About 6 months ago, the alarm chirp (I have my settings on Netherlands so the alarm chirps when locking and unlocking the car) started intermittently sounding muffled/faint - especially after going through the car wash. At some point, the alarm module dead shorted and killed the battery overnight. I went to the wrecking yard and got a newer style alarm unit from a 2009 CLK (just a part update - slightly smaller unit and a higher pitch on locking/unlocking). Problem solved, right?

About 5 months go by with no problems until about a month ago, the battery went dead again after messing with the phone system wiring from the arm rest to the main harness. Nothing too invasive - I just swapped out the harness and swapped it back again. (I'm still working a problem with the phone module installation). No obvious codes other than low voltage warnings in nearly every module. Charged the battery and cleared the codes.

Everything was fine for a couple of weeks and it happened again with no action from me. Charge, clear codes and move on...

Tonight my wife and I just went out to dinner (I ran the car through the wash on the way there), came out and it wouldn't start. Just a click. Interior lights at full brightness. The car acted like the transmission was not in park. Battery voltage was at 12.8V. I put my fully charged battery jump box on it and still no start.

At this point I noticed the lights suddenly and rapidly fading and I started getting erratic warnings on the display. I hooked my jump box up again (the cable has a voltage meter) - it was down to 5.4V. I had to jump it just to lock the car to go back into the restaurant to wait for a tow truck.

I came back out and unlocked the door with the metal key since the battery was acting fully drained by then. This set off the alarm (it has its own battery) upon opening the door and the interior lights came on full brightness. I turned off the alarm with the key fob and it responded. I figured I'd try it one more time and it started up like nothing happened. Cancelled the tow truck and drove it home - battery was charging at 14.4V on the way. Codes again are mainly undervolt in various modules (front and rear SAMs, EIS, overhead and upper consoles). The sunroof needed re-normalization.

Setting the alarm off obviously reset something that was drawing heavily on the battery though. Ideas?

(Cross posting on C55 thread.)

 

sounds like an alarm siren battery issue. You can replace the siren, or solder in new batteries. If you are unable to properly diagnose with a computer, soldering in these batteries is a 15 dollar try. Google this issue and you may see that it is your problem.

 

I haven't had time to do much diagnosis work, but some more data points: the dome light isn't coming on now with the door open despite the switch being in the middle position. If I manually switch it on, it works. Also, there is a 0.07A current draw on the battery that doesn't go away after a few minutes. Something isn't going to sleep apparently (or is that a normal draw?)

Since the light isn't turning on with the door and the sunroof took multiple tries this time to re-normalize, could it be the overhead console? One of the codes that I'd assumed had to do with low voltage in the module: "The supply voltage of the sensors WM61 (Hall effect sensor1) ans WM62 (Hall effect sensor 2) has short circuit or Open circuit." This can be cleared and goes away.