No accessory power after engine start. Recent alternator/battery change

Craig M. • 26 September 2019

Hi all. I'm a recent purchaser of a 2003 Mercedes Benz C320. The vehicle had been sitting for approximately one year with a completely dead battery. I jump started the vehicle after a few tries and got it home.

I realized that the alternator wasn't charging at 14.2 or better, and the battery wouldn't take a full charge because of a dead cell, so both were replaced, and the car is now starting and running perfectly.

The issue I'm having now is the following: With the key in position (2 accessory), I have complete power inside the vehicle ie, horn, turn signals, wipers, radio, windows, ac, etc.

As soon as I start the car, I lose all power to everything inside. Again, everything functions properly until I actually start the car, and I am at a complete loss as to why this is happening. ANY advice would be greatly appreciated because other than what was mentioned above, the car doesn't have any other issues. Thank you in advance for your time.


Hi Craig,

The C320 has M112 engine and 722.6 transmission which are very reliable. Overall it is a great car, with the exception of a few minor electrical issues.

The problem that you have will be hard to diagnose without a good scanner. Here is one thing that I would like you to try. Connect jumper cables to another car and let the other car running. Then try to start your car. Then check if the systems you described are still working.

I am wondering if there is a significant power drop at startup that is causing these systems to be turned off.

Where are you located in US?

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  • Thanks for your response. I am contacting you fron Belleair Bluffs, Florida. I had the vehicle scanned this morning and it came back with the following codes: P0600 Serial communications link error. P207D-04 Incorrect message from (CAN). Control unit N73 (EIS/EZS communications fault. P203E-04 No CAN message from Instrument cluster-Message fault. P2031-04 No, or incorrect message from controlunit (N80) Steering column module. Communications fault. 

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  • This could is most likely a ground issue for one of the modules or a corroded terminal. I would start by inspect the ECU.

    Did the mechanic clear the codes? Which one of these codes are showing as CURRENT and which one are STORED?

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  • I cleared the codes this morning with a borrowed scanner. I am going to go check them again and I will get back to you. Where is the ECU located on this particular car? 


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  • Is it a possibility that it may need to be flashed, or reprogrammed because of the amount of time it sat with a dead battery, as well as changing the battery and alternator?


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