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sprinkles08, ASE Certified Technician
Category: Car
Satisfied Customers: 21439
Experience:  ASE Master and Advanced level certified. Factory trained with 15 years dealership experience
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C180 Komp Classic SE Auto: XXXXX XXXXX Amedee,My wife

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Good Afternoon, Amedee,

My wife Mercedes a C180 Komp Classic SE Auto, 2004 has a problem in that the battery (a new one) goes flat when the car is not used for 48 hours or more.
My mechanic says the battery is fine and that something is switching itself on and running the battery down. He cannot tell me what it might be and says that a Mercedes Main Dealer may have trouble too. He suggests an Auto Electrician who will use an oscillator to ascertain the problem. He advises that it will cost approximately £1000 to fix it!
Do you have any ideas?

Hello and welcome to JustAnswer!


Until the problem is diagnosed there's absolutely no way to guess how much it's going to cost to repair it, so don't get to worried yet.


If it reliably dies in two days each time it's parked then the draw may be there all the time. It could definitely be intermittent though.


The problem could be something like a trunk or glovebox light coming on but more draws are caused by control module related issues on modern vehicles.


The modules communicate with each other on communication bus networks. If one module stays awake when the car is parked then not only will that module still draw power but it will keep others on the bus awake and they will draw power too.


Internal control module failures can cause problems like this or inputs to modules can cause it also. For instance a door ajar switch showing input will not only turn on interior lamps when it happens but it will wake up multiple modules.


What needs to be done is a draw test to first see if it is happening. If an ammeter shows a draw then it can be tracked down at that time. This can be done by checking voltage drop across fuses to see what circuit(s) have current flowing through them, or by removing fuses one at a time and seeing when the draw drops. When a circuit is identified as being involved in the draw then components on that circuit can be concentrated on.

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