
S1	30A	Green	Sleeper Blower Motor (Rear HVAC)
S2	15A	Blue	Sleeper Interior Lighting
S3	20A	Yellow	Refrigerator Power Feed
S4	15A	Blue	Sleeper 12V / USB Outlets
S5	20A	Yellow	Inverter Prep Power

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How to test a fuse

When you are stranded at a truck stop or out on a haul, quickly finding a blown fuse can save your day.

1. Visual Inspection: Pull the suspected fuse using fuse pullers or needle-nose pliers. Look at the U-shaped metal filament inside the plastic casing. If it is broken or scorched, the fuse is blown.
2. Using a Test Light: This is the fastest method for an ASE technician. Turn the ignition ON (or activate the circuit). Ground the clamp of your 12V test light to bare metal. Touch the probe to the two small metal testing tabs on the top of the fuse while it is still plugged in. If it lights up on one side but not the other, the fuse is blown.
3. Using a Multimeter: Set your multimeter to continuity (the beep setting). Touch the probes to the top testing tabs of the fuse. If you hear a continuous beep, the fuse is good. If it is silent, replace the fuse.

Always replace a blown fuse with one of the exact same amperage. Never put a 30A fuse in a 10A slot—this is how electrical fires start in heavy-duty trucks.

Common blown fuses on this vehicle

• 12V Outlets & CB Radio Fuses: Drivers constantly plug in high-draw coolers, heavy-duty phone chargers, or powerful CB radios. Overloading these circuits will frequently pop the 20A Yellow fuses in the dash PDC. Check your accessory cords for shorts before replacing the fuse.
• Trailer Lighting & ABS Fuses: Given that the 389 constantly hooks up to different trailers, it is very common for a badly wired trailer to blow the tractor's trailer tail, marker, or ABS fuses. If you lose trailer lights immediately after hooking up a new load, check the Under-Hood PDC and inspect the trailer pigtail connection.
• A/C Compressor Clutch: If your cab loses cold air on a hot summer haul, check the 15A A/C clutch fuse. The magnetic coil on the clutch can degrade over time and pull too much amperage, blowing the fuse.

Electrical diagnosis on a multiplexed Class 8 truck can get complicated quickly. If you are replacing the same fuse multiple times, you have a short to ground that needs professional attention. Find a trusted shop on Repairius.