CJG

I read here yesterday about a part (module) failure likely caused by jump-starting the battery. If this is something you should avoid, what does one do if they discover they have a dead battery? Must you be towed to a dealership, which seems ridiculous?

Also, I think I recall seeing a small battery in the trunk near the spare tire. What's this for?

Thanks!

psbenz12

Some cars have anti-theft failsafes that can be triggered by losing all power for an extended period of time. I don't know what the C has, but I bet there's something.

Have to be Towed? Use the old school manual key to get in the car and the hood should be 100% analog so you can at least change the battery or jump it.

I def saw mentions of a small secondary battery for powering critical components. I also some some stuff about hooking up a charger while removing the battery to prevent full drain. I didn't really look for one, but last time I used my spare I didn't see one. Even without a backup I would think it has to sit long enough to completely drain the battery and residual charge if it were to trigger something.

Couple weeks ago I put a new battery in my C and didn't do anything extra. Since the battery wasn't completely dead I figured it had enough residual to keep things up while I hook up the new one. I had no issues at all even my radio presets, bluetooth profile and tripometer were not affected. I don't remember if I had to reset the clock.

Jump start causing issues? I don't know about that, I would say more than likely a complete power drain would be your cause, over the jump start.

Seems a little too unethical to be the case, but maybe MB does program it to trigger something on a jump to get you in the door and sell you a battery... I doubt it tho.

An example of a failsafe on the C that I can confirm is- if you trigger the alarm the trunk locks up until you use your fob again. Great because you can leave valuables in the trunk with a low risk of having it stolen, but it totally sucks when you lock your keys in the trunk and don't having folding seats.

dchar

I think people were having issues after jumpstarting because they hooked up the leads directly to the battery rather than the designated spots near the battery.

My sister had an '09 C300 that needed a new battery. I stopped and waited in a parking lot while the key was in the ignition for less than 30 mins and it drained the battery. Luckily someone in the lot had jumper cables and I knew where to correctly connect the leads. The car was jumpstarted with no issues. I even replaced the battery on my own without hooking the car up with no issues; just keep the cables from touching anything when they are disconnected.