For the last 6 months, I have had to replace the battery 3 times already. the door locks would not work and the displays would not show. I figured it was an old battery. Now I'm figuring from the last 2 batteries that there's something draining the battery. Any idea of what it may be?
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batteries will self discharge over a period of time especially in cold weather
however 3 batteries replaced in 6 months raises issues
first question is why would they not charge up on a charger or a run in the vehicle
second question is the battery of sufficient size to run the electronics
have you had the alternator bench tested to check for correct out put, regulator operation and rectifier problems
check drive belt condition and tension
the wire could be a ground wire but I would think that if it is significant then something would not be working correctly
Merecedes keys have to "relearn" the car once the batteries either fail/die, or have the batteries removed. The only thing to do is take it to a garage and have them relearn the key for you. It doesn't take too long and shouldn'tcost more then an hours labor.
Programming When Key batteries completely discharged. Key batteries not replaced within 60 seconds. Key added or replaced. DO NOT press remote control buttons during battery replacement. How Unlock vehicle. Enter vehicle. Close driver's door. Switch ignition ON and OFF. Remove ignition key. Press and hold unlock button Fig. 1 . Press lock button Fig. 1  briefly 3 times within 10 seconds keeping unlock button  depressed. Release unlock button Fig. 1 . Some models: LED Fig. 1  flashes slowly for 10 seconds. Some models: Central locking system locks and unlocks rapidly to indicate correct programming. Some models: Repeat above procedure if central locking system does not work and LED does not flash or LED flashes continuously
To change batteries remove panel back of key then screws on plastic bar
Batteries are available from your local motor factor shop
You might not need new batteries try re programing as described above
Unlock vehicle. Enter vehicle. Close driver's door. Switch ignition ON and OFF. Remove ignition key. Press and hold unlock button. Press lock button briefly 3 times within 10 seconds keeping unlock button depressed. Release unlock button. Some models: LED flashes slowly for 10 seconds. Some models: Central locking system locks and unlocks rapidly to indicate correct programming. Repeat above procedure if central locking system does not work and LED does not flash or LED flashes continuously.
don't trust the new battrery, have it tested, i went through 4 aftermarket batteries in 6 months, gave up and purchased the factory batt, never had any further issues to this day, Kragen's are the worst.