In our last issue of Keep Those Trannys Rolling, we looked at the Lexus “Dynamic Torque Control AWD system and what made it tick. In this issue, we will look at what it takes to diagnose the Lexus “Dynamic Torque Control AWD System. As in our last issue, we will continue with the 2020 Lexus RX350 equipped with a 3.5l engine and a U881F transmission. Now, let’s reacquaint ourselves with Lexus’ Dynamic Torque Control AWD System basics.
This is similar to many other AWD systems you’ve already worked on. The essence of the AWD system is:
- A single-speed transfer case differential (Figure 1): It’s attached to the transaxle and delivers power to the rear differential.
- An Electro-Mechanec Coupler, EMC (Figure 2): It’s part of the read differential and controls the torque to the rear wheels. It can fully lock when driving on a slippery surface or connect at any percentage based on the road surface or while turning a corner.
- An AWD ECU (Figure 3).
Before getting too involved in a diagnostic, check the simple things first. Inspect all four tires and make sure they’re all the same size, have the same (or similar amount of wear), and are from the same manufacturer. While manufacturers have the same size tires (based on the tire’s specifications), they are not guaranteed to be exactly the same. It could be that the customer had tire damage and replaced one tire while it still has three worn-out tires. Having a spare tire falls into the same category; it could trigger an AWD ECU malfunction. Alert the customer if it has any of these conditions, and don’t go further than a code check for your diagnostics. Even low tire pressure can result in an AWD ECU malfunction, so pay attention to the tires first.
The BCU will set a C1337 (Tire Diameter is Not Uniform) if it detects a problem with tire size, but we’ve all seen computer problems where a condition is not enough to set a code but bad enough to cause a problem.
Now, let’s look at diagnostic codes specific to the AWD ECU. Note that codes from other systems (like a C1297 “Steering Angle Sensor”) will affect the AWD ECU. Check and repair all other systems codes before diagnosing codes specific to the AWD ECU.
The codes from the AWD ECU are pretty basic. Refer to the 4WD ECU connector terminals pin locations (Figure 4) and wire colors (Figure 5) for the following tests. Note: The chart only includes terminals and wire colors you’ll need for AWD ECU testing.
C120C- Linear Solenoid Power Supply Malfunction:
- This indicates low voltage across the linear solenoid power and ground wires (pins 1 & 2). This is generally an internal ECU problem but can be caused by loose pin connectors or damaged solenoid wires.
Inspect the pins and solenoid wires for damage. If they check okay, then measure the ECU supply voltage and ground across pins 3 and 4. Expect the same voltage as you would across the main battery terminals (A difference of 0.1 volts is acceptable). If there is a significant difference, then do a voltage drop test on the ground wire. It’s likely that a bad ground or low battery voltage is the cause of this code. A fully charged battery should measure 12.66V (KOEO). A low battery or one that’s unable to hold a full charge can also be the problem, even though no other battery-related issues exist. If the battery voltage is okay, then it indicates a failed AWD ECU.
C1241- Low/high Power Supply Voltage:
- This is similar to C120C (above) but is specific to the ECU input supply. Perform the same test as C120C.
C1298- Linear Solenoid Circuit:
- This is similar to C120C (above). Perform the same test as C120C. If it tests okay, then measure the resistance of the linear solenoid. The resistance specification for the linear solenois is 2Ω. If the linear solenoid is within specifications, then replace the AWD ECU.
C1299- Cancellation of 4WD Control:
- Indicates that the 4WD ECU has detected a malfunction that will interfere with the 4WD ECU operation. It’s recommended to check the 4WD ECU for “Reason for Cancellation.” This code is usually set with a companion module code. Repairing the companion module code and clearing the 4WD ECU should correct this concern.
Well, there you have it. We took another look at Lexus’ Dynamic Torque Control AWD System. We discussed the two sub-systems and the 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 bit of knowledge of Lexus’ Dynamic Torque Control AWD System, you should have no problem Keeping Those AWD Trannys Rolling down the road.
