FAW Atmospheric Temperature Sensor Logic Assembly: Why These 4 Critical Components are Essential for Engine Performance

Welcome to this critical technical guide on the FAW Atmospheric Temperature Sensor Logic Assembly (Assembly 553). This is the complete, high-precision environmental sensing system for the FAW Jiefang CA4251P66K24T1A3E5 6×4 diesel semi-trailer tractor. This assembly is a primary “logic” input for the truck’s main Engine Control Unit (ECU).

The FAW Atmospheric Temperature Sensor Logic Assembly is a set of components with one vital “logic” function: to precisely measure the temperature of the *ambient air* outside the truck, before it ever enters the engine’s intake or turbocharger. This data is critical for calculating air density.

This in-depth guide is an essential resource for professional diesel technicians and parts specialists. We will deconstruct the entire FAW Atmospheric Temperature Sensor Logic Assembly into its 4 serviceable components: the ‘Temperature sensor assembly’ (3602155-60D-C00), the ‘Atmospheric temperature sensor bracket assy’ (3602520-90R), and the critical, vibration-proof fasteners that hold it in place. A failure in this FAW Atmospheric Temperature Sensor Logic Assembly can lead to poor performance and failed emissions.

Core Component: The ‘Temperature Sensor Assembly’ (3602155-60D-C00)

The heart of the FAW Atmospheric Temperature Sensor Logic Assembly is the ‘Temperature sensor assembly’ (3602155-60D-C00). This is the electronic “nerve ending” of the system. It is a high-precision thermistor.

A thermistor is a “thermal resistor.” Its “logic” is that its electrical resistance changes in a precise, predictable way based on its temperature.

• **Cold Air:** Resistance is high.
• **Hot Air:** Resistance is low.

The truck’s ECU sends a low-voltage signal (e.g., 5 volts) to this sensor and reads the “return” voltage. By measuring this change in resistance, the ECU can calculate the *exact* temperature of the ambient air with an accuracy of one or two degrees. This single piece of data is one of the most critical inputs for the entire engine management system. A faulty sensor in the FAW Atmospheric Temperature Sensor Logic Assembly will feed incorrect data to the ECU, leading to a cascade of performance problems.

The “Logic” of Air Density: Why This Sensor is Critical

The FAW Atmospheric Temperature Sensor Logic Assembly does not just tell the truck the “weather.” Its “logic” is to determine **air density**. This is the core principle of engine tuning.

A diesel engine needs a precise **air-to-fuel ratio** to run efficiently, create power, and meet emissions standards. The ECU calculates this ratio by *mass* (how many kilograms of air vs. how many kilograms of fuel). However, the engine’s “Mass Air Flow” (MAF) sensor can be fooled.

**Cold Air is Dense:** When the truck is operating at -20°C, the air is very dense. Each cubic meter of air contains a large *mass* of oxygen.

**Hot Air is Thin:** When the truck is operating at +40°C, the air is very thin. Each cubic meter of air contains a much smaller *mass* of oxygen.

The FAW Atmospheric Temperature Sensor Logic Assembly provides the “correction factor.” The ECU takes the airflow reading from the MAF sensor and then “corrects” it using the temperature data from this sensor. This gives the ECU the true “air density,” allowing it to make critical “logic” decisions for:

• **Fuel Injection:** If the air is cold and dense, the ECU will inject *more* fuel to match the available oxygen, creating maximum power. If the air is hot and thin, it will inject *less* fuel to prevent over-fueling, which causes black smoke, high emissions, and wasted fuel.
• **Turbocharger Control:** The ECU uses the air density reading from the FAW Atmospheric Temperature Sensor Logic Assembly to calculate how hard the turbocharger must work to achieve its “target” boost pressure (MAP).
• **Emissions Control:** The Selective Catalytic Reduction (SCR) system’s efficiency is temperature-dependent. The ECU uses this ambient temperature data as a baseline for its complex models that control DEF (urea) injection and DPF regeneration.
• **Cold Start Logic:** On a cold start, the ECU uses this sensor’s reading to determine the “cold start” strategy, such as glow plug duration and cold-idle RPM.

A failed FAW Atmospheric Temperature Sensor Logic Assembly will cause the ECU to “default” to a “limp mode” or a pre-set value (e.g., 20°C). If the truck is actually at -30°C, the ECU will be under-fueling, resulting in a severe lack of power. If the truck is at +40°C, the ECU will be over-fueling, causing black smoke and high exhaust gas temperatures (EGTs).

Core Component 2: The ‘Bracket Assembly’ (3602520-90R)

The ‘Atmospheric temperature sensor bracket assy’ (3602520-90R) is the “logic” of *placement*. This bracket is just as critical as the sensor itself.

The FAW Atmospheric Temperature Sensor Logic Assembly must measure the temperature of the “ambient” air, not the air inside the engine bay. The engine bay can be 80°C or hotter than the outside air.

This bracket’s “logic” is to mount the ‘Temperature sensor assembly’ (3602155-60D-C00) in a “neutral” location, such as:

• Behind the front grille, but *in front* of the radiator and intercooler.
• Inside the wing mirror housing.
• On the chassis rail, away from the hot exhaust.

This bracket is designed to hold the sensor securely, isolating it from the “heat soak” of the engine block and radiator. If this bracket is broken and a technician “zip-ties” the sensor to the radiator support, the FAW Atmospheric Temperature Sensor Logic Assembly will fail. The sensor will read the *radiator’s* temperature (e.g., 90°C) instead of the *outside* air temperature (e.g., 10°C). The ECU will then “think” the air is hot and thin, and will cut fuel, resulting in a severe “low-power” complaint. This simple bracket is essential to the function of the entire FAW Atmospheric Temperature Sensor Logic Assembly.

Core Component 3: The Fastener System

The FAW Atmospheric Temperature Sensor Logic Assembly must be secured against the constant, high-frequency vibration of a diesel engine. The parts list calls for two specific, vibration-proof fasteners:

**’Hex flange bolt, coarse rod (Standard Type)’ (Q1841025) – 1 unit:** This is the bolt that secures the ‘Atmospheric temperature sensor bracket assy’ to the truck’s chassis. The “flange” head is an integrated washer, which is a “logic” choice to prevent the bolt from vibrating loose. It spreads the clamping force over a wide area and “bites” into the bracket.

**’Hex flange nut’ (CQ39610) – 1 unit:** This is the nut that pairs with the bolt. It is also a “flange” nut, providing a wide, secure base. This is likely a **lock nut** (e.g., a “Nyloc” or “Stover” nut) with a prevailing-torque design. This “logic” is critical, as it grips the bolt’s threads and *cannot* back off from vibration. A standard nut would fail, allowing the entire FAW Atmospheric Temperature Sensor Logic Assembly to fall off or into the engine fan.

FAW Atmospheric Temperature Sensor Logic Assembly Parts List

The following table provides the complete, detailed breakdown of all 4 components identified in the ‘Atmospheric Temperature Sensor Logic Assembly’ (553) diagram for the FAW CA4251P66K24T1A3E5 tractor.

Technical Specifications and Assembly Context

The specifications for the FAW Atmospheric Temperature Sensor Logic Assembly are defined by its function as a critical environmental input for the engine’s ECU. The FAW Atmospheric Temperature Sensor Logic Assembly is essential for performance and emissions control.