Hi Dave, thats quite a large drain you have there! Finding a drain can be a time consuming task as you will need to isolate different circuits to find the drain. I would always start with a full function test of the vehicle to see if anything is not working as it should - seat adjustment, seat heaters, heted screen, windows etc etc. Test as much as you can to see if anything has failed.
After that you would usually start to pull each fuse in turn to see if the drain goes when that fuse is removed. The problem with many systems on modern cars is they are often powered by an unfused feed!
If the drain is still there then the next step is usually to unplug the main power to each SAM unit (built into front & rear fuseboxes) - these power up many systems & it may help isolate to a component off of one of these.
The seat heater control unit is a possible failure, SAM units themselves can have internal faults, various problems with COMAND or telephone modules can cause drains - the list can go on!
It may be a job for an auto electrician or dealer but if you want to start looking further yourself then start with what I have listed & let me know how you get on.
Best regards, Paul
Hi and thanks for your reply. I had already started removing fuses before it got too dark and I'll carry on doing the same tomorrow. I'll let you know of my progress and if I don't get anywhere, I'll have to put it into the local dealer if I can get the car there. Regards, Dave
Hi Dave - no problem, good luck with it & let me know how you get on.
Thanks Paul, I charged up a new battery and as soon as I connected it, there was a big spark and the terminal voltage dropped quickly to around 11 volts. I checked the currently flow and it was 18 amps.
With my somewhat historical knowledge of vehicle electrics there are only two things that use this sort of current......starter motor or alternator.
Going to ring my local garage tomorrow and see if they can send someone out as there is no way it is going to start as it is.
Hi - other systems can cause a large power drain if there is a dead short, good luck with your repairs & please let me know if you need anything else.
I got your email and thanks. The reason I've not accepted your answer yet is that I still haven't resolved the problem. I've had to call in the RAC who are probably going to tow it to my local Mercedes dealer. I will keep you informed.
Hi Dave - no problem, will look out for your reply. Let me know if you need anything else.
Hi Paul, unfortunately even my garage has been unable to help and the car has had to be recovered to the Merc dealer as apparently only they have the necessary software to be able to trace the fault. I will keep you informed
Hi Dave - apart from being able to see if any codes are stored theres not much software on our diagnostic tools that will help with a drain like this! Is good old fashion electrical diagnosis with an ammeter attached. On the plus side dealers will have more experience of the electrical systems & all wiring diagrams to help solve the problem!
Hi Paul, Good news and bad news I'm afraid.
The good news is that they found the problem and have fixed it. The bad news is that it was the heater booster which had "blown" and shorted to ground. Cost to repair, just over £1k.
Thanks for your help. I'm not sure if an acceptance is justified in this case, however I will keep the £20 "in the pot" and if I need assistance in future, you will be my first port of call.
Hi Dave, no problem - glad you got your car fixed.