No accessory power after engine start. Recent alternator/battery change - Mercedes-Benz Forum
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post #1 of 2 Old 09-25-2019, 07:31 PM Thread Starter
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Vehicle: 2003 Mercedes Benz C320
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Cool No accessory power after engine start. Recent alternator/battery change

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.


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.

Any thoughts?

Last edited by Craig M.; 09-26-2019 at 11:20 AM. Reason: Update
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post #2 of 2 Old 10-03-2019, 07:39 PM
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That's weird... My first thought would be to disconnect the batt for a bit and see if that fixes it. And check to be sure all your fuses are good.
Did it do this on the original drive home, or right after the alt change, or some time after that?
Does the power go away the instant you turn the key to start, or does it actually wait until the eng fires up? Or does it dim first then loser power? Basically it would help to explain in detail exactly what and when it does it. And what if you were to turn the key off while running, or driving, then back to #2 ? That way you're turning it from off to #2 but not starting it, so I'd imagine you'll have power. Either way it's a clue to finding the problem, which however weak is more than you had before.

Many cars will kill power to many or all things while cranking to ease the batt load, and allow you to start when the batt is low. So maybe that function, no doubt feeding a relay, is the prob. Like maybe it kills the relay (maybe more than one), then waits until it sees xxx rpm before it powers the relay again. Kinda like a starter button in a keyless car that actuates the starter, but kills it when it sees xxx rpm. Or it could be looking for the key to return to #2 but it's not seeing that.
I suppose it could be in the key switch itself. In older cars the key assy itself was the switch for things, which draw a lot of current so the switches wore out over time. With newer cars the key should only actuate a relay(s) so I'd assume virtually no electrical wear. It could also be actual mechanical wear from something like some dumb bimbo having a ton of keys or whatever crap on the key chain that continually stressed the switch.

I would also pop open the fuse box and look for what is and is not powered when off, when on key #2 , then finally when running. Basically I'd be looking for the relay(s) that supply power in #2 , but stop working after you start. From there you can trace back to what is supposed to power it. Or you could power it from something else, like the computer, injectors, fuel pump etc which do stay powered. A bit redneck but I'd do it before I'd pay mass $ to the dealer.

If all fails I'd consider powering all that stuff via a separate relay that you install and actuate via one of the aforementioned sources, or you could power said relay with a toggle switch on the dash. It all depends on how far you're willing to go into redneck territory.
In my truck I have a master kill switch, which can handle a a ton of power and uses a big plastic red key. So when removed nothing gets power at all, zero, nadda. Good anti-theft device too, I think. Point being you could use one of those to power everything if needed, but I'd prefer a teeny little switch and a relay.
So go poke around and tell us what you find....
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