W211 battery going flat

[bruces670]

my battery keeps going flat for some reason,charge rate is good,battery is good so can only presume there's a discharge somewhere.
A quick google mentions either power seat or the battery control module

Anybody any pointers?

 

[alexanderfoti]

You need to identify the circuit by measuring the current drain, as it could be many things!

Once you have an ammeter in series, it should be pretty clear after you start pulling fuses.

 

[bruces670]

Thanks that's my next step I just wondered if there was a common fault that was a go to fix

 

[mersum1es]

Like you said electric seats is one, but perhaps one more common is Comand which keeps running. You can hear is it is not going to sleep. Electric heater draws battery dry very fast, yet another place could be breather heater (if fitted in UK). ANY module (ie door control module) which is not shutting down may keep the whole CAN bus awake and that flattens battery... only way to find out is measure the current draw (sometimes fault codes may find the faulty control module). Voltage regulator is one culprit which is often forgotten but it fails rarely nowadays (in a way that current leaks).

 

[bruces670]

what is command exactly?

 

[EmilysDad]

what is command exactly?

Google is your friend ...... it's basically your radio & sat nav

https://www.carkeys.co.uk/guides/what-is-mercedes-comand

 

[RUBBER_UNDIES]

Put your multi meter in series on the pos. Dont start the vehicle! Then pull fuses one by one noting what it's for whilst watching for a voltage drop.

This can potentially save you a load of head scratching and time.

 

[alexanderfoti]

Put your multi meter in series on the pos. Dont start the vehicle! Then pull fuses one by one noting what it's for whilst watching for a voltage drop.

This can potentially save you a load of head scratching and time.

Be careful though, most Mb's will kill more than 10 amps with koeo and pop fuses/kill multimeters.

 

[bruces670]

I've replaced the battery and it was fine for a week or so and yesterday it wouldn't start,would the STAR diagnostics be able to confirm a battery drain and its location?

 

[alexanderfoti]

I've replaced the battery and it was fine for a week or so and yesterday it wouldn't start,would the STAR diagnostics be able to confirm a battery drain and its location?

No, it won't. Certain fault codes may give an idea as to which control module. It could be, but it needs diagnostics and current measurement to determine the location.

 

[DaveE320CDI]

When i had same problem with my W211 it was the Alarm Siren! caused by water damage being positioned in the front wheel arch, replaced the siren and no problems since, about £100 from Mercedes Newcastle on Ebay,
Dave

 

[bruces670]

It's going to be a nightmare to find!

 

[alexanderfoti]

It's going to be a nightmare to find!

Not. Impossible. You just need to be methodical and have an ammeter in series with the battery.

It also helps to have a fuse ammeter.

 

[bruces670]

What's a fuse ammeter

 

[alexanderfoti]

What's a fuse ammeter

Its an ammeter that goes in place of the fuse, easily showing what current the consumer for that fuse is drawing.

 

[bruces670]

two things that seem to come up as troublesome are seat module and battery control module,just been out to have a nosey and I can't seem to see my bcm in the boot,I just see a fuse board with the battery leads going to it?

And the other thing is the seat module,I'm guessing it's a seat removal job to access that is it?

Thanks
Bruce

 

[alexanderfoti]

two things that seem to come up as troublesome are seat module and battery control module,just been out to have a nosey and I can't seem to see my bcm in the boot,I just see a fuse board with the battery leads going to it?

And the other thing is the seat module,I'm guessing it's a seat removal job to access that is it?

Thanks
Bruce

BCM is only on dual battery cars, if yours has 2, then its in the power lines to the battery in the boot.

The seat control module is under the seat, should be removable without removing the seat.

 

[bruces670]

I don't know if mine has two batteries,I've not looked yet,thanks Alex

 

[mercmancdi]

I had one and believe it or not the 3 boot lights were not going out when lid was shut and with me not driving every day it was draining the battery, the car usually started , just about.
It was the switch in the catch.

 

[bruces670]

I had one and believe it or not the 3 boot lights were not going out when lid was shut and with me not driving every day it was draining the battery, the car usually started , just about.
It was the switch in the catch.

I sat in the boot on Saturday and shut the lid to check that lol