Stepper motors

The purpose of this test is to investigate throttle stepper motor positioning signals at varying engine idle speeds.

How to perform the test

View connection guidance notes.

1. Use manufacturer data to identify the switched earth terminals at the stepper motor multi-plug.
2. Start the engine and allow it to idle.
3. Minimize the help page. You will see that PicoScope has displayed an example waveform and is preset to capture your waveform.
4. Start the scope to see live data.
5. Connect PicoScope Channel A. by probing each of the switched earth circuits in turn.
6. With your waveforms on screen stop the scope.
7. Stop the engine and turn the ignition off.
8. Use the Waveform Buffer, Zoom and Measurements tools to examine your waveform.

Note

Dependent on motor type there may be up to four signal circuits.

The engine must be below normal operating temperature to capture the cold idle signal.

Example waveform

Waveform notes

The stepper or stepper motor is a small electromechanical device that allows either an air by-pass circuit or a throttle opening to alter in position depending on the amounts that the stepper is indexed.

Invariably it will be used to control the idle speed when an idle speed control valve is not employed. The stepper may control an 'air bypass' circuit by having 4 or 5 connections back to the Electronic Control Module (ECM). The earth's enable the control unit to move the motor in a series of 'steps' and the contacts are earthed to ground via the ECM.

The stepper motor may also be attached to the throttle housing. A small control rod will move onto the throttle lever and adjust the butterfly opening in very precise increments.

The individual earth paths can be checked using the oscilloscope. The waveforms should be similar on each path. Variations may be seen between different systems.

Waveform Library

Go to the drop-down menu bar at the lower left corner of the Waveform Library window and select, Idle speed stepper motor voltage.

Further guidance

The stepper or stepper motor is a small electro-mechanical device that allows either an air by-pass circuit or a throttle opening to alter in position depending on the amounts that the stepper is indexed. Invariably it will be used to control the idle speed when an idle speed control valve is not employed.

Two popular types are described below:

4-wire stepper motor

With the 4-wire stepper motor, the first circuit being a set of contacts is known as the idle tracking switch. The other circuit is controlled by the ECM but only when the idle switch is closed. The second circuit maintains the idle speed when the engine is either hot or cold.

When the engine is cold the idle speed is increased to overcome the cold engine characteristics. When the throttle is released the stepper motor slowly returns the speed back to idle to avoid the engine from stalling.

The four terminals are as follows:

Pin 1 is the idle switch return and will be open circuit at idle.
Pin 2 is the idle switch signal and again, open at idle.
Pin 3 is the stepper motor signal positive at five volts.
Pin 4 is the stepper motor signal negative.

When testing there should be 4 to 6 ohms resistance between pins 3 & 4. Between pins 1 & 2 there should be infinity with the throttle closed, and closed circuit when the throttle is open.

5-wire stepper motor

The stepper may control an 'air bypass' circuit by having a 12 volt supply and a succession of 4 earth paths, as shown in Figure 2. These earths enable the control unit to move the motor in a series of 'steps' and the contacts are earthed to ground via the ECM.

The stepper motor may also be attached to the throttle housing, a small control rod will move onto the throttle lever and adjust the butterfly opening in very precise increments.

When used in either of these two examples, it will maintain the idle speed and avoid the engine from slowing regardless of electrical or mechanical loads. This component is also responsible for the increased idle speed when the engine is cold.

GT040-EN