Think I have it sorted out.

I pulled out my pile of books and started looking through the RNG "Volvo Problem Solver; Advanced Edition" (a blue three ring binder that looks like it was sold by Robert Gerhardt of Bloomingdale IL in the early 90s)

The first step he recommended was disconnecting the harness from the AMM and trying to start the car. Book indicates "if car starts with AMM disconnected, the AMM is bad." I tried that and the car started fine and idled normally. As I was reading how to further diagnose the AMM/confirm it was bad I noted the precautions about not probing from the front end of the harness due to the fragility of the connectors. That led me to visually inspect the connectors and I noted that one of them was bent open and had a missing prong. I gently bent the three remaining prongs back into place, hit everything with DeOxit, re-connected the AMM and the car now is running and idling normally.

I was planning on a frustrating day of troubleshooting in the garage and now have my Sunday back!

Oh, and this was on a 90 240 DL with LH2.4 FI.

Thanks to all for the suggestions.