Interesting read. A few comments-

The front diff is always open, so the front wheel torque split can be anything between 0/100% or 100/0% left/right. If one wheel is on ice, then it will get all of the torque and the other wheel will be stationary. The hallmark of any open differential.

The rear differential is 0-100% locking depending on the position of the lever arm that is controlled by the transverse PDAS lock (slave cylinder). With no hydraulic pressure, it is an open diff (0% locking). With full hydraulic pressure the lever arm fully extended by the slave cylinder, the split is 50/50%.

The front/rear center diff split is variable from 31/69% to a maximum of 50/50. The minimum torque to the front wheels is 31% when the longitudinal PDAS lock is in its rest position. The fully actuated position of the longitudinal lever arm moves the front torque to 50%.

When the PDAS switch is engaged by momentarily twisting the rotary switch (sand dune / snow drift / mountain climbing mode), the rear diff is locked (full hydraulic pressure) to provide equal torque to both rear-left and rear-right wheels, 50/50%. Also, the center diff is locked, full hydraulic pressure, to provide 50% total torque to the front wheels.

When the PDAS switch is held for 10+ seconds (until the chime and central informer warnings come up), the PDAS locks will stay in their rest positions and will not receive any hydraulic pressure actuation. In the rear, that means an open rear diff. For the front/rear center diff, that means always a 31/69% split. The ABS still functions normally.

I've been running autocross, pretty successfully, with the PDAS switched off for the past two years. I have no problems with the open rear diff spinning an inside tire when powering out of a turn (the usual open rear diff issue) as long as I squeeze and don't stomp the throttle. In that situation, the 31% torque to the front wheels is enough to fully accelerate with no inside rear wheel spin. And with an open rear diff there is no differential induced understeer. The perfect awd advantage in my opinion. In the wet, the tires slip more easily, but there is also load transfer in the turns, so the issue of inside rear wheel slip is still mitigated by the pull of the front wheels.

With the PDAS/ABS fuse pulled, the rear diff remains open, the front/rear split remains at 31/69, and the ABS does not engage. Also, the speedo doesn't work...

The C4 lightweight diff ***** are very simple. They are attached to master cylinders. Turning the **** moves the master cylinder piston, which pushes hydraulic fluid and moves the PDAS slave cylinder piston, which moves the PDAS lever arm. One manual **** for the rear diff, one **** for the center diff. The diffs locking lever arms can be manual moved to any position that the driver wants. For street driving, anything between fully open and fully locked is likely a very fast method to burn up the diff friction disks!

The accelerometers are still a mystery to me. They have the ability to provide absolute 2d g-forces to the PDAS controller. The driving dynamics of 0.xx g's in the snow compared to 0.xx g's on a dry surface are completely different. So who knows what the PDAS controller is doing with the accel info. Maybe someday the Bosch vault will open so that we can look at the code spec.

thanks for confirming my suspicion, editing my previous note.

I can definitely confirm that ABS continues to work after the PDAS is disengaged via holding the rotary switch and it won’t incur any mechanical issues. To clarify, momentarily rotating the switch, which locks the rear and center diffs, is only for low speed driving. Holding the switch until the alarms sound, which turns off PDAS engagement, is fine for any speed (driver ability being the limitation!).

More function info-

Occasionally when I start the car the pdas & abs warning lights stay on along with a warning chime. Usually coinciding with wet weather, and I recently noticed a break in the wire insulation on my front left wheel speed sensor. Ignoring the cause, this happened once at this last Saturday’s autocross in the rain and I was able to test out the result. Not only was the pdas in its passive mode (static 31F/69R split), but there was also no abs intervention. The hydraulic pump must have been working fine as the brake pedal feel was consistent and didn’t stiffen up.

Additionally, the heavy old C4 cabrio took fastest time of the day (ftd) for the morning session in the wet! That’s up against 2x gt3, 2x gt4, race prepped entries, national scca drivers/champions/etc. While everyone was complaining of their rear end getting loose as soon as they stepped on the gas, I was able to squeeze on throttle and accelerate through the course confidently. I have no illusions that there were many better drivers out there and that the awd is what made me the hero.

Of course, once the sun came out and the pavement dried off, I was back to my more typical position about 2 seconds behind the usual gt cars and national drivers. I’m hoping to narrow that gap next season by upgrading the car into the race class and more driver training.