SkampAZ

Hello everyone, long time listener first time caller. Maybe fellow owners can validate my thoughts on order of troubleshooting and conclusions thus far. I'm totally stumped. Any help from anyone else with experience on a "engine stalled/died, no crank no start, no communication from ECM and won't jump" problem please save my sanity and share your wisdom. Fabled and ultra rare bad ECU?? The concern: yesterday my 03 SL500's engine stalled/shut off while pulling up to a light, would not crank or start after. Would not come out of park, OBD port would not connect to scan tool to pull codes, but eventually today I found I can manually connect via the port to the SAMS, EIS & Body controllers etc. No codes except for low voltage. Key turns and all the lights come on, ABS/ESP alerts on the malfunction displays. All consumer electronics work. Key fob works central locking. The radiator fans come on full speed when I turn the key. What I've tried so far:-Could not jump with starter battery-If I understand correctly the Crank position sensor would not prevent cranking, only starting-Dont think the EIS is the issue since the key turns and everything works BUT no crank no start. No codes from EIS. -If the starter died that wouldn't have killed the car while driving or cause no communication with ECM.-Jump starting the starter battery did nothing, also charged both batteries and still no change. I don't think it's the alternator but I've heard these can be temperamental. -No blown regular fuses in either engine bay SAMs or the fuse block under the parcel shelf, none of the blade fuses are blown in either prefuse block. Checked all fuses and relays pertaining to Engine/body electrical and relays as well. None were bad. - Pulled alarm module and fuse, no change.- No corrosion in ECM connectors or the ESP/ABS module connectors either.- Pulled TCM connectors at TCM and cleaned them, no corrosion or anything. Was wet with wicked fluid from previously bad TCM plug at transmission, but never had problems and did it proactively. What's really confusing me is why the OBD port stopped connecting in conjunction with this issue. I've never encountered that before on other cars with issues other than ECU or grounding, which is making me think maybe my ECU just flat out died while driving. OR I'm just too inexperienced and under equipped to tackle a failed SAM and don't know what the give aways are for one.

I will likely ship it to the local Indy MB specialist here in PHX as it is way too dang hot to be doing this stuck in a driveway and I'm at wits end with this car lol Thanks for reading and replying if you have any thoughts 🙏