I know it sounds too easy but I have had one similar where the diagnosis was killing me. 4-Matic light, no codes...Turns out the brake light switch was open circuit and this was keeping the control unit in fits. Microprocessors understand connected to ground (binary 0) and connected to positive (binary 1) So open circuit doesn't fit into the program. I eventualyy found this by grabbing at straws after a complete hydraulic test. Now every time I have a difficult 4-matic I try a brake light switch just for the hell of it, though I have only had it be the fix for that one car. May just be worth a try in your situation, after all it is one of the cheapest components of the system