Comment by kortex
2 years ago
That definitely sounds like a loose connection, not an ECU problem. Usually it's a bad clamp on the battery terminal. Often it's the ground clamp in particular.
The easiest diagnosis is to rotate the cables on the terminal several times to rub off oxide build up, then leave them in an orientation so that the natural tension of the cable forces the clamp into good contact.
A "dead" (low voltage) battery will still cause some indicators/lights to come on when you crank, while a bad connection usually acts like zero volts.
I was about to say the same thing. My car (nowhere near as nice as a Merc) had the same issue. There would be oxide deposit on the contact points and the minuscule amount of rust and crack on the clamp would render it useless on a bumpy road randomly (1 in 10 times approx). It was scary the first few times but when I figured out the symptoms, it took me less than a minute to get it to start.