2018 Freightliner Cascadia — Fuse Box Diagram & Amperage
A complete guide to the engine (eERC) and cab (SAM) fuse box locations, diagrams, and common issues for your 2018 Freightliner Cascadia.
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Quick Answer
On the 2018 Freightliner Cascadia, the primary fuse and relay locations are the eFuse and Relay Center (eERC) located in the engine bay, and the SAM Cab fuse block located inside the cabin.
- Engine Compartment Fuse Box (eERC): This main power distribution center is a large black box found on the passenger side of the engine compartment, mounted to the firewall. It contains high-amperage fuses, blade fuses, and essential relays.
- Cab Fuse Block (SAM Cab): A secondary fuse block, the Signal Detect and Actuation Module (SAM), is located inside the cab. To access it, remove the panel on the passenger-side kick panel/footwell area.
Engine Compartment Fuse Box (eERC) Diagram
The eERC is the heart of your Cascadia's electrical system, housing fuses and relays for critical powertrain and chassis components. The 2018 model uses a mix of traditional blade fuses and modern electronic eFuses integrated into the module.
Important: Fuse assignments can vary significantly based on your truck's powertrain (engine/transmission combination) and optional equipment. The information below is a common configuration. Always confirm fuse numbers and functions by checking the diagram printed on the inside of your eERC lid.
eERC Fuses
| Fuse # | Amp | Color | Circuit / Component Protected |
|---|---|---|---|
| F1 | 30A | Green | ABS Pump Motor |
| F2 | 20A | Yellow | Engine Control Module (ECM) - Battery Power |
| F3 | 15A | Blue | Transmission Control Unit (TCU) - Battery Power |
| F4 | 20A | Yellow | Fuel Heater |
| F5 | 30A | Green | Trailer Power - Constant |
| F6 | 10A | Red | Common Powertrain Controller (CPC) |
| F7 | 15A | Blue | Ignition Switch Power |
| F9 | 30A | Green | ABS Solenoid |
| F11 | 20A | Yellow | Headlight - High Beam Right |
| F12 | 20A | Yellow | Headlight - High Beam Left |
| F14 | 10A | Red | HVAC Controller |
| F16 | 20A | Yellow | Optional Equipment (e.g., Qualcomm) |
| F17 | 10A | Red | SAM Cab Power |
| F21 | 20A | Yellow | Headlight - Low Beam Right |
| F22 | 20A | Yellow | Headlight - Low Beam Left |
| F25 | 30A | Green | Trailer Brake Controller Power |
| F26 | 30A | Green | Trailer Marker Lights |
eERC Relays
The eERC also contains critical relays. Their location is typically printed on the fuse lid legend.
- Starter Relay: Energizes the starter solenoid.
- Ignition Main Relay: Supplies power to ignition-switched circuits.
- A/C Clutch Relay: Engages the air conditioning compressor clutch.
- Fuel Pump Relay: Controls power to the fuel pump (on applicable engines).
- Horn Relay: Activates the horn.
Cab Fuse Block (SAM Cab) Diagram
The SAM Cab module controls most of the in-cab electronics and driver comfort systems. Access is via the passenger footwell kick panel.
| Fuse # | Amp | Color | Circuit / Component Protected |
|---|---|---|---|
| F1 | 15A | Blue | Front Cigar / 12V Power Outlet |
| F2 | 15A | Blue | Rear Sleeper 12V Power Outlet |
| F3 | 10A | Red | Radio / Infotainment System |
| F4 | 5A | Tan | Instrument Cluster |
| F5 | 10A | Red | Dome Lights / Interior Lamps |
| F6 | 20A | Yellow | Not Used (Common) |
| F7 | 5A | Tan | Airbag / SRS Module |
| F9 | 10A | Red | Mirror Heater |
| F10 | 30A | Green | Blower Motor (HVAC Fan) |
| F12 | 15A | Blue | Sleeper 12V Outlet #2 |
| F14 | 10A | Red | Diagnostic Port Power |
| F17 | 7.5A | Brown | Tail / Park / Clearance Lights |
| F18 | 7.5A | Brown | Brake Lights / Stop Lamps |
How to Test a Fuse
A blown fuse can cause a component or an entire system to stop working suddenly. Here’s how to check if a fuse is the culprit.
- Turn Off the Ignition: Before inspecting any fuses, turn the ignition key off and remove it.
- Locate and Open the Fuse Box: Use the guide above to find the correct fuse box.
- Identify the Suspect Fuse: Using the diagram on the fuse box lid, locate the fuse for the circuit that has failed.
- Remove and Inspect: Most fuse boxes will have a small plastic fuse puller tool. Use it to gently pull the fuse straight out of its slot. Hold the fuse up to the light. Inside a good blade fuse, you will see a small, intact metal wire linking the two prongs. In a blown fuse, this wire will be broken or melted.
- Test with a Multimeter (Optional but Recommended): For a definitive test, set a multimeter to the continuity setting (it often looks like a sound wave symbol). Touch the probes to each metal prong on the fuse. If the meter beeps or shows a reading of zero, the fuse is good. If it remains silent or reads "OL" (over-limit), the fuse is blown.
- Replace the Fuse: If the fuse is blown, replace it with a new one of the exact same amperage rating. Using a higher-amperage fuse can cause a fire.
Common Blown Fuses on the 2018 Freightliner Cascadia
While any fuse can fail, some are more prone to blowing due to the demands placed on their circuits.
- Cigar Lighter / 12V Power Outlets: These are among the most common fuses to blow. They are often subjected to high-draw accessories like phone chargers, GPS units, and small coolers. If your 12V outlet stops working, this fuse (typically in the SAM Cab) is the very first thing to check. Overloading the port with a multi-socket adapter is a frequent cause.
- Trailer Marker/Brake Light Fuses: The connection between the tractor and trailer is a frequent source of electrical issues. A short in the trailer wiring can immediately blow the corresponding fuse in your truck's eERC. If you connect to a new trailer and suddenly lose your trailer lights, check the fuses (F25, F26, F17, F18) before assuming a more complex problem.
- Headlight Fuses: A headlight that goes out could be a burned-out bulb, but it's always wise to check the fuse first. Because the left and right headlights have separate fuses in the eERC, a single blown fuse will typically only affect one side. If a new fuse blows immediately, you likely have a short in the headlight wiring that needs professional diagnosis.
If you find yourself repeatedly replacing the same fuse, it indicates an underlying electrical problem that needs to be addressed by a qualified technician.
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Before you pull a fuse on the 2018 Freightliner Cascadia
- Use the label printed on the panel cover or the owner's manual for your exact build. Layouts change with options, model year and build date, so a chart for another truck can be wrong.
- Replace a blown fuse only with the same amp rating. A larger fuse can let wiring overheat.
- A fuse that blows again right away means a short or failing component downstream. Find the cause before replacing it again.
- A good fuse doesn't prove the circuit works — relays, grounds and connectors fail too.
- Many heavy trucks have more than one power distribution point (cab, under-hood or near the batteries), and some circuits use circuit breakers instead of fuses. Some systems have 24-volt starting circuits; confirm the voltage before testing.
- Disconnect the battery before working near high-current fuses or cables.
Learn more: How fuses, relays and truck electrical systems work · Find mobile truck repair
More specs for the 2018 Freightliner Cascadia
Oil Capacity & Filter
Capacity, weight, filter PN, drain plug torque, change interval.
Fluid Type Specs
Engine oil, coolant, ATF, diff, brake, PS, DEF — with OEM spec numbers.
Coolant Capacity & Type
Total system capacity, OEM coolant spec, flush interval, bleed tips.
Transmission Fluid & Service
Trans model, fluid spec, capacity, filter PN, service interval.
Torque Specifications
Lug nuts, drain plug, caliper, axle nut, plug, key fasteners.
Brake Pad & Rotor Specs
Pad PNs, rotor size, min thickness, caliper torque.
Tire Size & Pressure
OEM size, cold psi, bolt pattern, wheel size, TPMS.
Battery Size & Group
Group size, CCA, AGM/flooded, dimensions, reset note.
A/C Refrigerant & Oil
R-134a or R-1234yf, charge oz, PAG oil spec.
Bulb Sizes (Headlight, Tail, Fog)
Headlight, fog, tail, turn, reverse, interior.
Factory Maintenance Schedule
Service intervals by mileage, severe-duty notes, known issues.
Towing Capacity & Payload
Max tow, payload, GVWR/GCWR, hitch class by trim.
Serpentine Belt Routing & Length
Belt routing, length, rib count, OEM + aftermarket PNs.
Wheel Alignment Specifications
Camber, caster, toe ranges (front & rear), adjustability notes.
Paint Color Codes
All factory colors with paint codes, sticker location, touch-up tips.
Wiper Blade Sizes
Driver/passenger/rear blade size, attachment, OEM + cross-refs.
Spark Plug Type & Gap
Plug PN, gap, torque, interval by engine.
Transmission & Axle Gear Ratios
Trans gear ratios, axle ratios, RPO codes, tow vs MPG impact.
Key Fob Programming
DIY programming steps, FCC ID, fob PN, battery, transponder.
Same spec — other model years
Maintenance & repair guides
2018 Freightliner Cascadia Maintenance Schedule →
Available maintenance guidance; confirm the manufacturer schedule.
Repair planning references →
Plan the scope of work; detailed procedures need configuration-matched evidence.
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