FAW Function Switch Logic Assembly: Unlocking 5 Essential Control Groups

Welcome to this definitive technical guide to the FAW Function Switch Logic Assembly. This component group is the primary Human-Machine Interface (HMI) for all non-driving, auxiliary functions on the FAW Jiefang CA4251P66K24T1A3E5 6×4 diesel semi-trailer tractor. It represents the complete bank of dashboard-mounted switches, frames, and covers that provide the driver with direct command over lighting, powertrain, safety, and auxiliary systems.

The “Logic Assembly” designation is critical. This is not a random collection of individual switches. The FAW Function Switch Logic Assembly is an integrated system of ‘Function switch frames’ (e.g., 3739120-B45, 3739125-B45) designed to be installed as a single module. This “logic” simplifies cab assembly, centralizes wiring harnesses, and ensures a clean, ergonomic, and serviceable layout for the driver.

This in-depth guide is an essential resource for professional technicians and parts specialists. We will deconstruct the entire FAW Function Switch Logic Assembly, breaking it down from its structural frames to its individual Minimum Serviceable Units (MSUs). We will analyze the logic behind the five main marked groups: Lighting, Interior, Powertrain, Safety, and Auxiliary functions.

Core 1: The Structural “Logic” – Frames and Covers

The foundation of the FAW Function Switch Logic Assembly is its physical structure. The parts list identifies multiple ‘Function switch frame’ components (3739120-B45, 3739125-B45, 3739130-B45, etc.). These are not just trim pieces.

The Frame Logic: The “logic” of these frames is modularity and serviceability. They are designed to “gang” together, creating a single, rigid bank that mounts securely into the dashboard. This modular design allows different truck specifications (e.g., with or without PTO, with or without diff locks) to use the same basic dash cutout, simply by populating the frames with different switches or “blanks.”

The Switch Logic: The frames are designed to accept specific ‘Three-function switch’ modules (e.g., 3739210-99R, 3739215A99R). This “logic” groups switches of a similar type (like lighting) into a single part number, simplifying the internal wiring. The ‘Function switch cover’ (3735021-B45) serves as the final aesthetic and protective piece, hiding the mounting hardware and giving the dash a clean, unified look.

Core 2: Lighting & Visibility Control MSUs (The “Longer” Module)

This group represents the largest and most frequently used set of Minimum Serviceable Units (MSUs) within the FAW Function Switch Logic Assembly. These switches control every aspect of the vehicle’s visibility.

Mark 1 – Hazard Warning (3726050-D04): This is the ‘Hazard warning switch assy’. It is the most critical safety switch in this group. Its “logic” is to be a standalone component, often with its own integrated flasher relay or a direct, high-priority link to the Body Control Module (BCM). It must function even if other systems fail. It is a primary MSU.

Mark 4 – Main Lamp Switch (3711045-D04): This ‘Lamp switch assy’ is the main rotary dial or rocker that controls the vehicle’s primary lighting: (Off -> Park -> Headlights). This MSU sends a low-voltage signal to the main lighting relays (housed in the fuse box), which then handle the high-amperage load.

Fog & Work Lamps: The assembly includes separate MSUs for ‘Front fog lamp switch’ (3732060-B45) and ‘Rear fog lamp switch’ (3732070-D04). This separation is a legal requirement in many regions. The ‘Work lamp control switch assy’ (3715070-B45) is also included, providing switched power (via a relay) to high-power utility lights on the back of the cab.

Mark 2 – Interior Lighting (3714025-D04): This ‘Interior lamp control switch assy’ is the MSU that governs all dome, map, and courtesy lights within the cab.

Horn Changeover (3721120-B45): A key “logic” switch, the ‘Horn changeover switch assy’ allows the driver to toggle between the standard electric horn (for city use) and the high-decibel air horn (for highway use). This MSU controls two different relay circuits.

Each of these switches is a distinct MSU. A failure (e.g., fog lights won’t turn on) is often traced back to one of these dashboard switches, which can be popped out of the FAW Function Switch Logic Assembly frame and replaced individually.

Core 3: Powertrain & Drivetrain Control MSUs

This group of MSUs in the FAW Function Switch Logic Assembly contains the heavy-duty “logic” switches that directly control the tractor’s powertrain and drivetrain components.

Differential Locks: These are the most critical off-road/low-traction switches. The assembly contains two distinct MSUs:

• ‘Axle differential lock switch’ (3834010-B45): This switch locks the differential *between* the two rear drive axles, forcing them to rotate at the same speed.
• ‘Inter-wheel differential lock switch’ (3834050-1508): This switch locks the differential *inside* the axle, forcing the left and right wheels to rotate at the same speed.

The “logic” of these switches is that they send signals to air-actuated solenoids (via relays) to engage the diff locks.

Fuel System Controls: The assembly includes two key fuel MSUs. The ‘Fuel preheat switch’ (3750055-1516) is essential for cold-weather operation, activating a heater in the fuel line or filter to prevent gelling. The ‘Fuel changeover switch’ (3806050-1516) is for trucks equipped with dual fuel tanks, allowing the driver to switch the fuel-draw from one tank to the other.

PTO Switch (3772030-B45): The ‘PTO (Power Take-Off) switch’ is a vital MSU for vocational use, allowing the driver to engage the transmission’s PTO gear to run hydraulic pumps or other auxiliary equipment.

Core 4: Auxiliary & Multi-Mode Function MSUs

This final group of MSUs within the FAW Function Switch Logic Assembly covers specialized and safety-related functions.

Mark 3 – Multi-state Switch (3735075-B45): The ‘Multi-state switch assy’ is an MSU whose “logic” is to provide more than a simple “on/off” command. This is typically used for functions like the engine brake (Off / Stage 1 / Stage 2) or a variable fan speed control. It provides multiple, distinct output signals to an ECU.

Hydraulic Cab-tilting (3818045-B45): This is a critical safety MSU. The ‘Hydraulic cab-tilting control switch’ activates the electric-over-hydraulic pump used to tilt the entire cab forward for engine service. The “logic” of this switch often requires a secondary action (like a “press and hold” or a safety cover) to prevent catastrophic accidental activation while driving. This switch is a key serviceable part of the FAW Function Switch Logic Assembly.

FAW Function Switch Logic Assembly Parts List

The following table provides the complete, detailed breakdown of all components identified in the FAW Function Switch Logic Assembly diagram for the FAW CA4251P66K24T1A3E5 tractor.

Technical Specifications and Assembly Context

The specifications for the FAW Function Switch Logic Assembly are defined by its role as the central HMI (Human-Machine Interface) for all driver-actuated auxiliary controls. This modular system is the bridge between the driver and the vehicle’s BCM (Body Control Module).