The operation of the stepper motor. The theory of this should be very similar for the MPi if not the same, though the MPi stepper is an air bypass valve and not a ‘throttle kicker’ as in the SPi.
The stepper motor consists of four windings. All the windings are commoned together at one end, this is the 12+ve supply, Brown/Pink wire. The other four ends of the windings connect to the ECU. The ECU then grounds these in a certain sequence to drive the motor in steps, each one of a predetermined angle of rotation. The max number of steps of the SPi motor is in the region of 180
When you turn off the engine, you should hear an audible clicking, this is the ECU driving the stepper fully closed. It drives it for 180 steps and a few more to make sure its closed, in doing this it knows where it is, zero steps, and can then count the steps open. After it has driven the stepper full closed and before the ECU switches off, It then opens the stepper to a point determined by the engine temp, ready for the next start. Once the engine is started it motors in relation to the engine temp and idle load of the engine. Idle speed is not directly controlled by the stepper.
I normally set the stepper up to show about 30 steps at warm idle with no load on the engine, IE no electrical devices on.
If one of the windings has not been working correctly, it misses a step in every sequence, but the ECU does not know this, so, say the ECU drives the stepper 40 steps from closed, what would happen in reality is the stepper would move 30 steps. The engine will not respond as it should so the ECU will drive it some more, resulting in the Service tool showing a high step count. the control will become unsteady.
I have found a couple of the steppers to have dry soldered joints on the small connection PCB on the motor terminals. It’s a simple fix, remove the stepper motor from its housing an re solder. Also worth checking that the winding resistances are all the same, if not, bin it and find another.
Corrosion on the plug terminals is also an other possible, as you have seen. Id check the solder joints just to be sure.