2005 Mercedes C240 Parasitic Drain

Excellent troubleshooting technique! GA

The only thing I would add is if the ammeter has a peak (Max) hold mode (one of the things digital does better than analog) then the OP can see if the intermittent taillight problem is the root. Moving the taillight wiring harness around while in this mode any brief shorts to ground will be detected.

Hi Del, This is a tough one. It intermittently drains the battery. For instance, one day it was driven and turned off (lights off, etc) and there was a large current flow from the battery. The next morning the car wouldn't start. My friend jump started it and it ran fine (to work). After she got off work, the car started fine and she parked it at home. We tested it and there was no current flow this time.

I have heard about problems with the power seat module on these cars, but I was wondering if someone has had the same problem.

Thank you.

It's a lemon, dump it....

I don't use my vehicles frequently, and the batteries running low was a constant problem....I have installed 5 watt solar trickle chargers to see if that works....will report findings....

What does "7. Taillight bulb warning message." actually mean? You say 'Bulb not replaced, but I don't think this has anything to do with the parasitic drain.' Is it possible that this warning message might be indicating a short rather than a failed lamp?

Have you checked for corrosion at the main contact points in the ignition system, or damaged wires, to make sure the starter/ignition system is getting full voltage when starting?

Mercedes has an information screen on the dash. When there is an error, a message comes up. The message she is getting is the taillight bulb warning, which means the bulb is burned out. The bulb is out (we checked it), but we didn't replace it yet. I don't think it has anything to do with a short, since it would blow the fuse. I was wondering if anyone has had a problem like this and could give me advices. Mercedes cars are wonderful to drive, but when there's a problem, it's a nightmare.

My C Class has an intermittent problem with the remote. Sometimes the remote will work and other times it doesn't. The problem arises when I try to unlock the door and it doesn't work. I then resort to using the key and the alarm goes off! I have to rush into the car and put the key in to turn the alarm off.

When you checked the interior lights did this include the light inside the boot(trunk) & any under bonnet lights? If in doubt, take the bulbs out. This also applies to kerb lights in the doors.

...Glovebox light as well.

Yes, we checked all the lights. We waited until her garage light turned off - the garage was pitch black = no lights on in the car.

Way to go. Good old fashioned diagnostics. That has worked for me many times.

Hi Fredski, I appreciate you giving me your advice. Our problem right now is that the parasitic drain isn't happening now. It randomly shows up and when it does, the car won't start the next day. There isn't a pattern we can find and as you know, until it occurs, we can't run tests.

This is why I was asking if anyone has had this issue with the Mercedes C-Class.

Nigh might be right. My wifes Mercedes had a trunk(boot) light switch that was faulty and the light would stay on or turn on intermittently. this drained the battery in the garage usually, because that is where she would open the boot to retrieve the groceries!

Hi Deefburger. Thanks for the tip. You gave me an idea - we'll pull the trunk light bulb and see if it solves the problem. It could be turning on by itself at night and drain the battery.

Thank you.

Also check the bulb in the glove box. I've had a problem where it would stay on occasionally with the door closed but was very hard to see.

We tried this and it isn't the glove box light. I have a Jaguar XJ that has this problem, but it has a neat feature - the glove box light is switched, so when the ignition is off, the light is off. I do need to get this fixed - I just need to get to the dealer to buy the part.

Thanks for your tip.

Another possible site is the cooling fan having sticky cut off switch (temp) - I had this on my Audi

Thanks Hugh. We checked the cooling fan and it wasn't running.

my dad had a Caddy a few years back with a similar problem, i used a 1 ohm, 10 watt resister and my VOM to locate the problem. this method will work to help verify drain and individual circuit. note: if you're drawing more than a few milliamps the resistor gets, quite hot!, remember if you have a mechanical clock it's a constant draw of a few Ma. your computer(s) always draw a little too. so after you hook up the resistor and meter you should get a volt reading across the resistor of a dozen or more Ma...if you have anywhere near a full amp or more you're looking for a short and not so much of an intermittent draw. heres an example http://www.youtube.com/watch?v=B__DqK90IIc

Thanks for the tips. For us, we can't duplicate the parasitic drain - we can't get it to happen. When it's in the shop, the mechanic can't duplicate the problem. When she drives the car, it happens randomly. The only way she knows that it happened is that the car won't start in the morning.

To find this drain you must make a tester 2 wires length doesn't matter and a bulb like a tail light bulb 2057 or 1157. Solder one wire to the bulbs connection and then solder the other wire to the other bulb connection after this is done take one of the battery terminals off and attach one wire from tester to terminal and with the other wire wrap it around the battery post if the light comes on you have a connection staying open. Start removing fuses from the fuse panel one by one until the light goes out which will expose the drain source. My money is on the dim fuse or the alternator which could still work but if the internal wire got hot and fused this make the alternator stay open slowly draining the battery. Also this tester can be purchase at part store if you don't have the parts cost maybe $5 - $7. Also before i forget test with the car off not running and the door close on the vehicle. This will pinpoint your problem fast.

I have a friend with the same problem on his Mercedes. Dealer said it was the seat module also. He just disconnected it to save the money. He said it was easy to disconnect it. Try it for 2 weeks and see what happens.

Or, follow Fredski's posts. Good luck.

Hi Bob, Thank you for your response. I'll have the mechanic do the same thing and tell my friend to see if she has the problem again.

I'll let you guys know if this works.

No need to thank me, Just send me some pictures of our past presidents.

A while back I worked at an Olds dealership. A common cause for dead batteries in the 98 series was a fold down set of trays that illuminated an area under the tray. Not easy to see in daylight. And when left on for extended time the bulbs would turn black, further masking the problem. Sooo, check for any blackened bulbs, These may have been burning an extended time, and could be the result of a non switching light.

How old is the battery. If more than 2 years replace. Should fix issue.

The old battery was 3 years old and failed the test, so a new one was put in. Still have the same problem.

In addition to all of the other symptoms people have suggested, I think this vehicle is CANBUS equipped. This means that when you go to raise a power window, you actually send a digital command to a μp and that in turn sends a command to a module that interprets this and then raises the window. It could well be "random" as you suggest, it could also be some combination of actions that is occurring. Might even be a corroded window switch that when dampened sends out a command.

I suggest you get a factory service manual and a smart charger that can provide a float charge. This will end the dead battery scenario, until you isolate the problem. There may be an "owners forum" that offers help. You will NOT get it from Daimler, they are more interested in providing lucrative repair bills to their service department than assisting you who are about to circumvent that process.

Keep us posted

Think I'm wrong? Try getting the tool to replace a fuel injector on their diesels.

Visor mirror lights?

To all those here, I want to thank you for your recommendations and comments.

As a follow up, here is what happened to my friend's car. After a couple weeks of opening the switch at night (the mechanic added a switch in line to the red + terminal) and closing it in the morning, she had no parasitic drain. The consensus was that the voltage regulator was the culprit, so the mechanic replaced the entire alternator with a Bosch rebuilt vs changing just the VR. The part was around $250 vs over $600 for a dealer part. So far, there is no problem and my friend loves her car again.

For all of you who called it right, great job!

If the alternator was the suspect, using the same installed switch on the battery lead from the alternator would have confirmed it. And just for giggles, a relay installed in that line switched by the ignition circuit would have saved the cost of an entire alternator, as would have changing the VR. But, the good thing is that it is behind now, and you did not have to spend a fortune on the repair. Good for you.

We thought about the relay, but couldn't find one that would handle the current from the starter (the cable runs from the alternator to the starter).

Look up an ST80 . It is a constant duty solenoid able to handle all of the alternator's current. Next time.