I finally discovered the cure to my idle control problem on the 86 740 Turbo. I had inadvertently swapped the harness connectors on the throttle switch and the idle air control valve. They both have a Bosch 3-terminal connector, which I failed to label prior to removing during the dreaded wiring harness re-build. LABEL EVERYTHING BEFORE YOU REMOVE IT ALL!!

Moral of the story - this will FRY your ECU's idle control circuit (at least it will on the 86 740 Turbo). The car still ran, but no idle control. I was able to repair the ECU by opening it up, applying a jumper wire across the burnt circuit and also replacing a bad transistor that switches the idle valve on and off.

Saved me big $$ by opening the ECU and checking for obvious signs of damage (burnt tracks on the circuit board), but I wouldn't have known the transistor was bad unless my Electrical Engineer mate had checked it.

It wasn't fun to repair, but the car runs fine now!

Interestingly, the Volvo green manual (for B230 FT engine troubleshooting) suggests you check to see if the wire connectors are swapped, but it doesn't mention that if you start the car with the wires swapped, it will fry the ECU!