Electronic ISC

[sas95]

I've got a '95 Metro 1.0L, which has an all-electronic Idle Speed Control (ISC) system. It's not engaging with the throttle closed. The ISC plunger does not extend out to touch the magnet on the throttle lever (w/throttle closed). There's about a 3mm gap. Does anyone have experience with these? Is the ISC motor still usable?

I've read some other threads in this forum mentioning idle speed control, but they involve vacuum lines, which I think are for earlier years.

I am not getting a DTC for ISC, but realized there was a problem with a Tech 1 scanner hooked up. The 'IDLE SWITCH' value was always OFF, even with the throttle closed (throttle lever at stop and 1.5 degrees from TP sensor).

The magnet is on a screw, which looks like it can be extended out to reach the Hall IC. I'm still struggling to loosen the wierd stop nut holding the screw. Think it's a torx-type pattern. Really diifficult location to reach.

My manual has the following
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NOTICE: The ISC motor and throttle lever screw are preset at the factory. The ISC motor must not be removed from the TBI unit or disassembled otherwise damage to the ISC motor and/or the throttle valve may occur. If the ISC motor needs to be replaced, refer to "Throttle Body Fuel Injection (TBI) Unit Replacement".
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I want to avoid replacing the entire throttle body if I can help it.

[sas95]

Update: have played with the magnet-on-a-screw, and the plunger now extends to touch it. So, the electromechanical part of this ISC thing works fine with ignition on, as long as the engine IS NOT running. As soon as I start up the car, and when the ISC is really needed, it stops working! Maybe vibration? Voltage/power drop?

Continuing to investigate ...

[sas95]

OK, think I've experimented with this thing long enough. Even though the repair manual says the IDLE SWITCH parameter should be ON during what it describes as specific idle conditions - idle/upper radiator hose hot/closed throttle/park/neutral/closed loop/accessories off, the IDLE SWITCH is OFF in these conditions on my car. Only when I turn on the A/C will the IDLE SWITCH parameter go ON. The ISC motor then extends its plunger to contact the magnet screw, and moves the throttle up from 1.5 degrees to 2.9 degrees (todays conditions) to keep the rpm around 890. With A/C turned off, the ISC probe retracks away from the magnet. I notice that the ECM is controlling rpm in this situation, but probably through other means, possibly injection pulse width, timing. After I turn off the A/C, the engine rpm goes from 890 up to 1300, then down to 1100. All this without me touching the gas pedal.

Think I'm doing to leave the whole thing alone.

The reason why I've been diving into this is the manual says that when diagnosing a DTC,

1st - do an On-Board Diagnotic system check using a Tech I scanner, so that you can trust the ECM,

2nd - verify the 26 parameters coming from the ECM with a Tech I scanner are within expected bounds,

3rd - follow the fault isolation chart for the specific DTC you've got

If any of the 26 parameters are not within bounds, you're supposed to keep this in mind when in getting to the bottom of your DTCs.