2018 Peterbilt 389 Fuse Box Diagram & Amperage

As an ASE-certified master technician, I know firsthand that tracking down electrical gremlins on heavy-duty trucks like the 2018 Peterbilt 389 can be a daunting task. With a complex multiplexed electrical system, knowing exactly where your fuses and relays are located is half the battle. Whether you have lost your headlights, your CB radio has gone silent, or your trailer brakes are acting up, the right fuse box diagram is your best friend.

Below is a comprehensive guide to the fuse box locations, common circuits, and amperage ratings for the 2018 Peterbilt 389.

Disclaimer: Heavy-duty trucks are highly customizable. Your specific 2018 Peterbilt 389 may have slightly different fuse allocations depending on the engine (Cummins ISX / PACCAR MX-13), sleeper configuration, and ordered accessories. Always verify with the printed legend on your fuse box lid.

Quick answer

The 2018 Peterbilt 389 generally features three main fuse box and power distribution locations:

1. Main Cab Fuse Panel (Under Dash/Kick Panel): Located inside the cab, typically on the driver’s side, behind the clutch pedal / left kick panel area.
2. Under-Hood Power Distribution Box (PDC): Located in the engine bay, usually mounted on the firewall or driver-side inner fender.
3. Sleeper Auxiliary Fuse Panel: (If equipped) Located inside the sleeper cab, usually behind an access panel under the bunk or in a side cabinet, handling sleeper HVAC, fridge, and accessory power.

Main Cab Fuse Panel (Driver's Side Kick Panel)

This panel handles the majority of the interior electronics, lighting, ignition, and dash controls.

Relays located near this panel typically include: Cab Ignition Relay, Blower Motor Relay, and Accessory Power Relay.

Under-Hood Power Distribution Box (Engine Bay)

The under-hood fuse box protects critical powertrain components, high-amperage chassis circuits, and exterior engine management.

Relays located in this box typically include: Starter Relay, Fuel Pump Relay, A/C Clutch Relay, ECM Main Power Relay, and Aftertreatment Relays.

Sleeper Auxiliary Fuse Panel (If Equipped)

For trucks with a sleeper, this secondary panel isolates hotel loads from the main starting and driving circuits.

How to test a fuse

When a component suddenly stops working, testing the fuse is the first step. You do not need to pull every fuse out of the board to check them.

1. Use a 12V test light or digital multimeter: This is the easiest and most professional way to check fuses.
2. Turn the key ON: Ensure the key is in the "ON" or "Accessory" position so power is flowing to the fuse box. (Note: Some circuits are always hot, but turning the key on ensures all circuits are active).
3. Probe the test points: Every blade-style fuse has two tiny exposed metal test points on the top.
4. Check for power on both sides: Ground your test light to a good, bare metal chassis ground. Touch the probe to the left test point, then the right. If the light lights up on both sides, the fuse is good. If it only lights up on one side, the fuse is blown and must be replaced.

Common blown fuses on this vehicle

Heavy-duty trucks rack up massive mileage and operate in harsh environments. On the 2018 Peterbilt 389, here are the fuses we see blow the most often in the shop:

• Cigar Lighter / 12V Outlets (Fuse 4): Drivers plug in power-hungry inverters, coolers, or faulty phone chargers, quickly exceeding the 20-amp limit. What to check first: Unplug everything and look inside the socket for a penny or metallic debris shorting the contacts.
• Trailer Power & Trailer Brake (Fuses 13 & F10): A common issue when swapping trailers. Many older trailers have frayed wiring harnesses or shorted marker lights. What to check first: Inspect the pigtail connection and the trailer’s 7-way receptacle for green corrosion or crossed wires before replacing the fuse.
• A/C Clutch (Fuse F5): The A/C system runs constantly on these trucks. If the compressor clutch coil begins to fail and short out internally, it will blow this fuse. What to check first: Inspect the wiring harness running down to the A/C compressor for chafing against the engine block.
• Clearance / Marker Lights (Fuse 10): With dozens of marker lights on a truck and trailer, it only takes one cracked lens letting water in to short the circuit. What to check first: Do a walk-around and look for condensation inside the cab roof lights or trailer side markers.

Electrical diagnostics can be frustrating, especially when a fuse keeps blowing immediately after you replace it. If you're dealing with a persistent short circuit or an aftertreatment electrical fault, it's best to let a professional heavy-duty technician take a look.

Find a trusted shop on Repairius.