The purpose of this test is to evaluate the correct operation of the Idle Speed Control Valve (ISCV) based on the switching voltage and duty control in response to target idle speeds.
This method is illustrated in Figure 1.
The waveform shows the switched earth signal from the ECU. In this example the earth pin is switched to earth for about half the time: in other words, the duty cycle is about 50%. The ECU can vary the duty cycle in order to adjust the rotation angle of the valve. A greater duty cycle causes a wider opening of the valve.
This waveform is obtained from a 2-pin valve. Switching the control pin to earth pushes the valve open against a spring, and opening the circuit to the control pin allows the valve to return to its closed position. A 3-pin type of valve also exists, with two switched earth pins. Switching one pin to earth pushes the valve open, and switching the other pin to earth pushes it closed. Both earth pins of a 3-pin valve can be monitored at the same time using a dual-trace oscilloscope.
Probing the supply side of the valve will produce a straight line waveform at charging voltage.
Figure 2 - Rotary idle speed control valve
The function of the idle speed control valve (ISCV) is to control the engine's idle speed according to its temperature and different load conditions.
When the engine is started from cold, the engine management Electronic Control Unit (ECU) gives the engine cold start enrichment and increases the idle speed to about 1200 rpm. It is the ISCV that is responsible for this increase. As the engine reaches operating temperature, the enrichment is eliminated and the idle speed reduced to a predetermined level. This speed is maintained regardless of electrical loads on the alternator and to a certain extent mechanical loads, for example, when an automatic gearbox has drive selected.
The valve is controlled by intermittently switching the earth path to ground. It has a 12 volt supply and its switching can be seen in the example waveform. It may be possible to see a slight frequency shift as the valve opens to maintain the engine idle speed under high electrical demands.
The rotary ISCV is an electromechanical device that has a supply voltage either from the ECM or a control relay. It has 2 or 3 electrical connections: battery voltage and either a single or a double switched earth path. The duty cycle with which the earth path is switched is determined by the ECU to maintain a preprogrammed speed. The valve forms an air bypass around the throttle butterfly to provide a controlled air bleed within the induction tract, and is therefore susceptible to dirt and carbon-containing deposits impeding its performance. It is recommended that they are cleaned at the manufacturer's service intervals with a spray solvent to maintain their efficiency.
If the engine has an adjustable air bypass and an ISCV, it may require a specific routine to balance the two air paths.
AT033-4
Disclaimer
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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