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Marc 08-09-2000 09:23 AM

I have an interesting situation with an 83' 500sl. The car normaly starts right up no problem. Last friday I took the car for a short errand, (about 5 miles) when returning to the car the battery was dead. I jumped it, drove it the 5 miles home, turned it off, battery dead. I stuck it on a 2amp trickle charge overnight and it started fine the next day. I put a multimeter on the battery with the car turned off and it read about 12.7 then started the car and it went to about 13.5 maybe 13.6 at idle, revving the car to 2000 or 2500 had no affect at all. We don't drive this car alot but I took it out each day since just to make sure it would start. Everything is fine. Then yesterday I remembered that a couple of days before this incident I had driven the car at night, so last night I took the car for about a 20 minute ride with the lights on, got back home, turned it off, Bingo! the battery is dead. Now first, I've only had the car about six months but the battery looks new, the lights do not get brighter when the accelerator is depressed so, that along with the reading on the multimeter, is the charging system working? If not would it be voltage regulator or alternator? Now I'm concerned that I may have a larger problem in that I think it unusual that the headlights would discharge the battery in that short of period with the car running and producing 13.5 - 13.6 volts.

Any comments or suggestions would be greatly appreciated.

BTW, charged the battery again and it starts right up. Maybe I should just tape a good flashlight to the hood for night time excursions. ;)

makakio 08-09-2000 12:59 PM

Had similar problem to this Marc - turned out it was the battery. Funny thing was that it would sometime hold charge, sometimes not. Replacement fixed it though.

LarryBible 08-09-2000 01:25 PM

I'll put bucks on the battery being gone.

Good luck,

Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

WmHarlow 08-09-2000 01:59 PM

Had a truck like that.... it started fine but use of the radio, headlights, and blower would drain the battery in less than 20 min :(.... changed the battery and it was fine from then on.

Good Luck,

76 240D (W115) - 550K miles
78 300D (W123) - 200K+ miles

Jim H 08-09-2000 02:48 PM

I'd suggest that the wires and connectors from the alternator to the battery also be given the once-over. The voltage readings sound good, but remember you can read 12.7 volts from a battery through a 10 megohm resistor with a digital multimeter!

Is there a way for you to get the wiring and battery checked under load, preferably for *FREE*?

Good luck.

BCingU, Jim

'96 E300D 56kmi
'95 Audi 90 118 kmi
'92 GMC Suburban 138 kmi
'85 300SD 218 kmi

jerry o 08-09-2000 04:12 PM

A couple of years ago, I had an auto parts store put a new battery in my 87 260E only for it to last a few days and die. They ended up putting in 3 batteries all together before I took it to my mb mechanic. He replaced the brushes in the alternator and said that even though store had checked it under load, the alternator was giving false readings and that that was all it needed.

LarryBible 08-09-2000 07:17 PM

I think some of you are overlooking the fact that he reads 12.X volts with engine off and 13.7 volts with engine running. If the problem were alternator or alternator wiring, this would not be the case.

Good luck,

Larry Bible
'84 Euro 240D, 523K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles

Brian16V 08-10-2000 12:15 PM

Had the same problem with my MB when I first got it (plus battery *looked* brand new). Looking over the records, seems the previous owner had the same problems, but they never had the battery changed. Once I changed to a new battery, I've never had the problem again.

Seems like the consensus is: Get a new battery.

Good Luck!


be459 08-10-2000 11:53 PM

Without looking at the specifications, "the car running and producing 13.5 - 13.6 volts" looks to me a little low for normal operation. Are the battery terminals clean? Are the belts tight?


[This message has been edited by be459 (edited 08-10-2000).]

jonvee 08-11-2000 10:04 AM

Batteries store current NOT voltage. Checking the voltage can give an idea of the state of the charge, but does not measure the ability of the battery to supply current to the starter.

Have a 78 450SEL that I got from the PO because the car wouldn't start after sitting for a couple of days. I checked the voltage and it was OK(12v). Started the car and turned on the lights, voltage was 13.8v. Swapped the battery with one from my other car. Hooked up ammeter to check for current drain, only drawing enough for the clock.
Car sits for a couple of days, dead battery.

Time to put the thinking cap on and start from square one. What is the purpose of the battery? To supply current to the starter. Therefore the battery must not be storing enough current to start the car.

Current flow is affected by the resistance of the charging circuit. Check the cable from the alternator to the (+) side. OK. Check the cable from the (-) side to ground. BINGO! The ground cable is attached to the brace under the battery tray. Brace has rusted to nothingness. VERY high resistance path the ground, therefore low/no current to the battery.

Voltage OK, current not OK.



andy day 08-11-2000 10:43 AM

my money is on the battery !

johngray 08-11-2000 11:21 AM

I will add my vote on the battery. You positively can't tell how good a battery is by how it looks. Also, I have found that the absolutelious worst thing one can do to a battery is to let it discharge completely and leave it in that state for some time. That will kill a fairly new battery. 13.7 on charging voltage is on the low side of normal but should be enough. Good luck.

Marc 08-11-2000 05:40 PM

Thanks for all of the posts guys. That is exactly what I wanted to see, past experience.

I've had the typical old battery problems before with other cars, but I never saw one that would start the car just fine, over and over, yet when your driving it (read that when it is charging) with the lights on, the battery goes dead.

I still question the charging system. The car is idling just about right 750rpm (In park), I don't think the charging system should be producing it's maximum output at this speed. From the other posts I've read the voltages should start to go up at about 2000rpm. Note I had no change at all when increasing rpms.

Terminal connections are clean and tight. I am going to trace and check the wiring. If I don't find a definitive answer I'll probably replace the battery, alternator, and voltage regulator.

I bought this car for my wife to drive and for me to fix. (gets me out of the office) It's really not a daily driver so I expect I will replace everything by the time I am done. I did a lot to it over the winter and we thought we would drive it on the few summer days we have, so I don't want it to leave her stranded.

Thanks again for all of the information.


Gfranks 08-11-2000 10:52 PM

This may be a total shot in the dark but your description sounds remarably like how my problems started. I had almost the same exact symptoms. I have traced the problem down to a bad ignition switch and a short in the radio system. I first found that my ignition switch was keeping circuits hot, when the key was removed, that should have had no current. After fixing that I checked each of those circuits and found high draw on the radio circuit. I pulled the radio and voila, no more problem. Unfortunately It looks like I did not catch it early enough as it looks like it took out several other items too. Oh, the morning I had finally narrowed down the problem to the ignition switch I walked out in the garage to be greeted by the antenna going up and down randomly. Gee wonder why my battery was dead, huh!

LarryBible 08-13-2000 06:25 PM


The only other thing you might check before changing the battery is to see that there is no engine off current drain. If you don't have an ammeter, even a low current ammeter on a multimeter would work. If you have an ammeter place it in series with the apparently fully charged battery and see if there is any appreciable current drain with the ignition switch and everything off. I would be concerned if there was over about 100 to 150 milliamps. You can check by looking for a spark when touching the battery cable to the battery in pitch dark with everything turned off. There is risk involved in doing this, because if you're battery is in bad shape, it can explode with the introduction of a spark. This is a remote possibility.

If you find no current dran, do yourself a favor and REPLACE THE BATTERY!

If it's making 13.7V with engine running, measured at the battery, the charging system is working. Many alternators, depending on battery charge state will not increase voltage when engine is revved up. This isn't a '56 Chevy with a two brush generator. One of the big reasons which led the manufcaturers to alternators over generators was their ability to charge the battery at idle speed.

Change the battery and relax, Good luck,


[This message has been edited by LarryBible (edited 08-13-2000).]

[This message has been edited by LarryBible (edited 08-13-2000).]

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