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Here is the Wiring Diagram:

enter image description here

I am trying to understand how this servo motor calibration process (see screenshot of BMW software) is supposed to work at electrical level. In particular:

  1. there is 2-pin classification resistor attached to the servo - What does it do? Does it measure something? How to know if it is good?
  2. what are IMP, SM_A, SM_B, BR and DIR pins on the servo harness?
  3. is there a way for me to somehow test if servo motor is up to its specification by testing it outside car?

For more context, that is a transfer case servo motor (aka VTG actuator) that engages clutch pack in BMW X3 ATC400 transfer case that distributes torque between front and rear axles. Few months ago I replaced a completely worn off plastic gear (not visible in the video below) in the servo motor and was still able to calibrate it successfully. However, few days ago the car was shuddering at slow, sharp turns. So I decided to disassemble servomotor the second time to lube up the gear with grease. Now the calibration consistently fails.

I took the servo motor out of transfer box and I can see that the gear is able to spin with no load. So it is not like servo motor is completely dead. Few members on BMW forums suggested that they replaced the servo motor to solve the issue.

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    \$\begingroup\$ Maybe the grease was a bad idea? Maybe the low speed shudder was not caused by a faulty servomotor? DIR probably means direction. I don't know if it is an input or output. SM_A and SM_B appear to be the connections to the motor itself. Don't know about the rest. The classification resistor is probably just a resistor that tells the VTG actuator what kind of system it is connected to. Like different models have different configuration resistors. You could try measuring it with an Ohm meter (disconnect first). \$\endgroup\$ – mkeith Apr 11 '19 at 6:33
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    \$\begingroup\$ @mkeith According to repair instructions I had to apply the grease on plastic gear. However, yes, I have it on my todo list to remove grease to see if behavior changes. Yes, It could be that low speed shudder is not caused by servomotor. However, I had to replace the old, worn gear in it because it made the dashboard look like Christmas tree (4x4, ABS, Airbag lights on). Replacing it made things go back to normal. However, the shuddering started to happen after replacing it. But again shuddering may have been masked all the time because worn gear effectively made my car 2wd instead of 4wd. \$\endgroup\$ – user389238 Apr 11 '19 at 18:07

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