How To Turn On All Wheel Drive

So, you're looking to understand how to engage all-wheel drive (AWD) in your vehicle? Whether you're tackling a tricky repair, considering a modification, or just want to understand your vehicle better, knowing the ins and outs of your AWD system is crucial. This article will break down the process, covering the key components, operation, and basic troubleshooting. We'll focus on the practical aspects, assuming you have some existing mechanical knowledge and are comfortable working on vehicles.

Why This Matters: Understanding Your AWD System

Understanding how to engage your vehicle's AWD system isn't just about flipping a switch. It's about understanding the entire drivetrain, how power flows, and what can go wrong. This knowledge is invaluable for:

Troubleshooting: Diagnosing AWD-related issues like binding, slipping, or failure to engage.
Maintenance: Performing necessary maintenance to keep your system running smoothly, such as fluid changes in the transfer case and differentials.
Modifications: Making informed decisions about modifications like lift kits, tire sizes, or even swapping in different differentials.
Preventative Care: Understanding operating limitations of the system to prevent damage by avoiding conditions like extreme heat or traction lock-up.

Key Specs and Main Parts

The specific parts involved in engaging AWD depend heavily on the type of system your vehicle uses. However, some common components include:

Engine: The source of power for the entire drivetrain. We're assuming a traditional internal combustion engine (ICE) here.
Transmission: Transmits engine power and torque to the drivetrain. Can be manual or automatic.
Transfer Case: This is a critical component in many AWD systems. The transfer case splits power between the front and rear axles. Some systems have clutches or viscous couplings within the transfer case to vary the torque split dynamically.
Front and Rear Differentials: Differentials allow the wheels on each axle to rotate at different speeds, which is necessary for turning. They also receive the power split from the transfer case (or directly from the transmission in some systems).
Axles and Driveshafts: These components transmit power from the differentials to the wheels. A driveshaft connects the transfer case to the front and rear differentials. Axles connect the differentials to the wheels.
Actuators: These are the mechanisms that physically engage or disengage the AWD system. They can be electric motors, vacuum-operated devices, or even mechanically linked levers.
Control Module (TCU/ECU): Many modern AWD systems are electronically controlled. The TCU (Transfer Case Control Unit) or the ECU (Engine Control Unit) monitors various sensors (wheel speed, throttle position, etc.) and commands the actuators to engage or disengage AWD as needed.
Sensors: These provide data to the control module. Common sensors include wheel speed sensors, throttle position sensors, and steering angle sensors.

Understanding System Engagement Mechanisms

The method of engaging AWD varies. Here are the most common types:

Part-Time AWD: Typically found in older trucks and SUVs. Requires manual engagement, usually via a lever or a button. In this system, the front and rear axles are locked together when engaged, meaning it should only be used in low-traction conditions (snow, mud, etc.) to prevent driveline bind on pavement.
Full-Time AWD: Continuously distributes power to all four wheels. Often uses a center differential or a viscous coupling to allow for different wheel speeds, making it suitable for on-road and off-road use.
Automatic AWD: Engages AWD automatically when the system detects wheel slip. Often uses electronically controlled clutches or viscous couplings to vary the torque split.

How It Works: The Engagement Sequence

Let's walk through a simplified example of how an automatic AWD system might work:

The Driver Initiates: The driver typically selects an "AWD Auto" or similar mode via a button or dial.
Sensor Input: Wheel speed sensors detect that one or more wheels are spinning faster than others, indicating a loss of traction.
Control Module Decision: The TCU/ECU receives the wheel speed data and determines that AWD engagement is necessary.
Actuator Activation: The TCU/ECU sends a signal to an actuator, often an electric motor, that engages a clutch pack within the transfer case.
Torque Transfer: Engaging the clutch pack mechanically connects the front and rear driveshafts, transferring a portion of the engine's torque to the front axle. The amount of torque transferred can vary depending on the system's design and the severity of the wheel slip.
Traction Restoration: With power now being sent to all four wheels, traction is improved, and wheel slip is reduced.
Continuous Monitoring: The system continuously monitors wheel speeds and other parameters, adjusting the torque split as needed to maintain optimal traction.

Real-World Use: Basic Troubleshooting

Here are a few common AWD troubleshooting scenarios and potential solutions:

AWD Won't Engage: Check the fuse for the AWD system. Inspect the actuator for damage or malfunction. If it's an electronically controlled system, scan for diagnostic trouble codes (DTCs) using an OBD-II scanner.
AWD Binding or Clunking: This is often a sign of a problem with the transfer case or differentials. It could be due to low fluid levels, contaminated fluid, or internal damage. Have the system inspected by a qualified mechanic.
AWD Light Stays On: This indicates a fault within the system. Scan for DTCs to identify the source of the problem.

Important Note: Always consult your vehicle's owner's manual for specific instructions on how to engage and disengage your AWD system. Incorrect operation can damage the system.

Safety Considerations

Working on an AWD system involves working with several potentially dangerous components:

Driveshafts: Rotating driveshafts can cause serious injury if you get clothing or body parts caught in them. Always disconnect the battery before working on the drivetrain.
High-Voltage Components: Modern AWD systems often use electric motors and control modules that operate at high voltages. Exercise caution when working with these components. Disconnect the battery and allow time for capacitors to discharge before handling wiring.
Hydraulic Systems: Some AWD systems use hydraulic actuators. Be aware of the potential for fluid leaks and high-pressure sprays. Wear appropriate safety glasses and gloves.
Lifting the Vehicle: Always use jack stands when working under a vehicle. Never rely solely on a jack. Ensure the vehicle is properly supported before performing any work.

Final Thoughts

Understanding your vehicle's AWD system empowers you to maintain it properly, troubleshoot issues effectively, and make informed decisions about modifications. Remember to consult your vehicle's service manual for specific procedures and torque specifications. While this article provides a general overview, the specifics will vary depending on the make and model of your vehicle.

Now you have a better understanding of the inner workings of AWD systems and can tackle repairs with confidence.

We have a detailed schematic diagram available for download, offering a visual aid to complement this article. Please contact us if you would like a copy of the AWD schematic diagram.