First off, I am nowhere near knowing much about the FI control system. Reading the pictorial diagram in the Volvo Service Manual WDM for 1987-88, which probably covers the 1986 system, may help a little.

Here's my guess:

1. Check the innards of the 25 amp fuse holder. Have a replacement holder at hand, as the 21-year-old plastic will most likely crumble to a bunch of chips.
The insides get full of corrosion and crud, so you have have enough to get a little power through, but not enough for the complete load.

2. Check the main fuel injection relay. If it does OK while starting, but then drops out from loosing its ground, that would create the mess you have.

3. Did you already swap in a known good ECU?

4. The four wires from the grey connector are the usual suspects, they are a separate harness set. But - inside that main wiring harness are a gang of other wires, and while the insulation may not be deliberately environmentally friendly, over 21 years of hot-cold cycles it can go bad, too.

The main wiring harness is a huge long thing. Starts from the plug into the ECU, there's a small branch to the FI relay, then there's a piece that goes to the back side of the grey connector. Down below the I-manifold smaller harnesses come out of it. One to the IAC, one to the TPS, to the Hall Sensor, to the coil, to the Knock Sensor, and - I think - a long tail over to the Ignition Control Module.

Any of them can get screwed up and cross-connected.

I wish you luck, these challenges can be terribly frustrating.

Bob

:>)