Help, changed AUX battery now car won't start+more faults

[SpiceWeasel]

Hi, I have done something stupid and now car won't start, no central locking, no radio and rear lights error messages on dash.
I have a 2013 C200 and had the main battery swapped recently + coded to the car at Halfords. My stop start still wasn't working so I ordered a new Varta AUX14 battery. I have already had the aux battery out before I ordered a new one to check for ID marking etc and put it back with no issues. When I had a recent service we checked the battery capacity and it was only showing as 30%, original Merc batteries so 9 years old.

I removed old aux battery and was cleaning some gunk up around the battery tray and sprayed with some rust proofer, while I was doing this the ground lead that connects to the aux battery shorted out to the positive terminal on the small box (charge relay?) briefly. I removed the ground from touching and then fitted the new AUX battery. I went to start my car to test it and the car turned over but would not start. So I thought maybe it needed locking/unlocking again. So I shut the boot and doors and went to lock from the key fob and the car wouldn't lock/unlock, I had to get back into the boot with the emergency key.

I have checked all the fuses under the bonnet, one next to aux battery and the fuse box in the boot and everything is fine. I have rear right side marker lamp, rear left side marker lamp warning message on the dash, no radio and I can't lock/unlock the car with the fob or if I use the emergency key in the door/boot. Engine turns over and keeps turning over but won't fire up, I turned the key off when this happened for more than a few seconds.

I should of disconnected the main battery when changing the aux then I wouldn't of had this happen. I already had the aux battery out before without disconnecting the main battery and car was fine after putting AUX back in about a week ago.

Does anyone have any ideas or anything I can check? My Dad found something out about the pre fuse block (near main battery but is supposed to be non servicable) which has big fuses ( you can buy replacements for these). Feeling like a bit of an idiot at the moment. I have attached a photo so you can see where I shorted to. Is there another fuse box inside the car? Manual says it's on side of dash behind a cover but nothing either side.

Thanks

 

[mikrt]

See this

Help please Urgent? | Electronics and Audio

So I was disconnecting my 2013 W204 aux battery to charge overnight, but the earth wire I disconnected glanced the live cable underneath it (Uppermost black cable with red sleeve below battery cables) Connecting the battery relay control module. After the spark and lots of cursing, Half the...

forums.mbclub.co.uk

Sounds similar to my issue. It's a little fiddly but fixable.

 

[SpiceWeasel]

See this

Help please Urgent? | Electronics and Audio

So I was disconnecting my 2013 W204 aux battery to charge overnight, but the earth wire I disconnected glanced the live cable underneath it (Uppermost black cable with red sleeve below battery cables) Connecting the battery relay control module. After the spark and lots of cursing, Half the...

forums.mbclub.co.uk

Sounds similar to my issue. It's a little fiddly but fixable.

Thank you mikrt. I am going to have a look at that pre fuse assembly near the main battery tomorrow. Looks like "Wilco" sell mega fuses, looked at eurocarparts and Halfords as well. I still can't believe I actually did it, there was some dry gunge near back of battery tray so I was just cleaning that up when I must of knocked the lead. Hopefully just a fuse blown, I will update if I find anything tomorrow.

 

[mikrt]

Thank you mikrt. I am going to have a look at that pre fuse assembly near the main battery tomorrow. Looks like "Wilco" sell mega fuses, looked at eurocarparts and Halfords as well. I still can't believe I actually did it, there was some dry gunge near back of battery tray so I was just cleaning that up when I must of knocked the lead. Hopefully just a fuse blown, I will update if I find anything tomorrow.

I know exactly how you feel.
Confident you'll get it sorted though

 

[SpiceWeasel]

It's alive, it was the 200A midi fuse the was blown in the pre fuse assembly. Bit of a nightmare to get the assembly out though, and a fair bit of swearing involved when doing the whole job

Thanks a lot for your post/guide mikrt....and to everyone who contributed. The job is very fiddly but much cheaper than going to Merc to repair. The fuse cost me £3 from an auto electrical store.

I do have a few tips that might make it easier if anybody else needs to do this job:
1. Disconnect both batteries, and remove the main battery as this will give you a bit more room to work on the pre fuse assembly. Make sure you wrap a cloth or something non conductive around the ground battery terminal lead in the boot, you don't want to repeat this job again . You can leave the rear battery in just remove the ground.
2. Undo the P-clip holding the air con pipe nearest the wing as this will allow you to move the pipe slightly towards the rear of the car and gain a bit of wiggle room to undo the top nut nearest the hinge point for the top cover of the pre fuse assembly... the bit you can lift up that has the jumper cable point. I found the pipe was just in the way to remove the last nut nearest the battery.
3. The clip that holds the pre fuse assembly to the body is a pita to get off. Once you have the top 3 cables undone pull out the two top clips away from the body (circled on photo) and lift up the pre fuse assembly. It won't move up very much but try and keep it up, then you need two screwdrivers to go down the holes (arrows on photo) nearest the pre fuse assembly to release 2 of the same type of clips you just undone previously. Now the tricky part is to keep the plastic clip assembly pulled up while at the same time pulling the 2 lower clips away from the pre fuse assembly with the screwdrivers and then you need to try and slide the pre fuse assembly downwards so the top hooks come away from the clip/pre fuse assembly. You can then remove the whole clip and carry on. See photos attached to see what I mean.



4. There is a cover that hides the cables that you need to remove, then I found you can remove the two big cables first and both cable wire assembly clips. You can twist the assembly into the space the battery was a little bit which will make removing/refitting the other 3 cables a bit easier.

5. You will need to cut a cable tie underneath the pre fuse assembly to give you more room with the cables. You can do this before step 4 if you can get to it....not easy to do which is why I took off the 2 big cables on the side first to gain a bit of wiggle room.

6. Once you have the pre fuse out you need to remove the cover, again a bit tricky. If you look on the back there are a few rectangular holes on the edges, if you get a small'ish flat head screwdriver in the holes you can pry away the locking clips and pull that part of the cover away. Just work your way around carefully, it's a bit fiddly but not to bad once you know how.

7. My last tip is to take plenty of photos, just to make sure nothing goes back in the wrong place or back together wrong. And make sure you check all the fuses in the pre fuse assembly. Check everything twice to make sure when doing this job, and everything is cliped back in properly.

I hope this helps someone else out who finds themselves in this situation