In this issue of Keep Those Trannys Rolling, we will look at Lexus’ all-wheel drive (AWD) system. For this example, we will look at the 2020 Lexus RX350F, equipped with a 3.5L engine and a U881F transmission. Lexus uses an all-wheel drive (AWD) system called the “Dynamic Torque Control AWD System.” Now, let’s take a look at Lexus’ Dynamic Torque Control AWD System.
DYNAMIC TORQUE CONTROL AWD SYSTEM
The Dynamic Torque Control AWD System uses sensors, switches, and solenoids from various modules throughout the vehicle to control the AWD system operation. The Dynamic Torque Control AWD System comprises two sub-systems: a mechanical sub-system and an electronic control sub-system. The first sub-system (mechanical/ muscle) includes the U881F transmission, a one-speed transfer case, a driveline, an electro-magnetically controlled coupler (located in the rear differential). The second sub-system (electronically controlled/brains) includes the 4WD ECU assembly, the Engine Control Module (ECM), the Vehicle Skid/ABS Control Module (VSC/ABS), the Body Control Module (BCM), the AWD Lock Switch and the Instrument Combination Meter (ICM) assembly.
SUB-SYSTEM ONE (MECHANICAL)
Sub-System One (the mechanical sub-system) of the Dynamic Torque Control AWD System is the “muscle” of the AWD system and is pretty straightforward. This sub-system consists of:
- A one-speed transfer case (Figure 1) (attached to the U881F automatic transmission)
- A driveline
- An electromagnetic coupler attached to the front of the rear differential (Figure 2)
- A rear differential assembly.
A CLOSER LOOK AT SUB-SYSTEM ONE
As previously discussed, sub-system one of the Dynamic Torque Control AWD System is pretty straightforward. This sub-system uses the transfer case and the electromagnetic coupling subassembly (located in the rear differential) to transfer torque to the rear differential. Now, let’s take a closer look at each of these components.
- Transfer case: Transmits torque to the rear differential by way of the driveline
- Electromagnetic coupler: varies the amount of torque delivered to the rear differential
- Rear Differential: transmits torque from the transfer case and electromagnetic coupler to rear wheels
SUB-SYSTEM TWO (ELECTRONICALLY CONTROLLED)
Sub-System Two (the electronically controlled sub-system) of the Dynamic Torque Control AWD System is the “brains” of the AWD system and is quite complex. This sub-system consists of:
- 4WD ECU assembly (Figure 3)
- ECM (engine control module)
- VSC/ABS (vehicle skid/ABS control module)
- BCM (body control module)
- ICM (instrument combination meter assembly)
- AWD Lock switch (combo switch)
- Wiring, Connections, and Data Lines
A CLOSER LOOK AT SUB-SYSTEM TWO
As previously discussed, sub-system two (electronically controlled) of the Dynamic Torque Control AWD System is quite complex. This sub-system uses the 4WD ECU (brains of the AWD system) to collect data from various modules throughout the vehicle and monitor and control the AWD system. Now, let’s take a look at each of these components.
- 4WD ECU: controls the AWD system by varying the current to the 4WD linear solenoid based on data obtained from the ECM, VSC/ABS, BCM, ICM, and AWD Lock Switch
- ECM (engine control module): crankshaft position sensor, throttle position sensor, park/ neutral position sensor
- VSC/ABS (vehicle skid control/ anti-lock brake control module): wheel speed sensors, stop light switch (BOO), steering angle sensor, acceleration sensor #1(airbag), acceleration sensor #2 (yaw rate)
- BCM (body control module): parking brake switch
- ICM (instrument combination meter): AWD warning, AWD Lock indicator lamp
- AWD Lock Switch (combo switch assembly): switches between “AUTO” mode and “LOCK” mode
DYNAMIC TORQUE CONTROL AWD SYSTEM OPERATION
As previously discussed, the Dynamic Torque Control AWD System uses data from various modules throughout the vehicle to control the AWD system operation. These components allow optimum torque transfer to the rear wheels during AWD operation. The 4WD ECU controls the AWD system by varying the current to the 4WD linear solenoid (located in the electromagnetic coupler assembly) based on data obtained from the ECM, VSC/ABS, BCM, ICM, and AWD Lock Switch. The 4WD ECU uses seven (7) different programs to control the AWD system. These programs are:
- Starting Control: controls the torque to rear wheels when accelerating from a stop.
- Normal Control: controls the amount of torque to rear wheels to maintain stability when accelerating while cornering, reduces torque to rear wheels to improve fuel consumption.
- Control when Locked: controls the amount of torque to the rear wheels to allow for maximum traction and performance
- Cornering Control: uses pre-torque control programming based on driver input, yaw rate feedback, and vehicle status to achieve maximum tracking while cornering.
- Braking Control: cancels AWD control to improve braking effectiveness
- VSC Cooperative Control: achieves smoother acceleration and stability while cornering on slippery surfaces
- Safety Control: disables AWD operation and transfers all torque to front wheels. Turns on the AWD indicator lamp and sets the codes for the 4WD ECU.
DIAGNOSING THE DYNAMIC TORQUE CONTROL AWD SYSTEM
The Dynamic Torque Control AWD System is equipped with an AWD malfunction indicator lamp in the instrument combination module (ICM). If the 4WD ECU detects a malfunction in any of its companion modules that will interfere with AWD operation, the 4WD ECU will disable the AWD system and illuminate the AWD malfunction indicator lamp. Diagnosing the Dynamic Torque Control AWD System starts by connecting your scan tool to the Data Link 3 connector (DLC3) in the left lower dash area. When checking for codes, it would be recommended to scan each module individually instead of doing a complete vehicle scan. Complete vehicle scans DO NOT always pick up everything. Before accessing the 4WD ECU with your scan tool, it would be recommended to check for codes in the engine control module (ECM), vehicle stability/antilock brake control module (VSC/ABS), the body control module (BCM) and instrument combination meter (ICM). It is recommended that all codes in the companion modules be documented and cleared before accessing the 4WD ECU. Once you’ve documented and cleared codes in each module, you can access the 4WD ECU and check for codes. Here is a list of codes you may find in the 4WD ECU.
- C120C- Linear Solenoid Power Supply Malfunction
- C1241- Low/High Power Supply Voltage
- C1280- Engine Circuit Malfunction
- C1296- ABS Malfunction
- C1297- Steering Angle Sensor
- C1298- Linear Solenoid Circuit
- C1299- Cancellation of 4WD Control
- C1337- Diameter of Tire is not Uniform
- U0073- Control Module Communication Buss Off
- U0100- Lost Communication with ECM/PCM “A”
- U0126- Lost Communication with Steering Angle Sensor Module
- U0129- Lost Communication with Brake System Module
As you can see from the 4WD ECU code list, only four (4) codes (high-lighted) are related to the AWD system. Dynamic Torque Control AWD System diagnostic routines include checking 4WD ECU powers, grounds, data lines (Figures 4, 5 & 6), and the 4WD linear solenoid. All other codes are related to companion module concerns and must be addressed through the companion module. It is recommended that you refer to your specific vehicle repair information for specific details.
Well, there you have it. We took a look at Lexus’ Dynamic Torque Control AWD System. We discussed the two sub-systems and their individual components that allow this AWD system to maintain AWD stability, traction, and performance. We discussed the diagnostic routines needed to keep this AWD system on the road. With a little knowledge of Lexus’ Dynamic Torque Control AWD System, you should have no problem Keeping Those AWD Trannys Rolling down the road!
