XF Tom's "intermittant no crank" resolution...
 

Thanks to the forum I homed in on the relay module as the source of my problem. I tried the suggested resoldering solution to no avail. Car sat on roadside for three days while I took relay module home for analysis. Found intermittant to be the coil of the engine (center) relay. Replaced relay with a completely different type; a four-pole double-throw in a plastic box. Wired all four poles in parallel (land also wired all four normally-open contacts in parallel) to maximize the current handeling capacity. Perfect fit in the available space where defective relay was removed. Works like a champ! Here are some tips: Use an ordinary 9volt battery (like in your smole detector) and a couple of clip leads to test the relay coils. The relays work fine on 9 volts. When you test the engine relay coil, both it and the relay to the left of it should close. Don't assume if you hear a relay click when you go to the "start" position of the key switch that the engine relay is closing! The one to the left of it closes too and you will hear it click. This puzzled me and threw me "off track" for awhie. If you decide to buy a new module, go to Mercedespartscenter.com. The part number is 170 545 03 05 and costs $156.00

2008 coupe, stock (except for relay module), 11980 miles; bought new from Chrysler in 2009.

Re: XF Tom's "intermittant no crank" resolution...
 

Like I have said before, as these cars age, we will see more and more of this.

There is a thread around here somewhere showing a suitable, solder-in replacement relay.


Anyway, good for your for finding it on your own!