I do not have a stock ecm for my bike I converted it to EFI from carb using all new harley parts except for the T Max I borrowed a buddy's stock ecm and tried it it does the same thing. I know what you are thinking that his ecm wont crank it because of the difference of the TSSM but I used a stock ecm from a 03 SERK when I was doing the swap and they will activate the guages and cycle the fuel pump and even crank the bike for 2 to 3 seconds until the ecm finds out its not programmed to this TSSM I am going to plug my T Max up on my buddys bike and see if it will run his bike maybe this will eliminate a problem with my T Max
I'm probably way off here as this comes from a single bike and incident several years ago. But I thought I remembered the following from a bike with an ECM and TSSM that weren't mated (though there's not a specific mating on the Marelli bikes, just the Delphi bikes).
Guy I knew had a Road King whose TSSM took a dump. He got a used TSSM. Pretty sure I remember the bike still starting and the signals and other lights still working. Just no alarm function nor ability to programs fobs to the alarm.
Don't take this as gospel. Was a long time ago and only one weekend afternoon he was telling me about it. But that's the recollection.