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Stan
Stan, Mercedes Technician
Category: Mercedes
Satisfied Customers: 3526
Experience:  Technical support engineer
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Mercedes c220 cdi 2001, heavy battery drain when car is left

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Mercedes c220 cdi 2001, heavy battery drain when car is left standing, battery is ok and alternator is charging at 14.5volts, have not done a current drain test but a very heavy crack comes off earth lead when it's reconnected back on battery, does any one know if there is a common problem on these before i look further in to it.

Diagnostics Engineer :

Hello,
Before going into rest mode, electronic contrlol units can draw up to 10 amps when battery is connected. That's why such test is not a good indicator on this car.

Diagnostics Engineer :

so it is important to measure the draw in rest mode.

Diagnostics Engineer :

Most common causes for drain are seat control units on this model

Diagnostics Engineer :

They are mounted to the seat frame, so it's quite easy to unplug all of their connectors

Diagnostics Engineer :

Regards, Stan

Customer:

Thank you for your advise stan, it's not my car, i've been asked to look at it for a relative as i used to work as an auto spark but thats 16yr ago, but things have changed abit since then! the car is also showing an srs seat belt restraint error on the dash, could this be related?

Diagnostics Engineer :

SRS ( airbag) is quite reliable and does not commonly cause any drains. But without a scan it is not possible to tell what is causing the light to be on.

Diagnostics Engineer :

Nowadays on Mercedes we have to connect the ammeter and then measure the CAN voltage, to determine if vehicle is asleep

Diagnostics Engineer :

If CAN is not asleep, control unit connectors must be disconnected from CAN distiributors to localize which ''bad child'' is keeping the kindergarden of control units awake)))

Customer:

i assume the ammeter has to be connected without the circuit being broken with use of a slave battery or using a clamp on meter then

Customer:

one thing i noticed was that there is a slight humming noise from were thr front interior light is

Diagnostics Engineer :

Take two longer wires. Tie one around the ground cable, and another around ground post of the battery. Connect them to ammeter leads. Let the car go into rest mode. Remove the ground cable from the post.
This way a circuit is not broken. Now disconnect the plugs from the light and it will be clear if the noise is affecting the current drain

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