ICV testing wiring question

[redcoupe86]

So this is for an 86 turbo, but I think this applies across the range in principle.

I just replaced all my vacuum hoses, the venturi (I'm not doing the delete), and a new ICV. I've double-checked all my connections and the routing. I've also done a smoke test on the system and there are no leaks. I also have a rebuilt throttle body a year or so ago and that was working fine up till this point. the TPS is also working fine based on the tests in Clarks.

I have an issue trying to set idle. Currently, it's running about 1100 RPM after start and after idling from cold for 5-10 minutes.

I set the stop screw on for the throttle plate, but when I plug in the jumpers across B/C in the test port nothing happens. The RPM remains constant - no change. Turning the idle mixture screw all the down doesn't change anything, the engine will continue at the same high RPM.

I'm wondering if the ICV might not be getting the signals it's supposed to get. So the question is how do you test the wiring and signal going to the ICV at the plug? What should I be looking for on a multimeter?

Assuming there are no vacuum leaks what else should I be looking for if not the wiring?

[944 Ecology]

Here is an explanation of the function of the three wire ICV (which this document calls an ISCV). It is from a BMW motronic discussion for a similar motronic system.

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ISCV (3 wire type)
The ISCV is a solenoid controlled actuator that the Motronic ECU uses to automatically control idle speed during normal idle and during engine warm-up. The ISCV is located in a hose that connects the inlet manifold to the air filter side of the throttle plate. Unlike the two pin ISCV which is controlled by a separate Idle speed ECU, the 3 pin ISCV is controlled by the Motronic ECU via ECU pins 33 and 34.

The ISCV is a DC motor that the ECU can rotate either clockwise or anti-clockwise. Rotating in one direction will open the valve and rotating in the opposite direction will cause it to close. A voltage supply is applied to the ISCV from the battery and the earth for the motor is made through two connections to the ECU.

Rotation of the motor in the appropriate direction is accomplished by actuating the motor through one or the other of the earth circuits. In reality the two circuits are opposed. This prevents the valve from being fully opened of closed in one particular direction. The valve will thus take up an average position that reflects circuit bias to be open or closed. Normally, this bias would be towards the open position.

A duty cycle can be measured on each earth circuit to determine the opening or closing time period as a percentage of the total time available. When an electrical load, such as headlights or heater fan etc are switched on, the idle speed would tend to drop. The idle ECU will sense the load and rotate the ISCV to increase the air flow through the valve and thus increase the idle speed. When the load is removed, the ECU will pulse the valve so that the air flow is reduced. Normal idle speed should be maintained under all cold and hot operating conditions. If the ISCV fails it will fail in a fail-safe position with the aperture almost closed. This will provide a basic idle speed.

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A multimeter would be almost useless to check the signal at the ICV. A scope would be necessary. Three channels would show the signals and ground from the ECU.