Mastering the Boss Snow Plow Wiring Harness: A Technical Troubleshooting Flow

Diagnosing electrical faults in a Boss snow plow wiring harness requires a systematic approach, especially when dealing with the high-amperage hydraulic solenoids and multiplexed lighting circuits. Winter environments introduce severe variables: road salt corrosion, extreme thermal contraction, and physical abrasion from road debris. This guide provides a senior-level, step-by-step diagnostic flow for the classic Boss 13-pin relay-based harness system, commonly found on straight-blade and older V-plow configurations.

Before grabbing a multimeter, technicians must understand that the Boss 13-pin connector is the primary failure bottleneck. Pin spread, dielectric breakdown, and micro-fretting corrosion account for over 70% of harness-related plow failures. Below is the definitive troubleshooting sequence to isolate faults between the truck-side harness, the 13-pin interconnect, and the plow-side solenoid valves.

Phase 1: Physical Inspection & Connector Integrity

Never begin voltage testing without verifying the physical state of the interconnect. The 13-pin plug operates in a high-vibration, high-moisture environment.

  • Pin Spread Check: Use a digital caliper to measure the male pin diameter on the truck-side plug. Standard Boss 13-pin male terminals should measure 0.156 inches (approx. 4mm). If any pin is bent or measures under 0.140 inches due to crushing, the female terminal on the plow side will not achieve adequate contact pressure, leading to voltage drop under load.
  • Corrosion & Fretting: Look for white or green oxidation at the wire crimp barrel entrance. Saltwater wicks under the insulation. If green patina is visible, the wire must be cut back at least 3 inches to reach clean copper.
  • Dielectric Application: The factory specification calls for a polyphenyl ether (PPE) based dielectric grease, such as Nyogel 760G. Standard silicone grease is insufficient for high-amperage solenoid pins and can wash out under pressure washing. Apply a 2mm bead inside the female sockets before mating.

Phase 2: The Boss 13-Pin Diagnostic Chart

Reference this pinout chart when performing continuity and voltage drop tests. Wire colors and AWG sizes are based on standard Boss relay-system manufacturing specifications compliant with SAE J1128 low-tension primary cable standards.

Pin Function Wire Color AWG Expected Signal / Voltage
1Main System GroundBlack100.0V (Continuity to Chassis < 0.2Ω)
212V+ Battery FeedRed1012.4V - 14.2V (Constant Battery Voltage)
3Right Turn / MarkerBrown1812V+ Pulsed (Flasher Module)
4Left Turn / MarkerYellow1812V+ Pulsed (Flasher Module)
5High Beam HeadlightGreen1812V+ (When High Beam Selected)
6Low Beam HeadlightWhite1812V+ (When Low Beam Selected)
7Lighting Common GroundBlack180.0V (Dedicated Light Circuit Ground)
8Raise / Lower SolenoidBlue1612V+ (Active only during Raise)
9Angle Left SolenoidOrange1612V+ (Active only during Angle Left)
10Angle Right SolenoidPurple1612V+ (Active only during Angle Right)
11Controller 12V+ FeedPink1812V+ (Ignition Switched or Constant)
12Controller GroundGray180.0V (Clean Logic Ground)
13Spare / AuxiliaryTan18N/A (Reserved for V-Plow Wings/Accessories)

Phase 3: Step-by-Step Troubleshooting Decision Trees

Use the following logic flows to isolate the root cause of the two most common Boss snow plow wiring harness failures.

Symptom A: Solenoid Clicks, but Plow Fails to Raise or Angle

A clicking solenoid indicates the control module is successfully sending the 12V trigger signal through Pins 8, 9, or 10, and the ground path via Pin 1 is functional enough to energize the coil. However, the hydraulic pump motor is not receiving adequate amperage.

  1. Step 1: Perform a Voltage Drop Test on Pin 1 (Main Ground). Set your DMM to DC Voltage. Place the red probe on the truck battery positive terminal and the black probe on the plow-side solenoid ground stud. Activate the raise function. A reading above 0.5V indicates severe resistance in the 10 AWG Black ground wire or the ring terminal crimp.
  2. Step 2: Check Pin 2 (Main Power) Voltage Drop. Place the red probe on the battery positive and the black probe on the solenoid power input stud. Activate the raise function. A drop > 0.3V means the 10 AWG Red wire is compromised, or the truck-side mega-fuse holder has internal corrosion.
  3. Step 3: Load Test the Motor. If both power and ground voltage drops are within spec (under 0.3V), bypass the harness entirely. Jump 12V directly from the truck battery to the pump motor terminal. If the motor still fails to spin under load, the fault lies in the hydraulic pump motor brushes or armature, not the wiring harness.

Symptom B: Plow Lights Fail to Switch from Truck to Plow

The Boss headlight isolation module relies on detecting a voltage drop or current draw to switch the relays. If the plow lights remain dark or the truck lights do not cut out, follow this flow:

  1. Step 1: Verify Lighting Ground (Pin 7). The 18 AWG Black wire on Pin 7 is dedicated to the lighting circuit. If this pin suffers from micro-fretting, the plow headlights will attempt to back-feed through the hydraulic solenoid ground (Pin 1), causing erratic behavior or blown fuses. Test continuity from Pin 7 to the plow headlight shell.
  2. Step 2: Isolate the High/Low Beam Pins (5 & 6). Disconnect the 13-pin harness. Use a fused jumper wire to apply 12V directly to Pin 5 (Green) and Pin 6 (White) on the plow side of the connector, using Pin 7 as ground. If the lights illuminate correctly, the plow harness and bulbs are intact; the fault is in the truck-side isolation module or relay pack.
  3. Step 3: Inspect for Back-Feeding. If the plow lights glow dimly when the truck lights are off, check for melted insulation where the 18 AWG lighting wires cross the 10 AWG power wires near the solenoid cover. Heat from the solenoids can melt the SAE J1128 insulation, causing a short.

Phase 4: Solenoid Coil Resistance Diagnostics

When the controller sends the command but you hear no click from the hydraulic valves, the issue is either an open circuit in the harness or a burnt solenoid coil.

Disconnect the 13-pin harness to isolate the plow. Set your multimeter to Ohms (Ω). Measure the resistance between Pin 8 (Blue) and Pin 1 (Black/Chassis Ground). A healthy Boss hydraulic valve coil should read between 3.5 and 5.5 ohms.

  • Reading is OL (Infinite): The internal coil winding is broken, or the spade connector on the solenoid has vibrated off.
  • Reading is < 1.0 ohm: The coil has shorted internally. This will draw excessive amperage, potentially frying the truck-side controller relay or blowing the harness inline fuse.
  • Reading is > 10 ohms: Severe corrosion exists inside the 13-pin connector at Pin 8, or the wire is broken inside the insulation near the mold.

Repair Standards and Best Practices

When repairing a damaged Boss snow plow wiring harness, standard electrical tape and butt connectors will fail within one snow season. Adhere to these professional repair standards:

  • Wire Selection: Always use SAE J1128 compliant stranded copper wire. Never use solid core wire, as the vibration from the plow mounts will cause work-hardening and rapid fatigue failure.
  • Splicing: Use adhesive-lined dual-wall heat shrink crimp terminals. The inner meltable polyamide adhesive seals out calcium chloride and brine solutions, preventing capillary action from drawing moisture into the copper strands.
  • Connector Terminal Replacement: If a pin is corroded, do not simply clean it. Use a terminal extraction tool to remove the pin from the 13-pin housing. Crimp a new nickel-plated terminal using a closed-barrel ratcheting crimper to ensure a gas-tight connection.

Authoritative References

For ongoing technical support, wiring schematics, and warranty information, always consult the manufacturer and recognized automotive standards bodies: