These stepper motors are both Unipolar motors, they can be identified by having five or six wires. Rover stepper motors have five wires but are actualy six wire motors.

There are two types of stepper motor commonly used in automotive applications, Bipolar and Unipolar. Bipolar motors can be identified by having four wires or four connections. Unipolar can be identified by having five or six wires or connections.

The construction of the unipolar motor winding can be considered in two ways. First is, there are two seperate windings, each with its own center tap, there by, three connections per winding (six wires). The other way to look at it is, there are four individual windings, these windings are paired in series (two pairs of windings) again to give three connections per pair (six wires again)

The center tap connections in unipolar configuration are the common (+ve) and the the remaining four connections are the grounds. Rover connects the two common conections together to give a five wire motor. This, as said, is the same for the SPi Stepper or the MPi Valve.

The construction of the Bipolar motor winding is much simpler than the Unipolar. There are two seperate windings two connections each (four wires)

To convert the SPi motor from unipolar to bipolar to work with the stepper driver chips likely to be found in aftermarket ECUs is simple. Remove the motor from its housing in the throttle body, you will find a small PCB connection board on the motor. Two rows of three connections with the middle pair being the common and only one wire attatched. Simply remove this single wire and then with a needle file, score out the track on the PCB that joins the two rows on this 'common' connection. You now have a bipolar stepper motor. Refit it into the throttle body and set the max steps to extend in the ECU to 190. I'll get round to documenting the step sequence soon

The MPi stepper valve is a little more difficult to modify simply because of the way it has been assembled. There is an easier way to achieve a bipolar stepper valve and for its simplicity, I will discus this now. If anyone wishs to modify the unipolar Rover MPi valve, I can discuss this with them at that time. The easiest and simplest method to achieve a bipolar stepper valve for the MPi Mini requires the purchase of a Magneti Marrelli stepper motor for a Peugot. This simply fits the bottom half of the Rover MPi valve housing, two screws and jobs done.

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