[oab_au]

Jim thinking about it, the ECU is the only thing that can turn the engine off, without posting a code.

So looking at the only connection between the two, the P and N signals, that are sent from the gear box selector switch to the ECU. It is used there so it can alter the conditions to suit the idle, and mixture position.
When they are selected on the shifter, the ECU inputs are grounded (pulled low). When neither of them is low, then the car is in gear, ready to go.

Where I see a connection to this, that there is a fault in the wiring that is raising the voltage on these lines, above the ground that it should see. So when you added the mod relay, that added resistance, between the source and ground, raised the voltage of the signal, to signal a high, when it was a low.

When neither of the two inputs, N P, to the ECU are low (connected to ground), it knows that you have just put the car in drive, so it raises the idle speed to handle the converter load. Your quote that “it raises the revs before it stops”.

If you can find the spot where the power is shorting to the starter, or the wire from the CC to the starter connection, wherever it is connected in.

In the mean time, if you can find a relay, that can handle, at least 30 amps and has a winding drawing about 1 amp, that may pull that CC line closer to ground and stop the problem.

Harvey.