[peterboattransporter]

Just changed the AGM battery on my B class and it wont start ,read the procedure after I done it ,how stupid is that .
it just lights up and tires to crank ,i think I have blown something like a diode or something on the positive terminal .
I have just left it overnight hoping that it might cure itself .
When I disconnected the battery I took the positive off first also the ignition key was inside vehicle .There was still a circuit live when i pulled off the terminal as it sparked

 

[plinker17722]

peter Is this your auxhiliary battery for stop start ? Try charging up the main battery, or check the voltage in it first...

 

[peterboattransporter]

No , it was the main large heavy battery under the bonnet

 

[plinker17722]

So AGM battery is for stop start not main battery .

 

[dedgar]

First thing I would have done is scan for codes.

 

[peterboattransporter]

Thanks done that and nothing, no pending nothing stored.
I am thinking that I may have damaged the battery positive terminal junction box .
When a start attempt is made the exit terminal shows 4.3 V ,now i am sure that it should be the full 12v as this is a starter motor type cable .The relay in the box can be heared and felt but nothing happens ,Any information would be helpfull

 

[plinker17722]

Newish Mercedes , like you have , will need special equipment to find out just whats been damaged .You may not find a quick fix from the forum . But you never know .What you did it might have been done before by some one else .Just foundthis video could just be your problem

 

[peterboattransporter]

Thanks for that info ,I think that might be the problem but I would want to be certain as the module will be very expensive .O have had the car from new so I have only had mercerdes service it and changed the brakes myself

 

[DEVOURS]

Thanks plinker17722



Some of these vids can really save B owners a ton of $$$..if you are handy with a few tools. Hope to hear that your problem was solved using that video.

 

[peterboattransporter]

There are 2 AGM batteries ( Advanced Glass Matt ) one main one under the bonnet about 100amp which does the stop ,start and a very small auxilary in the passenger footwell 1.2 amp which is probably for the clock ,radio,alarm maybe

 

[peterboattransporter]

The video did help a bit ,I am 90% certain that it is the positive battery terminal module .I have taken it off and examined it closely and you can see that the 3rd. terminal is squwed to one side and a slight break in the plastic housing I think its a pyro fuse . ,I will dismantle it after I have sorted it out

 

[peterboattransporter]

FOLLOW UP
I have to report that the car is all fixed now ,I was able to purchase a used Positive terminal module form a breaker in Lancashire who purchases Mercs and BMW from auctions then breaks them up and sells the bits .
If I had been brave enough I could have made up a short lead to jump across the 2 terminals bypassing all the quick blow fuse system but I was mindfull of creating a bigger problem than I had .
With the new battery working the stop start now works ,you need a main battery with more than 90% to make it all work,I can see the logic in that.thanks for the help

 

[plinker17722]

peter thats good news, another happy bunny,