FAW Fuse Box Logic Assembly: The 11 Essential Serviceable Units Revealed

Welcome to this in-depth deconstruction of the FAW Fuse Box Logic Assembly (Part No. 3722010-92W). This component is the primary electrical nerve center for the driver’s cab in the FAW Jiefang CA4251P66K24T1A3E5 6×4 diesel semi-trailer tractor. It is not merely a fuse holder; it is an integrated “logic” hub that manages, protects, and switches nearly every single electrical function within the cab, from the instrument cluster to the power windows.

The design “logic” of the FAW Fuse Box Logic Assembly is to centralize power distribution, circuit switching, and circuit protection into a single, serviceable module. This assembly takes high-amperage feeds from the battery and main power lines and intelligently distributes that power to dozens of individual circuits, each protected by a specific-amperage fuse. It also houses the critical relays that allow low-power dashboard switches to safely control high-power devices.

This technical analysis is an essential resource for professional technicians and parts specialists. We will deconstruct the entire FAW Fuse Box Logic Assembly, breaking down the main housing and identifying every Minimum Serviceable Unit (MSU) it contains. We will analyze the three types of plug-in relays (4-pin, 5-pin, and main), the five different ratings of fuses, and the specific service tools designed for its maintenance.

Core 1: The Structural Core (3722010-92W) & Hardware

The ‘Cab fuse & relay box assy’ (3722010-92W) is the primary structural component, the “large purchase item” that forms the foundation of the FAW Fuse Box Logic Assembly. Its design is more complex than it appears.

Internal Logic: The “logic” of this housing is its sophisticated internal busbar system. Molded within the high-grade, heat-resistant polymer is a network of copper pathways. These busbars take the main power feeds (from the battery and ignition switch) and distribute them to the “hot” side of all 56 fuse sockets and 12 relay sockets. This design eliminates dozens of potential failure points, increases reliability, and simplifies the main cab harness.

Sealing and Mounting: This box is located inside the cab, but it is still designed to be a sealed unit, typically with a gasketed cover (sold separately) to protect the sensitive connections from dust and moisture. The ‘Cross recessed pan head combination screw’ (Q2360640) is the hardware specified for mounting the assembly securely to the cab’s bulkhead, ensuring it is grounded and resistant to vibration.

Core 2: The “Logic” in Action – A Deep Dive into the Relays (MSUs)

This is the “longer” and most complex section of the FAW Fuse Box Logic Assembly. The relays are the electro-mechanical switches that perform the “logic” of the system. They are all plug-in MSUs. This assembly uses three distinct types, each with a specific function.

1. The Workhorse: ‘Four-pin relay assy (20A)’ (3711030-240)

The parts list calls for **eight (8)** of these. A 4-pin relay is a simple “on/off” switch. Its logic is to use a low-power circuit (from a dashboard switch) to control a high-power (20A) circuit. The low-power switch energizes a coil (Pins 85/86), which creates a magnetic field that “pulls” an internal switch, connecting the high-power-in (Pin 30) to the high-power-out (Pin 87).

The presence of eight of these in the FAW Fuse Box Logic Assembly indicates it is the central control for all major cab accessories: HVAC fan, wiper motor, power sockets, window lifters, auxiliary lighting, etc. Failure is common (stuck “on” or “off”) and they are designed to be swapped for easy diagnosis.

2. The Smart Switch: ‘Five-pin relay assy (20A)’ (3741070-240)

This box contains **two (2)** of these. A 5-pin relay is a “changeover” switch. It has two “out” positions: one “Normally Closed” (Pin 87a) and one “Normally Open” (Pin 87). When the relay is “off,” power flows from Pin 30 to Pin 87a. When the relay is energized, it “changes over,” breaking the 87a connection and making the 87 connection.

This “logic” is essential for functions that need to *switch between* two states. The most common example is the headlight circuit (switching from low beams [87a] to high beams [87]) or for systems that need to default to “on” and be turned “off” by a signal.

3. The Gatekeeper: ‘Main relay assy’ (3722110-671)

The assembly includes **two (2)** ‘Main relays’. These are the most critical switches in the entire FAW Fuse Box Logic Assembly. A main relay’s “logic” is to control power to *entire banks of fuses* at once. It is the gatekeeper.

One relay is typically triggered by the “Accessory” key position, sending power to comfort items like the radio. The second is triggered by the “Ignition” key position, powering all mission-critical systems (like the instrument cluster and ECU power). A failure of one of these two relays will cause a massive, system-wide outage (e.g., “half the cab is dead”), but is still a simple, plug-in MSU.

Core 3: The Protection Grid – Fuses & Service Tool (MSUs)

The most numerous MSUs in the FAW Fuse Box Logic Assembly are the blade-type fuses. Their “logic” is to be the sacrificial weak link, failing instantly in an overcurrent event to protect expensive harnesses and components from melting or fire.

The population of 56 fuses shows the complexity of this cab:

• 5A Blade-type fuse (3722011-50): 9 units. For very low-power circuits, such as ECU memory, interior LEDs, and the “control” side of the 12 relays.
• 10A Blade-type fuse (3722012-50): 32 units. This is the vast majority, protecting all standard cab circuits: individual lights, instrument cluster, telematics, radio, sensors, etc.
• 15A Blade-type fuse (3722013-50): 6 units. For medium-load “comfort” circuits, like cigarette lighters, 24V power sockets, and audio amplifiers.
• 20A Blade-type fuse (3722014-50): 7 units. These are paired with the 20A relays to protect high-load circuits like the HVAC fan, wiper motor, and power windows.
• 30A Blade-type fuse (3722017-50): 2 units. These protect the highest-load circuits in the cab, such as a main feed for an inverter or the main power for the entire window/door control module.

Finally, the ‘Puller clip’ (171937579) is a simple but critical MSU. Its “logic” is serviceability, providing the technician (or driver) the correct tool to safely remove and replace fuses without damaging the fuse or the box. Its inclusion in the assembly is a key design feature.

FAW Fuse Box Logic Assembly Parts List

The following table is the complete, 11-component-type breakdown for the FAW Fuse Box Logic Assembly (3722010-92W) for the FAW CA4251P66K24T1A3E5 tractor.

Technical Specifications and Assembly Context

The specifications for the FAW Fuse Box Logic Assembly are defined by its role as the central, high-density power distribution and switching unit for all cab-based electrical systems.