Ford F350 Auxiliary Switch Wiring Guide for 2018 Model Upfitters

Begin by locating the auxiliary relay module beneath the dashboard on the driver’s side–this compact block houses eight fuses and twelve pins, designated for secondary circuits. Trace the red and black cables from the predefined harness to pins 87 (power) and 85 (ground), ensuring a secure 16-gauge connection. Avoid splicing into the OEM fuse box; instead, route cables directly to the battery’s positive terminal with a 30-amp inline fuse placed within six inches of the connection point. This prevents voltage drops and interference with factory systems.

For momentary controls, wire the switch through pins 30 (input) and 86 (output) of the relay, bypassing the ignition feed unless continuous power is required. Use a dual-pole solenoid rated for 150 amps if integrating high-draw accessories like winches or hydraulic pumps–standard relays will fail under sustained loads. Verify all connections with a multimeter set to continuity mode before energizing; resistance above 0.5 ohms indicates corrosion or improper crimping.

Label each circuit with shrink-wrapped tubing marked in alphanumeric codes matching the vehicle’s original documentation. For fleets, add a diode network to prevent backfeed between multiple modifications. Test operation in sequence, starting with low-current devices (lights, compressors) before engaging heavy machinery. Keep a schematic of all modifications in the glove compartment–standard Ford manuals omit aftermarket wiring.

Modifying Auxiliary Control Circuits in Heavy-Duty Ford Models

Start by locating the OEM relay panel beneath the dashboard on the driver’s side–Ford designates this as fuse block #2. Remove the lower trim cover using a T20 Torx driver to expose the auxiliary connectors labeled C1059 (gray) and C1060 (black). These harnesses interface directly with the factory-installed command modules, supplying 12V reference and ground for custom components. Verify battery voltage at pin 30 (red/light blue) of relay slot R15 before tapping into any circuit; unexpected parasitic loads here can trigger the body control module’s overcurrent protection.

• Use crimp connectors rated for 16 AWG minimum–Ford specifies TE Connectivity PIDG style for auxiliary loads.
• Splice into the output side of relay R15 (red/light blue) for switched power; never connect directly to battery feed (orange/white) unless installing a dedicated fuse.
• Ground custom circuits at G101, a chassis stud behind the left kick panel; scrape paint and apply dielectric grease to prevent corrosion.
• Label every connection with automotive-grade heat-shrink tubing containing printable identification–include pinouts and load specs.

Programming adjustments are required if retrofitting LED beacons or hydraulic pumps exceeding 15A draw. Access the integrated power relay module via Ford’s diagnostic software, selecting “Auxiliary Output Configuration.” Assign the desired switch position (e.g., “Upfitter Switch #2”) to “High-Side Driver Mode” for momentary functions or “Low-Side Driver Mode” for continuous operation. Failure to configure these settings will result in erratic behavior, including backfeeding into the CAN bus and triggering DTCs U0100 or P0700.

For heavy-duty winches or compressors, install a secondary relay harness–use a Bosch 0 332 019 150 for inductive loads above 20A. Connect coil control to the auxiliary harness’s switched output (purple/yellow, pin 87a) and run heavy-gauge power wires (4 AWG minimum) directly to the battery through an ANL fuse holder. Route all cabling through existing grommets in the firewall bulkhead, avoiding sharp edges and heat sources like the exhaust manifold. Secure bundles with nylon zip ties every 6 inches, using loom tape for additional abrasion protection.

Locating the Proper Auxiliary Control Connection on a Ford Super Duty

Start with the central junction box beneath the dashboard on the driver’s side–Ford labels it “BCM” (Body Control Module). Peel back the plastic cover; the factory auxiliary harness is grouped in a 12-pin black connector tucked behind the fuse panel starter bundle. Count three connectors from the passenger-side edge; the middle plug carries the auxiliary channels.

The connector shell bears raised alphanumeric codes; match the socket marked “P5” to the harness plug. Pin assignments align left-to-right as viewed from the rear of the socket, numbered 1–12. Pins 1, 3, 5, 7, 9, and 11 supply switched outputs; their mating wires follow a striped color scheme–white base with colored stripes corresponding to the switch bank legend on the dash panel overlay.

Channel	Pin	Wire Color (Stripe)	Max Current	Fuse Rating
Aux 1	1	White / Light Blue	20 A	15 A
Aux 2	3	White / Orange	20 A	20 A
Aux 3	5	White / Dark Green	15 A	15 A
Aux 4	7	White / Red	15 A	15 A
Aux 5	9	White / Brown	15 A	10 A
Aux 6	11	White / Tan	10 A	10 A

Ground reference is common across all channels–locate the M6 stud behind the same junction box, distinguishable by its bare braided strap terminating in a ring terminal. Verify continuity between each auxiliary pin and this stud before energizing loads to prevent damage to the onboard computer.

If the harness ends short of the intended load point, use 18-gauge primary wire spliced via solder-and-heat-shrink method. Match the fused current rating of each channel to the auxiliary component–oversized relays or compressors exceeding 20 A require direct battery taps routed through a 40 A fuse within six inches of the positive terminal.

Secondary Access Point

For rear-mounted equipment, trace the main harness conduit from the cab roof bow to the rear crossmember; a second 12-pin white connector appears at the frame rail near the trailer plug bracket. Label reads “CJB Aux Rear”–pins mirror the dashboard connector but supply fused outputs for liftgates and bed plugs without cab interference.

Apply dielectric grease to both plug interfaces during reconnection to prevent water ingress; Ford service bulletin TSB-18-23-9 confirms corrosion on these terminals as a primary failure mode for intermittent relay activation.

Step-by-Step Installation Guide for Auxiliary Lighting via Factory-Configured Controls

Start by identifying the pre-installed control module beneath the dashboard’s left panel–locate the connector labeled “C3300” on Ford’s integrated auxiliary system. Use a pinout map from the vehicle’s service manual to confirm terminals 30 (power), 85 (trigger), 86 (ground), and 87 (output) align with the OEM relay slot. If repurposing an existing circuit, verify voltage at terminal 30 with a multimeter (minimum 12.5V under load) to prevent voltage drop during activation.

Route a 12-gauge primary cable from the relay’s output terminal to the lighting assembly’s positive terminal, ensuring a direct path free of sharp edges or pinch points–secure with loom tubing near suspension components. Connect the auxiliary unit’s ground to a factory chassis stud within 18 inches of the relay to avoid ground loops; scrape paint from the attachment point to guarantee conductivity. For dual-light setups, split the output wire using a weatherproof splice connector before branching to each light.

Relay Integration and Circuit Protection

Mount a 30A mini relay in the factory relay center or a nearby dry location–use dielectric grease on pins to prevent corrosion. Wire the relay’s coil trigger (terminal 85) to the pre-mapped control output from the C3300 connector; confirm polarity matches the manual’s diagrams. Insert a 20A fuse holder inline with the power feed (terminal 30) no more than 7 inches from the relay to protect against short circuits. Test the circuit by activating the designated control–audible relay clicks and steady 13.8V at the light terminals confirm proper function.

For delayed-off functionality, integrate a timer module between the trigger wire and relay coil–program delays up to 30 seconds using the module’s dip switches. If pairing with high-intensity LEDs, add a 1Ω, 5W resistor in series with each light to mitigate electromagnetic interference. Route all connections away from exhaust manifolds and rotating shafts; avoid bundling power wires with signal cables to reduce induction noise.

Final Validation and Troubleshooting

Activate the system and observe light behavior: flickering or dimming indicates poor ground or undersized wire gauge. Check for voltage drop across the circuit using a load tester–losses exceeding 0.5V mandate thicker cable or shorter runs. Seal all connections with adhesive-lined shrink tubing or liquid electrical tape to prevent moisture ingress. Document modifications in the vehicle’s service log, noting fuse ratings and relay positions for future diagnostics.

Common Mistakes When Connecting Aftermarket Accessories to Secondary Control Modules

Ignoring current draw specifications leads to immediate failures. Each auxiliary control point on modern heavy-duty platforms lists exact ampacity limits–typically 20-30 amps per circuit. Exceeding these values without a relay triggers fuse blows or, worse, melts factory harness insulation. Always verify accessory labels against manufacturer specs; a 150-watt light bar may demand 12.5 amps, yet some aftermarket units understate actual consumption by 30%. Use a multimeter to measure real-world current draw before final connections.

Skipping relay integration compounds wire strain and voltage drop. Directly powering accessories through toggle interfaces forces control contacts to handle load currents they’re not designed for, causing premature pitting or resistance buildup. Install a 30/40-amp relay between the power source and accessory, using the secondary control output as the trigger signal only. Ensure relay coil voltage matches system voltage–most automotive relays operate at 12V; industrial variants may require 24V.

Ground Path Errors

• Attaching grounds to painted chassis surfaces guarantees intermittent operation due to high-resistance connections.
• Bolt grounds to dedicated engine block studs or factory ground lugs, removing paint with a wire brush prior to fastening.
• Daisy-chaining accessory grounds through multiple components invites noise interference in sensitive electronics.
• Route all grounds to a single star point near the battery, avoiding shared paths with high-current devices like winches.

Neglecting circuit protection invites fire hazards. Insert an inline fuse holder within 18 inches of the power source terminal–14 AWG wire requires 15-amp fuses, 12 AWG needs 20-amp, and 10 AWG demands 30-amp. Avoid glass tube fuses for high-current applications; blade-type fuses offer faster response times under short-circuit conditions. For dual-battery systems, isolate accessory circuits from starting circuits to prevent voltage sag during engine cranking.

Panel Integration Pitfalls

1. Drilling mounting holes too close to OEM wiring bundles risks insulation damage; maintain a 2-inch clearance from any harness routing.
2. Using non-waterproof connectors (e.g., standard butt splices) in open-bed installations guarantees corrosion within months–employ heat-shrink butt connectors with adhesive lining.
3. Over-tightening control knob mounting nuts strips plastic housings–apply thread-locking compound and torque to 25 in-lbs.
4. Misaligned control cutouts prevent full panel engagement; use a stepped hole bit or Dremel for precise cuts.