Doing some additional reading online and will characterize the circuits...nothing novel yet, but laying the groundwork to start fleshing out the system logic.
- IMP = Sensor output (hall sensor sensing rotation of the motor shaft...need to figure out pulses per motor rev/gear output rev/full sweep of clutch from 0 - 100%)
- VCC = Sensor reference voltage...
- TEMP = Temperature sensor in motor to limit control system aggression (per
this xDrive link)
- DIR = ??? One source says it's "transfer case motor voltage", but it's a small gauge wire so I assume this is some feedback from motor when powered.
- SM_A = Motor Power 12V/PWM high side
- SM_B = Motor Ground (Brown wire is also pretty well universally ground on any BMW I've touched, and I think circuit "31" in the control unit is ground)
So here are a few open questions I'll be addressing via experimentation and documenting along the way:
- Is the transfer case motor power supply (SM_A) truly PWM drive like a two wire throttle controller, or is 12V DC sufficient enough to drive it to a position? Will driving it at 12V regularly cause the actuator to overheat (hence the temp sensor)?
- Is the servo motor controlled by an H-bridge (where SM_B can also go to 12V and SM_A to ground) to reverse motion on the motor?
- At least by hand I am not able to back drive the motor gearbox...but the wiring diagram showing SM_B being a part of the "31" circuit and very clearly SM_A going to circuit "30", it looks like it is only polarized in one direction...so does the transfer case clutch mechanism have enough mechanical advantage to back drive the center diff back to "open"? If yes, this implies the TC actuator would need a steady voltage applied to keep the diff locked, which maybe makes more sense why there is a temp sensor in the actuator, but seems inefficient.
- I've seen threads saying simply disconnecting the actuator results in 100% RWD/unlocking of the transfer case (among a bunch of other trouble lights)...is this because the car energizes the TC actuator to unlock when shutting down and disconnecting it while off keeps it open or is it because the actuator is allowed to be back driven...let's say we disconnect it immediately after 12V indexed it to 100% lock?
- I might need to get an o-scope, but will attempt to measure current of the actuator under various conditions...will need to figure this out to adequately design the current sense circuitry for the PIC.
I'll keep this thread up to date as things go along!!