The idle control valve is pulsewidth modulated. That means the battery voltage circuit is maintained by the controller pulsing the ground side of the valve. If you look at it on a scope (a graphical representation of voltage versus time) you will see a square wave pulse on off on off on off on off etc. The frequency will be fixed and the on off duty cycle will vary. The duty cycle refers to the amount of time the circuit is off versus on during the period of the cycle.

When the time on is greater the valve will close farther. It is basically infinitely adjustable within a range.

To analyse your problem the best way would be to view the system with a scope. A lot can be determined with a voltmeter. A 12v signal with a 50% duty cycle will look like 6v. The voltage you see across the terminals should be less than a 70% duty cycle as the system only goes to 70% either way 70% on is the slowest and 70% off is the fastest. If you get a big change in engine speed and you have no change in voltage you have a valve problem. If your voltage jumps around the controller is the likely culprit.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician