Forgot to mention that idling is all the time normal if I disconnect the speed sensor plug at the wheel. There is only no speed reading, no tacho reading and no automatic switch off from the turn signal. The rest is all OK. This is to make the headaches complete.
So by way of a bit of a summary and having re-read the Protune manual:
The IACV is driven by the ECU and if it's working normally without the speed sensor connected then that seems to rule out IACV itself being the problem.
The Speedo and turn signal cancelling works when the speed sensor is plugged in so the wiring between these would appear OK.
The ECU controls the IACV and at start-up it starts with a high idle and as the engine warms up it moves to a target idle speed set in the ECU. This idle is user set in the range of 900 - 1,100 rpm and as I recall is normally about 950 for our cars.
As you move off the idle speed is raised to improve drivability and it will return to the target value when you come to a halt. This is normal and your "good" car should do this if you coast to a halt in neutral.
There is an option for the ECU to use the engine temperature input to dynamically adjust the target idle speed, but I don't think this is used in our cars.
From the S&S manual - The vehicle speed sensor is routed first to the speedometer and then the speedometer creates a signal that is sent to the ECU.
So if the speedo is working OK it could be the problem is in the wire(s) from the speedo to the ECU. I would suggest unplugging the speedo and ECU and then doing a continuity test on the connecting wires.
