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  #1  
Old 03-31-2000, 09:37 PM
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Join Date: Apr 1999
Location: Apex, NC USA
Posts: 176
Three times recently I have come out in the morning to a dead battery. Apprently something is draining it but I am hoping for some tips and likely suspects to check. Can anyone suggest?

The battery is only 3 months old.

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1991 350 SDL
114,000 miles
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  #2  
Old 03-31-2000, 09:53 PM
metricman
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OK, you have a new battery.
First things first...does the charging system works properly? You are at the mileage where the brushes on the voltage regulator may be worn out. You will find this on the back of the alternator, mounted with two screws.

Beyond that, ther could be lots of things to drain the battery, from the antenna motor to a relay that stays energized. You need a multimeter or test light to try and find it.

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  #3  
Old 03-31-2000, 10:29 PM
Deezel
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Here is a quick trick. Disconnect your negative battery terminal. Use multimeter set to "volts" set to read up to 12V (use a cheap one in case you get your polarity backwards) and insert this in series with the disconnected battery cable. It has been a while since I have had to do this, but I think you should conncet the negative voltmeter lead to the loose negative battery wire, and connect the positive lead of the volt meter to the negative terminal of the battery. Make sure all systems are off. Voltage reading should be very low. Turn on the ignition as a test and voltage should read 12V. Shut ignition off and close door to eliminate dome light draw. If you have significant voltage reading (anything above 1/2 volt would be a concern I think, start pulling fuses until it drops off suddenly. Now you have the circuit identified and you can start figuring out where the exact problem is.


Becareful, safety first! A 12V battery won't shock you, but if you accidentally ground it against the car frame or with jewekly or tools, it will burn you or the car and/or the garage/house! Always disconnect the negative cable first to remove the ground circuit and then the positive if needed. renove rings watches and necklaces. Don't laugh, but have a hefty pair of dykes to disconnect the battery in an emergeny and have a fire exteinguisher handing as well.

Hope this helps!

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Deezel
87 300TDT
150,000 miles

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  #4  
Old 03-31-2000, 11:34 PM
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Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
This test: measuring the voltage between the batt cable and the batt terminal won't work on a modern car. Virtually all cars have some form of retained memory in control units that draw very little current but show full battery potential.

The proper test is to hook your multi-meter between the battery and the cable in the Amps setting. Do this with everything shut-off and with the ten amp scale (first - till you see the size of the load). You still could blow a tester fuse if a blower comes on.

Each car is rated on its maximum parasitic draw, but I would say your current flow should be less than 50ma. A trunk light bulb on will be it the neighborhood of 200ma.

If you have current above these levels disconnect circuits to narrow the search. Use the volt meter and identify the fuses that are powered when the key is off. Pull these fuses one at a time till you can see the current draw fall (on the ampmeter). There are probably some circuits that don't go through the fuse box so a wiring diagram is the next step. I would try unplugging the antenna, the alternator, the trunk light bulb and the fuses one at a time.

Of course all of this should take place after making sure the alternator is charging. If all you have is a volt meter a fairly good test would be to measure battery voltage at idle and 2000rpm. The voltage should be above 13 and below 14.8. If you turn on the headlights the load wil drop the voltage at idle but it still should be greater than the battery voltage with the ignition off.

------------------
Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician
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  #5  
Old 04-01-2000, 10:36 AM
Deezel
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Foiled again by modern technology! See what happens when you only drive old cars! It has been a while since I have had to do this, so I am glad you corrected me. Thanx!

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Deezel
87 300TDT
150,000 miles

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  #6  
Old 04-01-2000, 10:40 AM
mattsuzie
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First of all, I thought batteries were 13.2 V, not 12 V. I burned myself grounding my watch while working on an electrical system on a 68 model 6 years ago - got 3rd degree burns on my wrist and it got infected so do be very careful about taking off jewelry.

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'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)
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  #7  
Old 04-01-2000, 04:53 PM
LarryBible
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mattsuzie,

Batteries are 12 volts, or thereabouts. When the alternator is charging, they basically overvoltage to about 13.5 volts which charges them.

Your experience reminds me of something that happened to my Dad. He said that he got his wedding ring (there is no better conductor than gold) between a starter battery cable and the block. The ring got hot instantly, and he managed to get his finger out of the ring. I suppose that had he been unable to get his finger out, he would no longer have it. Safety first. Here that Deezel?

Glad you've healed from your burns,



------------------
Larry Bible
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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  #8  
Old 04-02-2000, 10:13 AM
mattsuzie
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Thanks Larry. Both your Dad and I are lucky to have survived with perhaps only a scar. Hair no longer grows on half of my wrist due to the burn, but since I wear a watch there now you would never notice.

It was not a happy experience. When the shock hit, I smelled meat burning, like a Barbaque, not knowing that I was the meal! I quickly took my watch off and the burn went through all the skin. Stupid me, I refused to immediately go to the hospital and it got infected. Before the infection went to my heart, I did go to the emergency room after showing a former nurse my wound, and they immediately put me on antibiotics in time.

I never never, ever, never, work on cars with a watch, not even to check the oil dip stick.



------------------
'89 420 SEL
'90 300 SEL
'84 300 SD (sold it)
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