battery discharge problem

I have a 2003 Mercedes E500 that has developed an intermittent battery discharge problem. I have unplugged both of the front seat modules to insure that they are not the issue. But that did not solve it. So I have now unplugged fuse f41 which is the central gateway control unit. Is that the correct one to disconnect the audio gateway? With that fuse unplugged i am now getting a check engine light and a SRS warning light. So am a bit confused here to say the least. Also is there somewhere i can find a fuse chart for this car. Mine is missing and it shocks me the manual does not have this. Thanks for any help!!!! I have ordered an amp probe to use in helping me but it has not arrived yet.

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Boki made a comment.
3 months 3 weeks ago

Hello! 

You need to check for parasitic drain. So, wait until your amp probe arrives.

When you have it, do the following:

- Remove the negative side battery cable from the negative battery terminal.

- Connect the black wire to the com input on a multimeter and the red wire to the 10A or 20A input on the multimeter. The meter needs to be able to read at least 2 or 3 amps for this test to work. Connecting the red wire to the mA input on the multimeter won't work and could damage the meter.

- Attach a multimeter (set the dial on the multimeter to measure Amps as per multimeters instructions) between the negative cable and the negative battery post. Wait a few seconds to several minutes for the car to go into sleep mode — i.e. when you make the contact with the ammeter, the car's computer systems "wake up." After a bit of time they will go back to "sleep."

- If the ammeter is reading over 25–50 milliamps, something is using too much battery power.

- ​​​​​​​Go to the fuse panel(s) and remove fuses, one at a time. Pull the main fuses (higher amp ratings) last. Perform the same steps on all relays found in the fuse panel. Sometimes relay contacts can fail to release, causing a drain. Be sure to observe the ammeter after pulling each fuse or relay.

- Watch for the ammeter to drop to acceptable drain. The fuse that reduces the drain is the draw. Consult the owners' manual or service manual to find what circuits are on that fuse.

- Check each device (circuit) on that fuse. Disconnect each lamp, heater, electrical device one at a time to find the drain.

 

Boki

 
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Phillip Jones made a comment.
3 months 3 weeks ago

Thanks Boki!!!  Will do. The meter i bought is for checking low current draw so it should be plenty sensitive to do this. 

 
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MercedesMedic made a comment.
3 months 3 weeks ago

Agree with BOki. You need to check for parasitic draw.

 
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Phillip Jones made a comment.
3 months 3 weeks ago

thanks just got the low current probe in the mail, will see what i can find

 
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Phillip Jones made a comment.
3 months 3 weeks ago

ok so i attached my amp probe to the negative battery cable and it is reading a 1.5 to 2.5 amp draw. Every once in a while the draw may drop to only .4 amps. I have pulled every fuse and every relay in the car and none of them changes it. Any further ideas? I am at a loss other than putting a battery disconnect switch on it.

 
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Boki made a comment.
3 months 3 weeks ago

Hi! 

Have you checked fuses in all three boxes (engine, trunk, dashboard)? 

 

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