Hi all,

Car is a 2010 C300 Sport with 21k miles--just bought it 2 months ago. I installed some no error LED license plate replacements yesterday, and then was able to drive it around all night last night (multiple times) without any problem. This morning, which by the way was very cold, I go to start the car just as always with no luck. Luckily caught the metro to work, the car is currently sitting in my garage.

Here are my observations:
- with key in ignition turned partially, electronics do turn on and work, radio and lights.
- when I turn to start there is a single click from behind the dash but no start at all.
- at the click, electronics very briefly and barely flash, but then remain on after failed start.
- I forgot to check to see if all the exterior lights were functioning, but I know the headlights were.

I've read a bunch this morning both here and Benzworld about potential battery problems like a dead cell or a messed up fuse, or even that the steering lock or ignition module could be the problem ($1800 fix apparently!?!). I've also read people recommending to mess around with the shifter or to keep putting the key in and out until another click sound, I'll try these later when I get home. My confusion about the battery solution is that on my 97' e320 when the battery is dead there are multiple solenoid clicks, not a single one, so I dont know if this single click is the same issue but is just different because of electonic start (and the fact that it is 13 years newer lol).

Anyone also have advice on maybe some multimeter tests I can try later to pinpoint the issue? Thanks in advance guys, I love the forums and all the information everyone contributes--very useful.

 

The first thing I would do is make sure it's not the battery. Before you go through a troubleshooting nightmare, just take the battery in to autozone or advance auto parts and have it charged. Once you're 100% sure it's not the battery, then you can start troubleshooting components.

 

I can guarantee you it has nothing to do with the LED bulbs. Added to the fact that you say that it got cold overnight and the car being an '10 with most likely original battery, it's almost a certainty you need a new one. You might even be able to start the car when you get home if the weather has warmed up a bit. But you should still take your battery to get tested.

 

It's still under warranty, would dealer check and replace the battery if that's the issue for no charge?

 

try calling them mate, would give you the answer you need.

 

Thanks guys, now just an update so as to help anyone else with the same issue (since I couldn't find anyone posting about the same case):

When I got home this afternoon, I decided to do some voltage checks.
- battery was basically dead, but still no sign of underlying cause
- voltage was about 12.5 until I opened a door and some electronics switched on then voltage would fluctuate around 10 or so (draining). Also the same clicking sound exactly when I opened the door.
- decided to charge it in my garage to see if I can even start the car at all. Throughout this time while my charges was hooked up every time the clicking sound would randomly occur (sometimes triggered by door opening etc) the voltage would change. Either it would be 15 volts charging fast if it's been sitting for a while then a click would happen and it would drop down to around 13.5 which while still charging indicated that something suddenly turned on and started draining simultaneously. The clicking would happen at random intervals for the 3 hours I spent on the car tonight, each time switching the mystery voltage load on or off.
- I opened up the trunk and unplugged those LEDs just in case it was actually the culprit and screwing up the computer. Still no dice, the clicking and draining would still occur.
- Car would start fine after a bit of charging, and alternator would charge battery with no problem while on.
- Found where the clicking was coming from! It is actually the larger black relay the is located in the hood fuse box directly between fuses #25 and #26. When I unplugged the relay while the car was off and charging, constant 15v was supplied and problem was gone even without the charger it was showing a healthy 12.6v. But plugged relay back in and problem seemed to come back and clicking would randomly happen again. Can't find out what the relay is for since it's not on the fuse diagram and can't find it online.
- I'm leaving the car alone overnight and will check to see if it drained again (LEDs not connected)

Looks like a weird electronic problem that might drag to the dealer. I'll keep updating this for the sake of anyone that has to suffer my headache.

 

It's either the horn relay or locking mechanism relay. I believe both of them are exactly the same. And the random clicking is most likely your security system trying to lock the doors after you've unlocked the doors without operating them within a certain time.

If you listen carefully, you'll hear the relay click everytime you unlock your car.

 

Can you tell us if this is the same setup as your fuse box?
The yellow one is the horn relay.

Attachment 447564

 

Yes that is it, except the one that clicks randomly (and I'm pretty sure it was not related to locking of the doors) is the larger square one between 25 and 26. I could actually feel that it was exactly that one when it happened as my finger was on it. Any idea what that one is for?

 

I can't find any info, but since you're under warranty, I'd take it in. A faulty relay is not an uncommon issue, they should also be able to tell you what the relay is for. If you're really curious just start checking electrical things around your car. Seats, mirrors, audio, cluster, lights, etc. With the relay out one of them won't work.

 

Alright guys, while I wanted to be able to give a better reason, I left it overnight after a charge both with and without the LEDs and it still died in the morning. So I took out the LEDs, took it to the dealer and they said the battery was defective with a faulty cell :P. Replaced free of charge.

So turned out to be totally unrelated. I suspect my previous owner just let it sit around a lot and the battery was neglected.

Still thanks for everyone's help! Now to deal with the flat tire I got this morning (the bad luck wont stop).

 

don't want to say "I told you so".......but.........