The purpose of this test is to evaluate the correct operation of the stepper motor based on the switching voltage and frequency present at each individual return path in response to target idle speed.
Plug a BNC test lead into Channel A on the PicoScope, place a black clip on the test lead with the black moulding (negative) and a Back-pinning Probe onto the test lead with the coloured moulding (positive). Place the black clip onto the battery negative terminal and probe the stepper motors individual earth paths in turn with the Back-pinning Probe as illustrated in Figure 1. If you cannot reach the terminal or plug with a probe, then you may be able to use a breakout lead or box if you have one available.
There are several wiring options for a stepper (see technical information). The operational conditions for the stepper motor will be dependent largely upon the engine's temperature and the electrical loads placed upon the engine.
The vehicle's ECM will activate the individual earth paths to move the motor in small increments, this in turn adjusts the idle speed to the ECM's required specifications.
With the example waveform displayed on the screen you can now hit the space bar to start looking at live readings.
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.
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:
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:
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.
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.
A stepper motor mounted on the throttle housing
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.
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