How the MK AWD System Really works.

[Gramps]

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:

Slipping on ice while backing up will send a lot of power to the rear axle.

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.

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.

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 gerotor 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.

[unibus]

Good information Gramps, Sounds so good maybe I should have gotten the 4 Wheel drive version.... Oh well.

Thanks for the post.

[07compasssport]

I read this to the wife since she drives the jeep the most. Her response was "as long as it works and does it's job I dont care to understand how it does what it does"

[CharBroiled]

heh guess that explains why I cant light the front tires off when they are on dirt and the rears are on tar.. it sees my WOT and transfers the power to the rears

[Dr. Grinch]

An aftermarket bias controller would be dope.
I wonder what the maximum balance is? 50/50?

[mdls4]

I drove on the beach here in Sth East Qld for the first time yesterday in the Compass. Was an experience i've never had in a car before!! Good for the most part, the rest just nerves when thinking I might get stuck!

I did take some pics, mostly of the family and the spot of fishing we did. Oh, and one hell of a sunburn!

Back to the topic... It was interesting changing between the three different modes of ESP. To be honest I couldn't tell much of a difference on the straight runs, the car seemed to want to drift left and right a lot but always corrected itself. It managed to pull out of some deep holes with soft sand which was very reassuring.

[CharBroiled]

that explains a lot.. I always wondered why I hardly ever got torque steer when WOT in its normal FWD mode. it automatically transfers most power to the rear before the power even gets there. nice.

the patriot handles very well at speed considering what it is, probably in large part because of its brains. I gotta watch out, its getting to be a little too much fun to drive, heh.

[ricku]

Gramps:
Very informative description. Thank you !!
So.....my '08 Patriot with FDII off road package is basically AWD all the time.
When I pull the 4WD lever, it's basically in 4WD LOCK for off road only and not for street use. Does that pretty much sound right for my Pat ??
Thanks again.

Rick

[Gramps]

Yes, both systems are an ON DEMAND AWD. The major difference between the FD I and FD II is your drive ratio that gives you your 19:1 crawl ratio. You can use the Lock for ANY adverse driving conditions to get you going, including in the snow on the street, however it's not recommended that you use it for normal driving conditions due to the extra wear on the coupling and tires and it's just not necessary.

The 4WD Lock with ESP off and in Off-Road mode is JUST for off-road, the other features are the same, very streetable.

[indianrefining]

Originally Posted by mdls4

I drove on the beach here in Sth East Qld for the first time yesterday in the Compass. Was an experience i've never had in a car before!! Good for the most part, the rest just nerves when thinking I might get stuck!

I did take some pics, mostly of the family and the spot of fishing we did. Oh, and one hell of a sunburn!

Back to the topic... It was interesting changing between the three different modes of ESP. To be honest I couldn't tell much of a difference on the straight runs, the car seemed to want to drift left and right a lot but always corrected itself. It managed to pull out of some deep holes with soft sand which was very reassuring.

Yeah, rub it in, mate! A "toasty" 8 degrees (F) here, this morning!