I left my Mercedes Benz in a parking lot this past weekend since I was going out of town. It was left in park, the radio was off, and the lights were off. When I got back, I put the key in and it wouldn’t even turn to start to car, the radio wouldn’t turn on, and the horn wouldn’t honk. I tried to jump the car but it did nothing. What is wrong?

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    Do you mean the ignition key wouldn't turn in the lock, or the starter motor wouldn't turn the engine? – DavidSupportsMonica Jan 28 '19 at 3:37
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    Test it with a fully charged batter, then see... – Solar Mike Jan 28 '19 at 4:58
  • So that was meant to say battery, just love my typos... – Solar Mike Jan 28 '19 at 9:22
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    Fully charged batters don't appear until baseball season. – John Canon Jan 29 '19 at 4:58
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    How old is the vehicle? Many older models have a mechanical lock between the steering column and the ignition switch (which the key turns). If it's wearing out, it can be common that it won't let you physically turn the key if the steering wheel has any load on it (i.e. if you leave it parked with the wheels turned slightly off center). If your problem is that the key won't physically turn, try jiggling the steering wheel from side to side as you attempt to turn the key. – dwizum Aug 1 '19 at 14:26

Your computer works 24/7 waiting for a START command. In your case more than the computer is working overtime.

The way you troubleshoot is temporarily replace one of the battery cables with a much lighter wire. Now all current flow is thru this wire, and any time current flows thru a wire, a voltage drop is observed. The higher the current, the greater the voltage drop.

Set your digital Volt meter to its lowest DC voltage scale. Attach your meter Leads to a point close to either end of the wire you installed. You'll see a voltage displayed because the computer along with the unknown device(s) are drawing current. So now start disconnecting fuses. If no change is seen on the meter, go on to the next fuse. Don't be surprised if you find more than one additional device drawing current.

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