HELP! 92 400E mysterious battery draw issue - PeachParts Mercedes-Benz Forum

amg280 · #1 06-01-2009, 10:45 AM

Guys, Im at my wits end. This car has a brand new high capacity interstate battery. Alternator is good. Runs and drives great. Most of the time is will start just fine. Then, every few days, the car will be DEAD. Not even enough charge in the battery to light the dome light. This problem is intermittent. We fully charged it 2 days ago. Went to start it this morning, nothing!

It was at my independants for a week, and he couldnt find anything. It didnt die while it was out there. It is weird because this problem is so intermittent. Whatever is drawing on it, is pretty severe to drain this battery so fast. Its gotten to the point now where I dont trust taking this car anywhere.

Where do I start? Please help.

gsxr · #2 06-01-2009, 11:00 AM

Unfortunately, I don't know of an easy way to diagnose this. You really need a couple of good meters.... first, a standard VOM with the capability to measure 10A current via the probes... and second, a clamp-type DC ammeter that you can use on the battery cables to do a quick check for a major current draw. Then you have to check it all the time. When you measure a large current draw at the battery via the clamp-type meter, then you have to start isolating one circuit at a time (pull one fuse at a time, etc - don't forget the external ones for the HVAC blower fan, heated seats, fuel pump, etc) until you can find the circuit which is causing the drain. If it's none of the fused circuits, you have to start checking the unfused items (alternator, starter, possibly the CAN modules, etc).

I have a Fluke 36 AC/DC meter, you can find them on eBay for $50-$100 or so, for example

this one went pretty cheap. Just clamp it around either main battery cable and you can see if there is a major current draw or not (normal should be less than 1 amp, anything over 1-2 amps will kill the battery overnight).

ejboyd5 · #3 06-01-2009, 11:49 AM

Did you have this problem before you changed the battery? If not, I would suggest that the battery itself is bad (although new), if so you will have to check all the circuits beginning with the diode in the alternator. Good luck.

amg280 · #4 06-01-2009, 11:52 AM

Yes we had this problem before we replaced the battery (twice). It would sit for a while and it would die (we wouldnt know), and this ruined a battery or two (dead cell). Could the alternator be draining the battery? How do you test it?

Thanks!

amg280 · #5 06-02-2009, 10:21 AM

I did notice that sometimes at night, after I lock it, the interior backlights (dash, window switches, etc) will flicker or pulse a bit. What controls these?

ohiomike · #6 06-02-2009, 11:39 AM

I had this in a 400E for a time. Take readings with a simple digital voltmeter from Radio Shack. Pull fused circuits on at a time . See if there is battery draw hr by hr from the battery. Especially touch your hand to the to the radio amplifliers in the trunk to see if they are warm (battery draw). Disconnect the power to radio antenna as they stick on instead of shutting off. You test these things and you will find the power draw. I bet it is the radio amplifier in the trunk. Good luck. The digital voltmeter shows readings in tenth and hunredths of volt which is what you want. Should cost under $20 and be battery powered.

david s poole · #7 06-02-2009, 02:34 PM

let's make this a little easier.there is a 99% chance that the draw is from one of the fuses that stay alive when key is off[radio antenna.central locks,courtesy lights etc.]pull those one at a time while watching the clamp-on ammeter that should find it pretty quick.

amg280 · #8 06-02-2009, 02:37 PM

Thanks for all the advice! Am going to dig into this over the next few days...

Anyone know why the interior backlights flicker when the car is off sometimes?

Shooter · #9 06-02-2009, 03:50 PM

Check your door jamb switches that they are turning off/pushing in completely. this may be causing the light flickering. Several times I've seen the cigar lighter causing a draw(the male portion) or money and crud in the bottom of the lighter causing a draw.

david s poole · #10 06-03-2009, 10:12 AM

but in a benz the cigar lighter is not powered with the ignition off.

dpkreuze · #11 06-03-2009, 05:33 PM

Just a WAG but I have seen the module under the left rear passenger seat bottom cause that symptom.

Genbiltstein · #12 06-05-2009, 06:14 AM

Intermittant or not you should be able to find the culprit. Take your time in doing so. It might take you several times during several start up cycles.

Disconnect the negative lead from the battery. Use a light tester and jam the point into the negative pole. Clip the other end onto the negative lead you disconnected.
If you are experiencing a short the light will light up.
Disconnect and reconnect one fuse at a time until the light goes out.

If it is intermittant and killing your battery pretty quick Then I want to blame an electrical motor somewhere. Winshield wiper motor for instance.
Maybe an alternator. Those diodes when they do short will cause amp draws.

Put a trickle charge on the battery to keep the charge up. Do disconnect the battery from the negative terminal.

Trickle charge plus time equals a healthy battery. I garantee it!

gsxr · #13 06-05-2009, 11:55 AM

Quote:

Originally Posted by Genbiltstein

Disconnect the negative lead from the battery. Use a light tester and jam the point into the negative pole. Clip the other end onto the negative lead you disconnected. If you are experiencing a short the light will light up.
Disconnect and reconnect one fuse at a time until the light goes out.

The clock always runs, and some other items also consume a small amount of power as well. These items could cause the light to turn on, even though they are not a "short" or a major drain. That's why you need to use a meter to measure the actual current draw. Otherwise you could end up chasing down a false "short".

Quote:

Originally Posted by Genbiltstein

Maybe an alternator. Those diodes when they do short will cause amp draws.

That is correct... if you can measure the abnormal current draw, try disconnecting the wires from the alternator, and see if the current draw goes away. If so, the alternator is bad. However, I'd be surprised if this were an intermittent problem... usually the alternator is either good or bad permanently.

Quote:

Originally Posted by Genbiltstein

Put a trickle charge on the battery to keep the charge up. Do disconnect the battery from the negative terminal. Trickle charge plus time equals a healthy battery. I garantee it!

Trickle chargers are great. No need to disconnect the negative terminal though, just don't turn the key on if you're worried about sensitive electronics. Most trickle chargers don't put out enough juice to damage anything (unlike bigger chargers, or "starter booster" units). If your car will be parked longer than 2-3 weeks at a time without being driven for at least 15-20 minutes, a trickle charger is highly recommended. They're only about $25-$35 or so. You can even get solar ones for cars parked outside.

Uncle Acky · #14 06-05-2009, 05:07 PM

I know your current draw could be from a number of things but.....I had this problem (92 300E) and after checking with an AMP meter in my case, when I unpluged the cable that runs from the radio amp unit (right side fender well) wire that runs down to the becker CD changer. Follow the wire you will find a 4 wire connector to unplug and see if you have the same problem I did. Mine too was intermitent and would drain the battery completely.

Good luck buddy,
Acky

amg280 · #15 06-08-2009, 11:26 AM

Well I found the culprit! Completely not what I expected it to be. I noticed the trunk light stopped working months ago. I thought it was just a bad bulb. I never cared too much to fix it. At any rate, the other night I noticed when I was closing the trunk very slowly, the light kicked on right before it shut. That also made the the interior lights stop flickering. Bingo.

Upon further investigation, the 4 wires that are wire tied onto the drivers side hinge of the trunk (license plate light power) were all stripped and bare (at the pivot point). When it would get open or shut, it disturbed them. The power was back feeding into the illumination circuit causing some of the interior lights to flicker. In addition, it was grounding out on the trunk metal, causing a pretty significant amp draw.

I repaired the short and everything seems perfect now. Im only getting .1 to .2 of amp draw now.

So be sure to check those wires to make sure they arent grounding out!

Thanks for all the great suggestions.