Compass 4WD L4-2.4L VIN W (2007)
Control Module: Description and Operation
Operation
OPERATION
The all-wheel-drive system requires no driver input or control. Under most driving conditions, it is passive and power is transmitted to the front wheels
alone. Unlike all-wheel drive systems that rely on pumps or viscous fluids to transfer torque, this system requires no front-to-rear slippage for activation.
This allows the system to transfer torque solely in response to accelerator pedal position. If the driver is asking for a lot of power, the system
immediately starts clamping the electronically controlled coupling (ECC), transferring a high percentage of power to the rear wheels. This avoids front
wheel slippage, as power to propel the car is transmitted through all four tires. This mode of operation is called open-loop operation in that there is no
feedback to affect the torque transfer.
A second, closed loop, operating mode uses feedback from the wheel-speed sensors to determine the appropriate torque transfer. When the front wheels
slip, the All Wheel Drive (AWD) Control Module tells the ECC to start clamping, sending power to the rear wheels. Attempting the same aggressive
launch described above with the front wheels on ice and the rear wheels on dry pavement, the ECC sends even more torque to the rear wheels to
minimize slippage and launch the vehicle. Both modes are always active with the closed loop mode layered on top of open loop mode to increase torque
to the rear wheels when needed to maintain traction in extreme cases.
Power to the rear wheels is modulated under the following conditions:
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Slipping on ice while backing up will send a lot of power to the rear axle
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Loss of traction while traveling at freeway speeds, for example hydroplaning on a puddle of water, will send very little power to the rear wheels
because the controller knows at those speeds a lot of power is not needed at the rear wheels
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A third condition, which is independent of the others, uses wheel speed differences to determine when the vehicle is turning in a tight circle. This
condition, which is indicated by a large discrepancy in side-to-side wheel speeds, causes the electronic control module to reduce torque to the rear
wheels to prevent binding in the driveline. The electronic control module is always checking for this condition as well.
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A fourth condition that is unique to this system is to influence vehicle dynamics. Other systems limit AWD to aiding traction or providing off-road
capability. The primary focus is on launching the vehicle or going off road at speeds up to about 25 mph (40 km/hr). Above that speed range,
they use it to limit wheel slip for traction. On this system, additional ECM calibration controls torque to the rear wheels for improved handling in
the 25-65 mph (40-105 km/hr) range. In this speed range, the system increases torque to the rear wheels during cornering with the throttle open to
make the car turn more easily - make the handling more neutral. This is more readily accomplished with an electronically controlled system, than
with viscous-coupling or generator systems that require some degree of front-to-rear slip to transfer torque to the rear wheels. Above 70 mph (113
km/hr), the control strategy provides minimal torque to the rear wheels under normal driving conditions to aid fuel economy.
The control module also interfaces with the Electronic Stability Program (ESP) and traction control systems. The interface allows the ESP system to use
the ECC to help gain control of the vehicle. For this purpose, torque transmitted to the rear wheels by the ECC can be reduced. This system is not
traction control. It only works on situations where front-to-rear traction varies, for instance, front wheels on ice, rear wheels on dry pavement or climbing
steep grades. AWD does not aid side-to-side traction. ESP does that through brake intervention on this system.
