[QUOTE=Frank Reiner;3244561]Unplug the cable from the throttle switch, and jumper the pins/sockets that correspond to switch pins #1 & 2. This will simulate a closed throttle switch of 0 ohms. Any difference in the idle, or in response to changes to the idle mixture?[QUOTE]

Did you make this test? What, if anything, happened?

I return to this because the duty cycle remains fixed at 20/80 (polarity sensitive), which indicates that there is a fault in the throttle switch or the wiring connecting the switch to the ECU and the Idle Speed Control Unit.

The improvement in idle characteristics that is experienced with the EHA disconnected indicates that the EHA is out of calibration. When both the EHA and the mixture screw are correctly adjusted, there will be almost no change in the idle when the EHA is unplugged.