1992 400E. Battery draining issue - PeachParts Mercedes-Benz Forum

amg280 · #1 09-24-2008, 08:54 AM

The past 3 weeks, if my car is left sitting for more than 2 days, its deader than a doornail in the morning. Not even a light turns on. Its got a brand new Interstate high capacity battery as well. Something is drawing on this thing big time. I have no idea what it could be. Last night I did notice something weird.

When I switched the lights off, and shut the door, the backlights for the window switches and some of the dash board (not instrument cluster) would flicker on and off multiple times a second. Very odd. Just for kicks, I unplugged the alarm module in the pass. kick panel and also the convenience module under the rear seat. The lights kept flickering.

Can anyone point me in the right direction. I have no idea where to look. Maybe a relay somewheres?

Thanks!

retroguybilly · #2 09-24-2008, 09:38 AM

You need to identify which circuit is causing the drain. With your ignition off, remove the positive battery cable and hook an ammeter in series with the battery and cable. See if you have a current draw. If so, then open the fuse box and remove each fuse, one by one, until you find the one that stops the current draw. Then you know which circuit it is and can trace it.

amg280 · #3 09-24-2008, 09:40 AM

Can a standard volt meter be used for this?

Will_w202 · #4 09-24-2008, 11:13 AM

Don't have an answer but do have moral support, as my 93's light panel above the rearview mirror is doing the same fast-flashing at certain times. Mine's probably headed to the point yours is so I will stay tuned.

G-Benz · #5 09-24-2008, 04:51 PM

Quote:

Originally Posted by amg280

Can a standard volt meter be used for this?

A standard voltmeter would work fine...you are looking for excessive current draw, so measure amperage, not volts.

Note: I had a Jaguar that would suck the battery dry within a couple of days. The cause was a faulty alarm brain...aftermarket issue. Yanked the offending alarm and all was well from then on.

As far as the "flashing issue", likely unrelated to the unusual electrical drain. It's usually a symptom of a bad or loose ground in the illumination system. Typically, the "bulb-out" indicator circuit. The flashing should be prominent when applying brakes while the car is at rest.

I had the "flashing" occur as well. The culprit was due to loose metal grounding strips in the taillight housings. They also caused phantom "bulb-out" conditions, even when all of the bulbs were the correct type, wattage, and tested fine.

amg280 · #6 09-24-2008, 11:35 PM

Does everything that is powered in the car run through the main fuse box under the hood. I always thought that some of the big stuff was on its own feed, with seperate fuses/relays, etc.

I will give it a try this weekend.

Thanks for all the advice.

vwbuge · #7 09-25-2008, 05:39 AM

I had an issue with my Subaru just like yours. I left the battery hooked up buy made sure it had a full charge. I removed all the fuses then started to insert them one by one while watching the voltmeter. I noticed a huge drop after I plugged in the power door lock fuse.....no more power door locks for me.

retroguybilly · #8 09-26-2008, 09:11 AM

Yes, most low-cost digital meters have a couple of current (ammeter) ranges, but they don't measure very high amounts of current. That's OK, though, because your drain is probably a pretty low-current one.

Don't confuse voltage and current. Current is proportional to the number of electrons flowing in the circuit, while voltage is proportional to the amount of "push" that each electron has. You always have to measure current in series with the circuit (e.g., as discussed above, by disconnecting the positive battery cable and hooking the ammeter up in a series with the battery post and cable.)

If you wanted to measure or monitor the charging voltage at the battery, you would of course leave both battery cables connected and simply connect a voltmeter across (i.e., "in parallel with") the battery terminals. (But what purpose would this serve, since your dashboard gauge does the same thing, doesn't it? If you had a problem with your charging voltage, wouldn't your dashboard gauge have already told you so?

An ammeter is a low-resistance device, while a voltmeter is high-resistance. In other words, NEVER connect an ammeter directly across the battery terminals. If you did, you would draw essentially infinite current across the short circuit and would blow up the battery, some of the circuits in your car, and/or ammeter, as well as perhaps starting a fire.

Remember, always measure current in series with, and voltage in parallel with, the circuit, or you'll destroy something!

amg280 · #9 09-26-2008, 10:10 AM

Thanks alot! Hopefully its not drawing that much current that it will overwhelm my multimeter. I had blown those little fuses inside the multimeter before, and I dont feel like making an unneccesary trip to radio shack at 8 at night!

amg280 · #10 10-01-2008, 08:54 AM

I located the problem. The convenience module under the rear seat was drawing power all the time. I took out fuse number 9 and that cured the problem. Since this is an older beater, im not fixing it. The radio is also on fuse 9 so I rewired it. All is well, except for no auto-rollup windows and no dome light. ehh...