When VIS motors are operated in situ the load of the flap or butterflies reduces the speed of the motor and prevents overrun when the micro switch switches the motor off at the end of travel. If you operate the VIS motors off the car they run so fast that they reach the end of travel and operate the micro switch but they are spinning so fast the cam rotates a bit more than it should and switches the end of travel micro switch back on and the motor carries on spinning. Many years ago I repaired one and operated it off the car and the above happened. I remember having to strip the unit down again to rectify it but I can't remember what I did to rectify it. If I have to run them off the car now I apply some load to the output peg/square hole with my hand to prevent overrun.
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