FAW Sub-Box Left Upper Centre Shaft Assembly: A Comprehensive Analysis of Its 4 Core Components

Welcome to this comprehensive technical analysis of the FAW Sub-Box Left Upper Centre Shaft Assembly. This is a critical set of components within the auxiliary “sub-box” (or range box) of the transmission for the FAW Jiefang CA4251P66K24T1A3E5 6×4 diesel semi-trailer tractor.

The FAW Sub-Box Left Upper Centre Shaft Assembly is the “counter-shaft” for the transmission’s auxiliary section. Its “logic” is to handle the immense torque required for the “high” and “low” range split, effectively doubling the truck’s available gears. This assembly, which includes the shaft, bearings, and locating washers, is essential for the transmission’s durability.

This in-depth guide is a vital resource for professional transmission technicians and parts managers. We will deconstruct the entire FAW Sub-Box Left Upper Centre Shaft Assembly into its 4 main serviceable part numbers, explaining the critical function of the ‘Welded countershaft assembly’ (1701515FA0L), its tapered roller bearings, and its precision locating shims.

Core Component: Welded Countershaft (1701515FA0L)

The backbone of the FAW Sub-Box Left Upper Centre Shaft Assembly is the ‘Welded countershaft assembly-sub box’ (1701515FA0L). The parts list specifies a quantity of two, which is a critical detail. This indicates the transmission’s auxiliary box uses a **twin-countershaft design** (or “twin-shaft”) for its range selection.

This design splits the torque coming from the main box, sending 50% through this “Left Upper” FAW Sub-Box Left Upper Centre Shaft Assembly and 50% through its (identical) twin. This dramatically increases the torque-handling capacity of the gearbox, allowing for a more compact and durable design.

The term “Welded assembly” signifies that the gears (e.g., the high/low range drive gears) are permanently attached to the shaft, likely via friction welding. This creates a single, incredibly strong component that is not serviceable in individual pieces. The entire FAW Sub-Box Left Upper Centre Shaft Assembly must be replaced as a unit if a gear tooth is damaged.

The Bearing System: Tapered Roller Bearings (33109-1 & 33109-2)

Each FAW Sub-Box Left Upper Centre Shaft Assembly is supported at both ends by a ‘Tapered roller bearing’. The parts list breaks this down into its two serviceable components:

• ‘Outer ring-tapered roller bearing’ (33109-1) – 4 units
• ‘Inner ring-tapered roller bearing’ (33109-2) – 4 units

The quantities (4 of each) perfectly equip the two ‘Welded countershaft’ (1701515FA0L) units with a front and rear bearing set. Tapered roller bearings are essential for this application. Unlike standard ball bearings, they are designed to handle extreme loads in *two* directions simultaneously:

**1. Radial Load:** The immense separating force of the helical-cut gears pushing away from each other.

**2. Axial Load (Thrust):** The “push/pull” force that is naturally generated by the angled teeth of the helical gears. A ball bearing would be destroyed by this thrust, but a tapered bearing is designed for it. This is a critical piece of logic in the FAW Sub-Box Left Upper Centre Shaft Assembly.

The ‘Outer ring’ (or “cup”) is pressed into the transmission’s rear housing. The ‘Inner ring’ (or “cone,” which includes the rollers) is pressed onto the journal of the FAW Sub-Box Left Upper Centre Shaft Assembly.

Precision Logic: The Locating Washer (1701518-A0L) and Preload

The final component listed is the ‘Locating washer’ (1701518-A0L), with a quantity of two (one for each FAW Sub-Box Left Upper Centre Shaft Assembly). This is not a simple washer; it is a **precision shim**.

The most critical aspect of installing tapered roller bearings is setting the “preload.” This is the precise amount of “squeeze” or axial pressure applied to the bearings.

• **Too Loose:** If the preload is too loose, the FAW Sub-Box Left Upper Centre Shaft Assembly will have “end-play,” allowing it to move back and forth. This causes the helical gears to misalign, leading to a loud “whine,” high-frequency vibration, and eventual gear-tooth failure.
• **Too Tight:** If the preload is too tight (over-shimmed), the bearings will have no running clearance. This creates immense friction and heat, which will seize, “blue,” and destroy the new bearings in a very short amount of time.

This ‘Locating washer’ is a “selective fit” shim. During a rebuild, a technician uses a dial indicator to measure the end-play and then selects a washer of the *exact* thickness required (from a set of various-thickness shims, all under this part number “family”) to achieve the perfect, factory-specified preload. This shimming process is the “logic” in the FAW Sub-Box Left Upper Centre Shaft Assembly and is the most important step for a successful transmission repair.

Service and Diagnostics

A failure within the FAW Sub-Box Left Upper Centre Shaft Assembly is a major event. The primary cause of failure is almost always lubrication-related. If the transmission oil level is low, or if the oil is contaminated with water or debris, the bearings (33109-1, 33109-2) will be the first components to fail.

The main diagnostic symptom of a failing FAW Sub-Box Left Upper Centre Shaft Assembly is a loud, pronounced **whining or growling noise** that is directly related to vehicle speed, not engine speed. The noise will be present in *all* gears (both high and low range) and will change pitch as the truck accelerates or decelerates. This is because this shaft is “downstream” of the main gearbox and is always spinning in relation to road speed.

A secondary symptom is the presence of a large amount of “glitter” or fine metal shavings in the sub-box’s oil during a service. This is the hardened surface of the bearing or gear (1701515FA0L) “pitting” and “spalling” (flaking off) due to overload or lack of lubrication. Servicing the FAW Sub-Box Left Upper Centre Shaft Assembly requires a complete teardown of the auxiliary box, replacement of all bearings and shims, and a careful re-setting of the bearing preload using the ‘Locating washer’ (1701518-A0L).

FAW Sub-Box Left Upper Centre Shaft Assembly Parts List

The following table provides the complete, detailed breakdown of all 4 component types identified in the ‘Sub-Box Left Upper Centre Shaft Assembly’ diagram for the FAW CA4251P66K24T1A3E5 tractor. This FAW Sub-Box Left Upper Centre Shaft Assembly is a complete set for two countershafts.

Technical Specifications and Assembly Context

The specifications for the FAW Sub-Box Left Upper Centre Shaft Assembly are defined by its function as a high-torque, twin-countershaft system. The entire FAW Sub-Box Left Upper Centre Shaft Assembly set must be installed with precision preload.