92 500 SEL - battery drains when sitting for 2+ days
Installed new battery - mechanic checked alt output and said OK
How do I check/locate possible excessive current drain???

Although this may not be the fastest way, I had a similar problem with my 2002. What I did was remove one fuse at a time until the disharging stopped, which then identified the circuit with the problem. Turned out to be a pinched wire, which was easy to find once I knew where to look. Note that you only have to remove fuses with power across them when the ignition is in the off position. HTH

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1982 Mercedes-Benz 300CD
1982 Mercedes-Benz 240D - stick

Basically, one must view the current flow and separate the circuits involved until the culprit is identified.

More specifically, the current should be less than 50ma probably closer to 25ma, at rest with all control units asleep. Here, though lies the greatest difficulty. To get ammeter readings in that range one must be in series with the circuit. Since the circuit isn't known one must start at the battery and separate circuits elsewhere. Another problem exists due to the way the ammeter is inserted. To do it properly the ammeter must be inserted without disconnecting the circuit. Disconnecting the circuit in effect reboots the control units and can easily change the test results.

A suggested mechanism is to get a battery disconnect switch installed on the negative terminal of the battery. Run the ammeter circuit across the future open circuit (the disconnect carries the heavy loads and allows all operation and then disconnects the circuit usually with a rotary switch). Run the vehicle normally and when the igntion has been switched off the disconnect is openned. The minor currents still operating now flow through the bypass ammeter circuit. These currents could easily be in the 100ma or more range until all control units go asleep. The sleep function should take place in a few minutes.

Once the current is stable, the decision is made whether it is too great and to divide the involved circuits to find the defective one. Remember that interior lighting circuits in late model cars can take over an amp (1000ma). Most ammeters have fused circuits at about 2a (2000ma), anything left on will likely blow the ammeters internal fuse.

The easiest way to separate the circuits is to pull fuses. Eventually a power distribution tree should be viewed to be sure the separation is complete.

Go get em.

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Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician

BTW the ignition key MUST pull out the ingition lock put it on top off the vehicle and if vehicle have aftermarket alarm system remove it first before you do the test......just because 140 battery is in trunk you have to switch the light off after trunk is open or remove the light bulbs.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner

Any luck with your problem?

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1989 300CE Astralsilber/Schwarz
1992 300CE Schwarz/Schwarz

An easy way to insert an amp meter in series with the circuit without disruption of power is to hook meter up between bat. neg terminal and eng. gnd.
[ observing meter polarity].
Now, with everything off , disconnect the neg bat. cable at the ground end [ not at battery terminal]. The current will now go through the meter , which is simply in parallel with the ground cable, without power interuption...

What was happening on my 1984 300SD was the trunk lamp not shutting off when the trunk was closed.
You can check for this by opening the trunk and feeling the bulb immediately upon opening it... if it's way hot, this might be your problem.
Or, just pull the bulb out and see if your discharge goes away.
Mine was taking more like 3 or 4 days to fully discharge.... just a thought, Bob

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic

I go along with advice from Steve and MB Stars in general, but I don't measure current to find a key off current drain. I insert a 1 ohm 10 watt resistor in series and measure the voltage across. The reading should not go over .05 volts. If you do the math, it comes out the same as reading the current flow. Since I have to run this test often, I have put the resistor in a nice little cigarette sized box from radio shack with permanent leads and alligator clips big enough to hook to the battery.

Peter

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