First some backstory:

I recently have had a problem where my car wouldn't start until several attempts. Like a crank sensor issue. I changed the sensor and the car started right up and ran for 3 days until it dies while driving. I killed the battery trying to start it, towed it home and charged battery and it fired right up. The next day went to use the car and it wouldn't start again. Thought maybe faulty new crank position sensor so bought another one and it wouldn't work. Screwed around with the connector thinking that was the problem and even cut the connector of and crimped on different connector but still inconsistent start and not start then finally wouldn't start again. Had 3 or 4 injector codes which threw me off and a map sensor code.

Now 2 weeks later still trying to start the car.

I replaced the map sensor and replaced the cam position sensor and went to crank the car and it was turning over pretty slow. I thought the battery was low so charged it overnight on 6amp charge and tried again but same thing. Took battery to auto parts store and their machine said good battery, 13.7 volts and 780 cranking amps. Put back in car and same thing barely cranks over. Thought maybe I wore the starter out (car has 120k+ miles) so I got another starter and put it in and same thing, slow start. I then took the battery from my other car that I have been driving every day and same thing slow crank.

Brought voltage screen up in display and it says 12 volts then as soon as I start cranking it drops down to 10.2 and does the slow cranks. Did the exact same thing with both batteries. I happen to have another battery control module so I put that in and try again, same thing, 12 volts on display, drops to 10.2 while cranking slow.

I don't know what else to try? Does the front little battery have anything to do with the car running other than being backup for the braking system? I put a meter on it and it said 12 volts though. This car is killing me, I am at a loss on what is left that could be causing this slow cranking and it not firing up.


CLIFF NOTES: Changed battery, starter, battery control module in trunk, crank sensor, cam sensor, map sensor and car cranks over slow and will not start. Has fuel pressure and has spark while cranking. The spark is just weak from I assume it cranking over slow.

What else is there that could be messing up the engine from cranking correctly and firing up?

Fuel pumps and check the fuse junction box in the passenger footwell

Front (small) battery has nothing to do with cranking / starting the car, its for the accessories and related conveniences.
That's an odd issue you're having, its appears you have checked and changed everything that would cause this issue.

I'm pretty sure that is a myth that gets repeated often. The small battery is a dedicated SBC battery.

Check and clean starter and battery connections. Any bad/corroded junction will cause a voltage drop when the current begins to flow through it. What codes (if any) are you getting?

+1 on Hulk's suggestion of the front pre-fuse block under the floor board, starter motor connectivity goes through there. See attached.

The small battery located in the front is for convenience functions and also supplies adding voltage when starting if the rear battery is low through the BCM and is not strictly used for the SBC. Now that is a myth!

Most definetly wont "add voltage" when starting... its pretty hard to clearly state this as english its not my native language.

The whole reason why we need this backup battery is because of the brakes. If the alternator goes bad or the main battery, its there to suply power for a limited short time to get the car stopped, once again because the brakes need power. Of course in an event of total failure of alternator and main battery, the aux battery is dilivering power to all consumers still active on the car. the BCM job is to monitor when to charge the aux battery, switch off consumers when main battery is low and finally connecting the aux battery when the main battery runs out of juice.

Its very well documented here: http://bayhas.com/mercedes/w211/contents/INDEX.htm

Electrical-battery

I read a bunch of that. It says the small battery is short time back up for main electrical system failure.
I think that is the only thing I've learned today. Thanks.

my apologies if this has been answered or covered already ...

did you use an actual Bosch crank position sensor, or just an (o'reily, etc) equivalent? It is fairly common for "oriely's best" to be not good enough on this part in particular.

good luck and happy holidays to all,
Chris

I had the same problem check the voltage at the end of the main battery cable where it connects under the front carpet well on the passenger side. my battery cable was corroded inside the plastic covering.

I will check the battery cable at the box location mentioned in passenger area. I have not done that or even knew it was there. I also went and bought a brand new rear battery today so I have both front and rear new batteries now.

As for the crank sensor, I replaced it with a bosch when I first had the starting problems where the car would start sometimes on first crank then other times would take a few tries. I then had the car die on me while driving 3 days later and then killed the battery trying to start the car a ton of times and at that time the car was cranking at normal speeds until the battery was low. At that point I bought another crank sensor from Autozone and installed it (not a Bosch) thinking maybe the new bosch sensor was faulty.

My current problem now is just getting the car to crank at a fast rate like it should. I am hoping at that point when it does crank fast it will start and run since I have replaced all those sensors mentioned.

As for codes I haven't been able to read them again lately since the car is at my house. Before it was at a friends shop and got the map sensor and injector codes mentioned. I then drove the car home and drove it a few days before having all these problems I am now dealing with at home.

Just checked voltage at the trunk battery and it was 12.5 volts, I then went to the area in the pic posted of that connection point under passenger carpet and had 12.5 volts at the big black cable and also at the 2 smaller cables at the top of that. They all looked clean from what I could see too.

What do you mean by "inside the plastic covering?"

There is the rubber/plastic jacket of the cable, then the actual metal ring terminal crimped on the end of the cable. Do you mean you cut the jacket of the cable off before the ring terminal? Or do you mean there was corrosion on the ring terminal where it is connected to the post up in that plastic junction box?

My connection points look clean everywhere visible but I didn't go cutting the jacket if that is what you mean?

if you have not done so already, measure your voltage drop between the battery and starter while cranking. If the drop is excessive, measure it while cranking at any and all junctions (between battery and starter) to isolate the point of excessive resistance.

if you don't have convenient/easy access to a star system, you would do well to have a code reader that reads Mercedes specific codes (like an icarsoft i980, etc).

good luck,
Chris

Check all fuses in there. If you open up the part of the box on the left where the 2 small cables go, there are two fuses in there also, 150amp each

The starter is sorta kinda hard to get at lol. I just changed it and it sucks. It does drop though, like I said before I will click the key to 1st position and then hit the "reset" button 3 times for the gauges to bring up the voltage screen. It will then be reading say 12.5 or 12.2 or whatever and then as soon as I start cranking the voltage would drop down in to the 10s.

The code reader I have access to is a Snap On scanner (no clue what model) at my friend's shop.

I will look deeper tomorrow after work when I have some light. Thanks.

What you need to find is how much drop there is between the battery and the starter and where it is dropping. While it is mildly useful to know that you are monitoring your battery (at a yet, undefined point) and that you can see that the voltage drops (from 12.x to 10.x) it tells you little about where the problem is (once you have ruled out that your battery is not shot or your starter is not semi-shorted internally).

This is the basic process ...
If you can get one lead of your meter on the "+" terminal of your battery and the other on the power wire of your starter and read it while cranking, you should read less than .6vdc (that is the actual circuit voltage drop), If you read less, the explanation for the lack of starting is likely that your battery is bad .... if you read more (than .6vdc drop), you should isolate the "drop" to a specific connection by moving one of the leads down the chain of connections towards the other (ie move your lead on the battery to the next junction between it and the starter and repeat your test ... continue doing this until you get to the last junction before the starter) .



If you don't have access to the schematic for this circuit, I can dig it up for you in the WIS or ETM

Chris

oops, I should have looked more carefully in the thread. bbirdwell already posted some good info on where to check for starter circuit voltage drop .

You can use his diagram to assist as you check for voltage drop between the battery (G1 +) , the input of module F32 (input is "terminal E" in ETM), the starter side of F32 (called "termainal M" in the ETM) and finally terminal 30 of the starter "M1".

hope that helps,
Chris

Did you figure out the problem? I'm having the same issue.

Don't make any sense to me but the passenger side fuel pumps connector was melted some and causing bad contact I guess, the fuel pressure was there when I checked in the middle of all of this so I am at a loss. I brought the car to a friends shop since I go tired of fooling with it and told him to check a few things like the connector and sure enough he chipped away melted plastic and car fired up. Cleaned it up some where plastic was melted and the car ran and had good fuel pressure for a solid 30 min of idling. I don't understand the slow cranking though but once that was messed with it seemed to be cranking fine and would fire right up.

It has failed a few times since then though and I parked it and bought new oem pumps for the passenger side off ebay and that has been a nightmare in itself. Car has been down for 2 weeks now with no pumps. Place had picture of correct pumps listed and correct part number then I received different part number and pumps not the same.

They told me there were superseded with this pump setup they sent but the pumps have male nipples on them to connect hoses where our pumps have female holes where an o-ringed fitting goes in the pumps and a little plastic c clip holds them in place. Then there are hoses built in to the basket of our pumps that you then hook a quick connect hose to that goes over to other side of tank but these new pumps they sent didn't have any of that. It took me almost 2 weeks to finally get them to give me a return label after I told them I was leaving negative feedback and contacting my credit card. They kept telling me the pumps should work and I sent them pics of the pumps sitting next to each other showing they clearly will not work.

I ended up purchasing another set of pumps from elsewhere and they should be here any day.

wouldnt this be part of that whole "non-recall" TSB?

I am not sure but I am about to lose my mind. The 2nd pump setup arrived from a different place and they sent the wrong pump now, how in the hell can this be happening to me. They sent the driver side pump and ironically it has connections on it that look like it would connect to the other pumps I just sent back. I have a scary feeling that I am not even going to be able to get the correct pumps now for the passenger side to connect to the old driver side setup and am going to have to buy both due to the hose connections being wrong.

They sent me part number A 211 470 64 94

if they are oems pumps i believe they have been revised by MB with new design and i think you would have to change both of them with new p/n

Yes they are OEM MB parts purchased from businesses online, one from Ebay then one from Amazon. I wonder if I can get an aftermarket pump then that still is setup like the existing ones in the car where I don't have to buy both sides at over a $1000

In that case you have to look for older p/n pump