W126 idle speed control unit

[merc380sel]

I check the idle control unit which was located under the dash board today because the car was still idling around 900rpm after I replaced the idle speed actuator.
To my surprise, I noticed that the control unit had not been connected all this time! The plug was dangling near the box but not connected.
I connect the cable back onto the control unit, but the idle speed hasn't changed a bit.
I was shock to realize that the car can still run with the unit not connected!
Did the previous owner by-pass the system . Is this possible?
The car is very heavy on gas and idle rough.
Please help me with some advise.
I appreciate if Steve can also help me in this matter.