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Old 11-10-2000, 11:56 AM
450 SLC 5.0
Posts: n/a
Could anyone recommend a way to trace a voltage leak to its source? Or is there a relatively common problem that may be causing this? I have been experiencing the battery going flat after the car sits for more than a few days. I have removed the fuse to the climate control servo to see if it was not “parking” properly (and drawing voltage if it were stuck) – but that was apparently not the problem. The only other thing that is drawing any voltage while parked is a standard little Bosch relay that is part of the alarm system. (But I have had that for years without any prior problems, it doesn’t pull very much amperage.) And yes, the battery is not only tested & good, it was replaced as a result of this problem, yet I am still having the new one go flat after 3-4 days. I am puzzled and don’t know what else to look at.

I almost forgot - the car in question is a 1980 U.S. spec 107 chassis

[This message has been edited by 450 SLC 5.0 (edited 11-10-2000).]
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Old 11-10-2000, 12:36 PM
Registered User
Join Date: Jun 2000
Location: Saugus, CA USA
Posts: 2,012
Anything that is drawing enough power to kill your new battery in a couple of days may also be getting warm. Try feeling around for something warm, maybe crispy, after the car had sat for a little while and everything should be at the same temperature.
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Old 11-10-2000, 12:42 PM
Posts: n/a
What you need to do is use an ammeter in series with the battery, then start disconnecting things until you find the circuit that is drawing the current.

The problem is that most ammeters that are incorporated into multimeters are only good for a fraction of an amp. There are many newer digital multimeters with a 20Amp capability. It very well could be that your current drain is less than an amp, since it takes a few days to drain the battery.

First you need to ensure that the battery is being drained, rather than not being charged. Use a DC voltmeter across the battery to measure the voltage. Measure with a charged or mostly charged battery and the engine off, the voltage should be around 12. Then measure with the engine running, the voltage should then be about 13.5 give or take. If the voltage stays the same after starting the engine, the battery is not being recharged.

You can make an ammeter with a heavy .1 ohm resistor and a voltmeter and a simple calculation.

Best of luck,

Larry Bible
'01 C Class, Six Speed
'84 Euro 240D, manual, 533K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in
Mercedes automobiles
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Old 11-10-2000, 03:02 PM
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Join Date: Jul 2000
Location: Sunny Florida
Posts: 199
Here's an old trick from my car fixin days: Pull the positive battery cable and hook a test light (12v voltage indicator) between the battery and the positive battery cable. Start pulling and replacing fuses until the light goes out. When you isolate the circuit, put the fuse back in and begin disconnecting components or cable connections to isolate the source of the drain. It is the same as Larry's ammeter, only the light has the advantage of being easily seen from where ever you are in the car. Helps when you are under the dash or in the trunk and have to look up. Good luck!
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