The reason the ground is disconnected first is to "unground" the chassis to prevent short circuiting the entire system. And as for allowing the current to fully discharge from all the electrical components, i'm pretty sure that the current will flow to the chassis,and discharging it, since all the electronics are grounded through it.

Now for cars that are not using a chassis ground,ie a fiberglass car, disconnecting the positive lead first sounds ok because the current can only be discharged to the battery

from my understanding, it doesn't really matter which comes off first because either way the current flow is stopped, but disconnecting the ground first is safer to prevent shorting the entire system.

if someone with more technical knowledge/experience disagrees, please inform me because i would like to know if i am wrong

i think i should also mention that i am a senior studying electrical engineering

To avoid errors and synching windows ESP etc I made a memory saver (my Haynes manual suggests using one when disconnecting the battery EXCEPT when working around any airbag components)

Got an old 12V accessory plug, soldered a diode in the positive lead i.e. center one (pointing the >| on the diode towards the cig lighter) and crimped a standard female connector on each end of the 2' leads and expanded them a bit with a screw driver. Then plugged in the ends to my deWalt 12v XRP battery from my drill (make sure which is + and -; the most forward terminal is + on deWalt batteries) turned the key to the first notch and plugged in to my lighter (the lighter is a switched device and the circuit is cut if the key is out). After I disconnected my battery (neg first, then pos) and either worked on a item or when I changed out the battery, then when everything is done (new battery installed or worked on a component) re-connected and did not lose any info.

Variation: instead of a cordless 12v or 9.6v power-tool battery you can use a slave 12v car battery, or a 12v lantern battery, or instead of connectors at the end of the leads use a 9v connector and use a 9v battery. Problem with the 9v is that there is very little juice in those and should you open the door, have the trunk open, the lights go on it will drain it very quickly. The cordless tool battery gives you certainly more time to do things, the 12v slave car battery gives you all the time in the world to work on things

Note: DC Current "Conventional Flow" is taught Positive to Negative, but I think you may find the "Electron Flow" in DC systems as Negative to Positive. & Why the capacitors (condenser) are on the positive side of points in an old magneto or distributor (am I dating myself?).

Regardless I've read the whole thread, I easily will follow the MB manual. Neg off 1st rule, as been my practice. FWIW my $.02 any auto battery exchange any marque, a few thoughts for other DIY'ers to ponder, besides the usual safety glasses or face shield.

I was taught everything off, no load or draw, ( = no key in), so little to no sparks desired around a battery (DYI me = Automotive College Degree & practiced Aircraft Mechanic). Always pull ground - negative off first.

Opposite sequence has not been advised because if you wrench or ratchet away on the Positive nuts & Bolts 1st with the ground still on, your hood, any fender, bracket, brace all metallic surfaces may spark when you touch the opposite end of your metal tool to that. Read the extreme dangers of a rapid battery discharge elsewhere.

Also note if you are using a 12vdc supply jumped in while you change your main battery you have same dangers to that extra battery. You short under the hood that other battery may get real hot wherever it is sitting. So if you do that still watch your tools. Picture those rubber coated insulated electrician tools now.

i've MERCEDES C240. 2002.
My FUEL GAUGE VALVE is down & it's not showing the fuel level & not blinking also when i start engine. pls if u all can tell me what is the problem. Is it a fuse? (if it's a fuse how can i find that. i've not diagram of locations of fuses. where can i find that?) or something differnt?
THANK'S A LOT

This has got to be one of the craziest idea ever. Most be a newbie who never did it but would like to hear if works with others trying it first. It would be almost a guaranteed wholesale destruction of electronic units on the car.
Keys should never be in the ignition when doing battery work, Is that simple or what? without a key in the ignition then engine would be running for sure. The trivial reset actions after reconnecting the battery occur on most MB cars of this vintage . Windows, sunroof, ESP. The means to reset are simple. Just read the other postings that make sense in this thread,

Ok, I followed the advice here. Undid neg first then pos. Switched out battery and viola, nothing. I was in a hurry and frustrated because my daughter was requiring attention. Came back check out forum again and went back an hour later when daughter was asleep. Undid neg and got a spark, slid it back on and lights came on. Tightened it up, closed hood started car. I got the ESP warning, turned wheel stop to stop and then rolled down windows and rolled up and held for the 15 seconds. Everything works great. Thanks this is the second piece of advice I have used from this forum. Thanks everyone!

so can i use donor battery or not? or either way ok

I have a 2005 C230K with 14,000 original miles since new. The reason for the low mileage is that I stored the vehicle for 3 years when I went overseas (not military). I stored it in a non environmentally controlled climate, and would drive it once a year during my annual leave. Hence, I disconnected the battery intentionally during my time away. I would reconnect the battery and the only problem I experienced was the sunroof wasn't sync'd;specificslly it would not fully close upon the single click of the rocker switch. The problems described by others was non-existent. My car during this period had about 6,000 miles, so it was basically new. I did take it to Mercedes for an oil change and had them sync the sunroof.

Like I said, today I have just shy of 14,000 miles and had an A service only last year. The car smells and runs new, plenty of power and handles superb.

If anyone says storing a car is not good for it, well I just don't buy it. In fact I have the original tires on them today, and only the rears need to be change. Rubber is good, not cracking,etc. I was concerned about the fuel in the tank and how that may affect the fuel system. I kept the fuel topped off and used additives when I stored it. Every year when I was back home, I connected the battery and it would kick over the first time, all the time.

I'm sold on Mercedes for life. I just recently bought a 2012 E Class Coupe. I still have the C230K.

I got a 2001c240 after selling my beloved 01 E320 ( c240) looked more appealing - (Worst mistake of my life) I love mercedes but think this might be the worst car ever made.
Long story short the fuel pump died @ 120,00 miles -
After calling the dealer I was told I had to replace both the fuel pump and the sending unit since the model no of the fuel pump was discontinued and the new one was not compatible with the original sending unit. (looking to spend approx $900-1200 on parts plus labor. Luckily after a lot of research I bought a used 2004 c320 fuel pump and sending unit for less than $200 from Apex auto parts (highly recomend this guys - dealt with Jim & Jeff). Used a Haynes manual for a DIY and had to use some modifications using my old sending unit to connect the pins - not hard - ( see dedicated forums on this) recomend changing the fuel filter at the same time since it may have been clogged.
The Creation of a monster
My car had been idle for a few days and had disconnected the battery - poor me!
After reconnecting the battery my car run ok but turn signals wouldn't work, trunk release, wipers, seat adjustment controls , back up lights, remote entry - You should have seen and change lanes like a mad man. Otherwise long story read forums that recomended changing the canibus or something - bought and changed even the working fuses -no luck. remember I bought the car against my wife's advise - Called the dealership said they would charge $ 130 for a diagnostics then parts and labor. Boy was I trapped. So I make an appointment to go to the dealer for a diagnostic figuring if I know the root of the problem - I may be able to buy parts and fix the car myself. A few days before the appointment I was running late for a training session - and could not find my regular keys. So I go to the drawer and pick this one key that I dont like because the keyless entry had never worked - it was all black - must be the original key. After driving for like half a mile a weird thing happens - I see my wipers move once twice ... am excited ... check the signals ... they're working, try the seat adjustment thingies - they are working - excited cant wait to park and test every thing - all is working except the gas gauge - ( I forgot to mention that one) but right now I dont care abt thae gauge - Guess who I call 1st - ( Hint - I told you we were not screwed) and then 2nd. Have been driving for some time - and though the gauge is still dead - everything else works.
I f you encounter the same problem try driving with the mother key.
Good luck

Hi, I have a w203 c230 2006, following some 50,000 miles my OEM battery seemed to be on its way out, therefore I replaced with a like for like Bosch battery.

I followed the re-connection sequence detailed in the manual and no issues and faults, however some 400 miles and 3 weeks later, the same problem (not able to re-start car).

I have had my local garage look at the car and they carried out a full battery and alternator test, battery drain and re-charge, and they are unable to find any faults, as the alternator is registering normal.

Does anyone have an idea what might be causing this issue, thinking of taking into MB dealership, but cringing at the thought of what they might insist on charging or might find.

Sounds like your battery is being drained by a faulty seat module. Do a search on seat module. You can first disconnect them and see if the problem goes away and there are several fixes/workarounds listed if that's it.

I have done some research around seat module, and the only reference i can find relate to memory seats, however i do not have memory seats, therefore would i still have a seat module?

No, sorry for the false lead.

i recently replaced my 03 c230 battery and everything was ok. while out of town my neighbor informed my my fog lights came on during a thunderstorm. my car might have been struck by a bolt during the storm. when i got home i jumped the battery and the car started fine. a few minutes after turning the car off the fog lights came back on and will not turn off. any idea?

I put a new battery in my car and now it won't crank. Fuses all seem to be fine. Is there a reset that I need to do or something?

My old battery was completely dead, when I put the new battery in I did accidently attach the possitive first and now it won't crank.

youre suppose to do positive first on reinstall it goes when removing you do negative the positive then reinstall you do positive then ground.

So any idea why is won't crank now?

Is there a relay for the starter or a fuse hidden somewhere that may have blown. I tried using my other key to start the car, tried unhooking the battery and reconnecting it but it still won't crank. Has anyone else had this problem?

The old battery was completely dead. I removed it and got a new battery then pit the new battery in positive cable first. Everything else works fine just no crank.

check your fuses and relays in front and rear SAM. Make sure the battery is putting out enough voltage

Is rear SAM the fuse spot on the side of the dash? How do you check the relays? I'm missing the fuse diagram.

As for the battery. When I got the car it had the smallest battery that Advance sells, only filled half of the battery box. I had no problem with the little battery until it suddenly died. I took it to Advance since it was still under warranty and I got the biggest battery that they had which was supposedly the minimum that the car requires according to Advance. I can't remember the size but it's twice the size of the battery that was in there and fills up the battery box. I'll check on the size of the battery and may try to find a bigger one.

I don't think it's the battery. Where are the fuses and relays that I'm looking for?

Can someone help me,car was stored for 2 years replaced the main battery ck the lux battery okay recharged .lights coming on in dash . i think the car when in safe mode i have the trunk boutton light is red climate control knob light is red and also were the shifter boutton are in red,when i turn the key lights coming on in cluster . please help iam going crazy...........

Just thought I would weigh in with my experience. I changed the battery on our 03 c240 tonight. Car has 97,000 miles on it and had a Mercedes battery in it. Don't know how long it's been there but it had definitely been changed before. I swapped it for a Bosch 49-850B from pepboys. Cost was $108 and I found a $20 off coupon on their site so was a good deal. Unhook negative first and hooked it up last and did not run into any issues at all, only thing I had to do was reset the clock.

My problem after disconnecting my battery is that my seat heaters no longer work on either Side but were fine prior to disconnect. Any advise on resetting them.