What Sedans Are All Wheel Drive

Alright folks, let's dive into the world of All-Wheel Drive (AWD) sedans. Understanding how AWD systems work is crucial for any serious car enthusiast, whether you're planning on tackling some maintenance yourself, upgrading components, or just want to impress your buddies with your knowledge. This article focuses specifically on AWD systems found in sedans, which often differ in architecture and behavior compared to trucks or SUVs. We'll cover the key components, how they interact, basic troubleshooting, and, most importantly, safety considerations. Consider this your go-to guide for understanding the guts of your AWD sedan.

Purpose: Understanding the Heart of Your AWD Sedan

Why bother understanding this stuff? Simple. Knowledge is power. Imagine facing an unexpected repair bill, knowing you could have diagnosed and fixed the problem yourself. Or perhaps you’re considering upgrading your drivetrain for better performance. Without a solid understanding of the system, you’re essentially throwing money into a black box. This knowledge allows you to:

Diagnose Issues: Recognize symptoms of common AWD problems.
Perform Maintenance: Understand what components need regular servicing.
Choose Upgrades Wisely: Select the right aftermarket parts for your specific needs.
Make Informed Decisions: Decide whether to attempt a repair yourself or consult a professional.

This article will help you do just that.

Key Specs and Main Parts of an AWD Sedan System

Let's break down the core components of a typical AWD sedan system. Keep in mind that AWD systems vary from manufacturer to manufacturer, and even between models within the same brand. However, the fundamental principles remain the same.

Main Components:

Engine: The power source for the entire system. While technically not part of the AWD *system* itself, it's where everything starts.
Transmission: This is the gearbox that converts the engine's output into usable torque and speed. Crucially, in an AWD vehicle, it's designed to output power to both the front and rear axles.
Transfer Case (or Power Transfer Unit - PTU): In many AWD sedans, particularly those with transverse-mounted engines (engine oriented side to side), a transfer case or PTU is used to split power between the front and rear axles. This unit typically contains gears and clutches to achieve the power split. More modern systems may use electronically controlled clutches for variable torque distribution.
Front Differential: The differential allows the front wheels to rotate at different speeds when cornering. It's a set of gears housed within a casing, connected to the front axles.
Rear Differential: Similar to the front differential, but located on the rear axle. It allows the rear wheels to rotate at different speeds.
Driveshaft (or Propeller Shaft): This is a rotating shaft that transmits power from the transfer case/transmission to the rear differential. It's a long, sturdy component designed to withstand high rotational speeds and torque.
Axles: These are the shafts that connect the differentials to the wheels, transmitting power to make them turn.
Coupling (Center Differential or Viscous Coupling): Many AWD systems use a center differential or a viscous coupling to further manage torque distribution between the front and rear axles. A viscous coupling uses a fluid that becomes more viscous (resistant to flow) when subjected to shear force, allowing for a variable power split. More advanced systems use electronically controlled multi-plate clutches.
Electronic Control Unit (ECU): Modern AWD systems are often electronically controlled. The ECU monitors various sensors (wheel speed, throttle position, steering angle, etc.) and adjusts the torque distribution based on the driving conditions.
Sensors: Wheel speed sensors, throttle position sensor, steering angle sensor, yaw sensor. These provide crucial data to the ECU for optimal AWD performance.

Key Specs:

Torque Split: This refers to the percentage of engine torque sent to the front and rear axles. Common splits are 50/50 (equal distribution), or biased towards either the front or rear. Many modern systems have variable torque splits, adjusted dynamically by the ECU.
Response Time: The speed at which the AWD system can react to changing conditions and adjust the torque split. Faster response times lead to better traction and stability.
AWD System Type: Different AWD systems have different characteristics. Full-time AWD provides power to all wheels constantly. Part-time AWD only engages the rear wheels when needed. Reactive AWD engages the rear wheels based on sensor data and driving conditions.

Symbols: Understanding the Technical Diagram

Technical diagrams are essential for understanding how these components interact. Here's a breakdown of common symbols you might encounter:

Solid Lines: Typically represent mechanical connections, such as shafts, axles, or linkages.
Dashed Lines: Often indicate electrical connections, vacuum lines, or hydraulic lines.
Boxes: Usually represent components like the ECU, differentials, or transfer case.
Circles: May represent gears, bearings, or other rotating components.
Arrows: Indicate the direction of power flow or fluid flow.
Colors: Different colors may be used to differentiate between different systems or components. For example, blue might represent hydraulic fluid lines, while red represents electrical wiring.
Icons: Standard symbols representing specific components, such as a battery, a motor, or a sensor. You'll find a legend on the diagram itself, so familiarize yourself with it.

A good diagram will also include labels identifying each component, and potentially arrows indicating the direction of rotation for shafts and gears. Take some time to familiarize yourself with the specific diagram for your vehicle.

How It Works: From Engine to Wheels

Let's trace the path of power from the engine to the wheels. We'll assume a transverse-mounted engine setup, as is common in many AWD sedans.

Engine Output: The engine generates power, which is delivered to the transmission.
Transmission Gearing: The transmission selects the appropriate gear ratio based on driver input and vehicle speed, converting engine output into usable torque and speed.
Power Transfer to PTU: Power from the transmission is directed to the PTU (Power Transfer Unit).
Torque Split: The PTU splits the torque between the front and rear axles. In a reactive system, this split may be variable, based on sensor data. A front-biased system might send 80% of the power to the front wheels and 20% to the rear in normal driving conditions.
Driveshaft Engagement: The driveshaft transmits power from the PTU to the rear differential.
Differential Action: The front and rear differentials allow the wheels on each axle to rotate at different speeds, which is essential for cornering.
Axle Power Delivery: The axles transmit power from the differentials to the wheels, causing them to turn.
ECU Control (if applicable): The ECU constantly monitors sensor data and adjusts the torque split through the coupling (center differential or viscous coupling) as needed to optimize traction and stability. For example, if the front wheels start to slip, the ECU might send more power to the rear wheels.

Understanding this flow of power is crucial for diagnosing issues. For instance, a broken driveshaft will obviously prevent power from reaching the rear wheels, resulting in a loss of AWD functionality.

Real-World Use: Basic Troubleshooting Tips

Here are some common AWD problems and basic troubleshooting steps:

Problem: Loss of AWD function (front wheels spinning, rear wheels not engaged).
- Possible Cause: Faulty transfer case, damaged driveshaft, failed rear differential, or a problem with the ECU or sensors.
- Troubleshooting: Check for visible damage to the driveshaft. Listen for unusual noises coming from the transfer case or differentials. Use an OBD-II scanner to check for error codes related to the AWD system. Check fuses related to the AWD system.
Problem: Unusual noises (grinding, whining, clunking) from the drivetrain.
- Possible Cause: Worn bearings in the transfer case or differentials, low fluid levels, or damaged gears.
- Troubleshooting: Check the fluid levels in the transfer case and differentials. Inspect for leaks. Have a mechanic inspect the components for wear or damage.
Problem: AWD system engaging erratically or not engaging at all.
- Possible Cause: Faulty sensors, malfunctioning ECU, or a problem with the viscous coupling or multi-plate clutch.
- Troubleshooting: Use an OBD-II scanner to check for error codes. Check the wiring and connections to the sensors. Have a mechanic diagnose the ECU and coupling.

Always consult your vehicle's repair manual for specific troubleshooting procedures.

Safety: Handle with Care

Working on AWD systems involves dealing with heavy components, high torque, and potentially hazardous fluids. Here are some crucial safety precautions:

Secure the Vehicle: Always use jack stands when working under a vehicle. Never rely solely on a jack.
Disconnect the Battery: Disconnect the negative battery terminal to prevent electrical shocks or accidental activation of components.
Wear Safety Glasses: Protect your eyes from flying debris and fluids.
Use Proper Tools: Use the correct tools for the job to avoid damaging components or injuring yourself.
Handle Fluids Carefully: Wear gloves when handling fluids like gear oil. Dispose of used fluids properly.
High-Torque Components: Be extremely careful when working on components with high torque specifications (e.g., driveshaft bolts). Use a torque wrench to ensure proper tightening.
Never Work Alone: Have a helper present in case of an emergency.

AWD systems contain components under significant stress. Improper disassembly or assembly can lead to serious injury or vehicle damage. If you are not comfortable working on these systems, consult a qualified mechanic.