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Hi Brian,

I own a 2000 model W220 (S500) which has had a quiescent battery drain problem for quite some time which I am keen to resolve. My drain is quite high in that a good, fully charged battery will be exhausted if the car is left for 4-5 days. As the battery charge becomes progressively lower, sometimes leaving the car for only a day without being used results in the battery voltage dropping to below the starting charge value.

I have a multimeter which I plan to use to test each fuse with, as you've described in your guide, however I don't know if the multimeter is sufficiently sensitive to conduct the testing properly. I've attached a photo of the type of multimeter I would be using. Can you tell me if this would be okay to do the testing with?

Prior to doing the above I plan to test the battery draw after the car has sat idle for about 30 mins, as I suspect the draw will be greater than 1.6 amps which I believe indicates a potential fault on a CAN bus as opposed to a consumer unit. I am fairly sure my multimeter is at least capable of doing this test.

Thanks in advance for your advice!
 

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Discussion Starter #3
Hi chapzboy,
The photo of your dvm doesn't show how many digits it will read. Post another photo when measuring a 1.5V battery.
My DMM in DC voltage mode, ie when used as a Digital Volt Meter (DVM), is quite sensitive and will read 0.1999V or 199.9mV maximum on the lowest scale and hence is very good if used with an appropriate value resistor, to monitor small and large currents.
I have posted the full technique here.
https://w220.ee/DIY_High_Current_Sensor_-_Quiescent_Current_or_Parasitic_Discharge_Tests

This page is a subset of;
https://w220.ee/Battery#Checking_Parasitic_Battery_Drain which you should also look at.

If you cannot manufacture a current sense resistor out of coat hanger wire use the method whereby you measure across individual fuses. This does however require a very sensitive DVM, eg one able to read microvolts such as 39 microvolts or 0.039 mV or 0.000039 Volts. The method and typical results for various fuses are also in the first document above under section "Fuse Quiescent Current Draw".

Let's know how you go.
Brian
 

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I suspect the draw will be greater than 1.6 amps
Hi,

1.6 amps is waaaaaaay too high, even on a fully loaded V12 Car.

You need to be seeing 50 mA to an absolute max of 75 mA once the car has gone to sleep :wink
My V8 S Class has recently stood un started a couple of times for 2 - 3 weeks, due to me building a workshop extension, and it still fires up no problem at all :wink

HTH,

Cheers Dave
 

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Discussion Starter #5
As it says in the articles linked above when the car has been locked and has fully gone to sleep (takes at least twenty minutes) the quiescent drain from the car battery must not exceed 0.05A or 50mA.

Note also you cannot use a multimeter in current mode to measure the actual drain from the battery. You will blow the meter to bits. The only safe way is to fashion a measuring resistor and put it in the battery lead and measure the voltage drop across it.

Read the links.
 

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Hi,

Apologies, I got my decimal point in the wrong place in my head :laugh

I will amend my above post to avoid confusion, I should have said expect 50mA max 75mA :wink

Actually, regards using the amp meter, it will depend on the meter, I have a Fluke that measures up to 50A, so you can use that, lock the car and as the car goes to sleep it will change it's range to .000 amp.

Cheers Dave
 

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Hi Brian,

Sorry, I should have attached an image showing the values. Here it is.

My electrical knowledge is limited and I think this all may be beyond me. I was hoping to be able to test each fuse (without having to pull them out) to see whether a circuit is drawing an excessive load, but I suspect my multimeter isn't sensitive enough to do this. In any case, if my problem is a CAN bus not going to sleep, which I suspect it may be (due to the apparent level of parasitic draw), then there is no need to test each fuse, I believe?

It also sounds like you're saying it's too risky to use the multimeter to directly measure the level of parasitic drain at the battery (running the multimeter in series), as the multimeter will be fried if the PSE pump actuates, for example, is that correct?
 

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Discussion Starter #8
Your multimeter is sensitive enough to measure the small DC voltages across the fuses or across the DIY test resistor.
I would check the fuses first as it is easy to do. The MB fuses have exposed tabs at the top. Get everything set up and then after the car is asleep just measure the voltages across each fuse. Use the table in my first link above to get an approximation of the current flowing through each fuse without removing them.
All up the total current should be less than 0.050A.
As you obviously have an issue it should be easy to spot a large voltage drop across and hence large current draw through a particular fuse and hence identify which circuit is the problem.
Dave's suggestion of putting an automatic 30A multimeter directly in the battery lead will work but turn everything off first eg air con. And do not crank the engine.
When I first started investigating my battery drain issue years ago I tried this technique but couldn't lock the doors as the 12V supply browned out through the ammeter. What worked for me was putting the DIY resistor (a piece of fencing wire wound around a broom handle) in the battery lead and measuring the voltage drop across it. Of course you still cannot crank the engine though as the wire will glow red hot with the hundreds of amps required in the starter motor.
 

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Hi,

@ chapzboy

Brians post is a very well written, easy to understand way of doing this, and unless you have access to a digital 30amp + meter then the resistor / volt drop is the way to go :wink

All you need is the patience to keep leaving the car for 20 mins each time you wake something up :wink

Don't get too hung up on this CAN Bus thing, the CAN is a communication line, so the only real reason it would stay active is because one component won't sleep, because there is another faulty component that it is not seeing, so the first component keeps looking for it .........

Here's an example .........

At some time the boot, (trunk) of the car has got wet, so the TV Tuner has got wet and is goosed, (the duff component) ........ COMAND will keep looking for it, so COMAND keeps the CAN alive endlessly looking for it :wink

This is why you have to check at each fuse, until you find a specific circuit drawing too much, then by unplugging the components on that particular circuit one at a time you will find which one is faulty :wink

Here's a tip to keep your 20 mins waiting time to a minimum........

When doing the rear fuse box / SAM, I always remove the rear seat first and gain access to the SAM, then I switch off the Alarm motion Sensor, (Towing mode), and leave the Right Rear Door open, Making sure to close the lock latch and tape up any pin swithches a car may have That way you can lock the car with the fob and go in and out without waking the car.........

Do similair under the bonnet, (hood). Make sure to close the bonnet locks and any pin switches with the bonnet open :wink

NOTE:- When you are finished Be sure to pull the door handle / hood release before trying to shut them !!!!!!!!

HTH,

Cheers Dave
 

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Hi!! Quickly... in everyones opinion, or maybe a straight up fact, what is a VERY good, or even the best multi meter available out now? And possibly the store or stores that would carry it, or one that is MORE than acceptable too take care of all the needs we have for its use on these w220's?? I had one i thought was ok but was told it really wasn't very good. Purchased one at Harbor Frieght, and one at orioles auto.....Any ideas, please?? Thanks so much!!!
Power of Choice
 

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Thanks for the replies, Brian and Dave, this is all useful advice for when I embark on this. I will let you know how it goes, in due course.
 

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Discussion Starter #13
After my $400 Fluke failed a few months ago I bought my new one on fleabay for AU$15 about US$20 and it works fabulously well.
Search for
Portable LCD Digital Multimeter AC/DC Voltmeter Ohm Multi Tester 6000counts
Its a bit fragile but if you break it just buy another one. Won't break the bank.
 

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Hi,

I have to agree with Brian, I wouldn't have bought my Fluke, but it was bought for me by a company I used to work for a few years ago now :wink

That little Ammeter is very useful for this though, I bought it last year for doing a couple of cars with drains, and all I did was extend the leads and put "Crocodile Clips" on the ends :grin

Worked on the basis that was cheap enough to buy a new one if it breaks, but I have to say, it has proved very useful and well made :wink

Cheers Dave
 

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I have been very pleased with the Greenlee tester kit that I bought for less than $20 from Home Depot. Includes an excellent multimeter mounted in a rubber housing, and a couple of home electrical system testers that have proven very useful, all in a high-quality zippered fabric case.

I also have four or five of the cheap Harbor Freight multimeters scattered all around for quick-n-dirty checks...
 

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Hi Brian,
I have got my head around this more and will do the testing of the fuses in the next few days. I just wanted to check a couple of things before I do.

I'll obviously need to leave the bonnet open to test the left and right sill fuse boxes; is there any switch/lever I have to tape closed so the car thinks the bonnet is closed? I don't think there is but just wanted to check. I already know I have to keep the rear right door switch taped in to enable the rear right footwell fuse box to be checked.

In your guide, you make mention of a "Front Cabin Fuse Compartment". I've looked extensively online but can't work out which fuse box you're referring to, can you please elaborate?

Lastly, I will be using my multimeter on the 200m range, and comparing these values to the values in column 3 of your guide. My understanding is that different fuse ratings have different acceptable upper limits; for example a 5A fuse giving a 0.5 reading is acceptable, but a 7.5A fuse giving 0.5 is not acceptable.

I note your values are for 30mA, yet the Mercedes specification is for 50mA maximum, was there a reason you used 30mA instead of 50mA?

Thanks!
 

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1) I'll obviously need to leave the bonnet open to test the left and right sill fuse boxes; is there any switch/lever I have to tape closed so the car thinks the bonnet is closed? I don't think there is but just wanted to check. I already know I have to keep the rear right door switch taped in to enable the rear right footwell fuse box to be checked.

2) In your guide, you make mention of a "Front Cabin Fuse Compartment". I've looked extensively online but can't work out which fuse box you're referring to, can you please elaborate?

3) I note your values are for 30mA, yet the Mercedes specification is for 50mA maximum, was there a reason you used 30mA instead of 50mA?
Hi,

1) You also have to set the Door and Hood locks to the "closed position" using a round object like a screwdriver, as the locks have micro switches in them which send "Lock open / closed" signal to a variety of different modules
Also, don't forget to switch the ATA to "Towing / Off" position via switch on dash :wink

2) Forget that, your profile says your car is early pre facelift, so it doesn't have a Cabin Fusebox :wink

3) That figure is for the Total mA drain, seen at the Battery, which requires a different method of testing to which you are embarking on :wink That is measured when you are using Ammeter in series with battery Earth lead :wink

IMHO anything up to 75mA is acceptable, obviously 30 - 50 is better.
I say this as some cars have more Equipment than others, and have had extra stuff added over the years, (maybe tracker, USB charger sockets, etc etc.........
When the original MB figures were issued they were quoted based on "Equipment of the period" fitted to one specific model :wink

HTH

Cheers Dave
 

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Dave,

Where exactly is the hood/bonnet microswitch that I need to manually close while I have it open? Also, what do you mean by the ATA? I think you are referring to the tow-away alarm button on the console, do you mean I need to switch this on before turning off the car?

Given that my parasitic drain equates to 0.6 volts from the battery per 24 hours, would the value that I see at the relevant fuse be fairly high?

Thanks for all your advice with this, really appreciate it and I hope I can get to the bottom of what is a very annoying problem! :)
 

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Discussion Starter #19
Dave has answered all your main questions.
Using this method it is not a very big task to undertake.
Just do the preparation re car sleeping and then measure the voltage readings for each fuse.
Make sure you do it systematically ie draw a diagram or print out the fuse location diagrams (https://w220.ee/Fuses) and record the voltage drop across each fuse.
Then add them to a table/chart and upload here and we can help analyse the results.

BTW my update W220 must have a bonnet/hood switch because the other day I drove off the bonnet unlatched and the dashboard lit up red warning me. I don't where it is located though and it is certainly not obvious.
Brian
 

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Dave has answered all your main questions.
Using this method it is not a very big task to undertake.
Just do the preparation re car sleeping and then measure the voltage readings for each fuse.
Make sure you do it systematically ie draw a diagram or print out the fuse location diagrams (https://w220.ee/Fuses) and record the voltage drop across each fuse.
Then add them to a table/chart and upload here and we can help analyse the results.

BTW my update W220 must have a bonnet/hood switch because the other day I drove off the bonnet unlatched and the dashboard lit up red warning me. I don't where it is located though and it is certainly not obvious.
Brian
Mine is a pre facelift, and did exactly the same red message one day, when I moved it a few feet in my drive with hood up :wink

That's how I know it must have switch(es), like Brian says, they aren't obvious, I've never needed to look for them :wink
I assumed they are in the left and right hood locks, like on other vehicles.
So, now, on all Mercedes I snap them closed with the bonnet up when testing for Current Draw :wink

Remember to snap shut Right Rear Door lock too :wink

And Remember to unlatch them before slamming Hood / Door shut or you will likely damage something, I always tie a bit of red cloth / ribbon / tape around the Door Handle and Front Grille to remind me :laugh

Yes ATA = Anti Theft Alarm, and yes switch on dashboard, turn it off as per owners manual, so that the motion sensors inside the car do not detect your movements :wink

I cannot answer your question about 0.6 V drop overnight, I've never bothered measuring that:wink

IMHO that test is not worth doing as it depends on a variety of things like Battery age / condition, and how long is the "night" :wink
If I have a car with a drain, I simply get to and start pulling fuses, like I said, I always start at the Rear Box under Seat, as most of the cars I've had draining Battery have had Water Ingress damage to Modules or Wiring Plugs in the rear end of the car, which has caused the drain :wink

:laugh A Jaguar I did recently had a "Swimming Pool" under the Rear Seat Squab, and one of the Heated Seat power feed plugs was actually emitting steam as it was drawing current :laugh
Cause was a leaking bonded rear screen !!
Couldn't visibly see that until I lifted the Rear Seat Squab to get at the Fuses :grin

HTH

Cheers Dave
 
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