Car is 2003 SL55 with about 140k miles. Very well maintained.
Both batteries were replaced back in 2017, so they are not new.
When parked in the garage for more than a few days, the rear battery is always connected to a CTEK maintainer. The car always starts like a champ, but I do get the red battery warning light for an unknown to me reason with every start. The red light became very consistent over the last 5-6 months.
Both batteries show to be charging well when the car runs. I tested both with a battery tester a month ago and they both were healthy.

About 3 weeks ago, I tried to start the car, and it barely started and then died - it was cranking well, but it was sputtering, almost felt like it was not getting enough fuel and cutting off. I tried to start it 3-4 times, and it acted the same way. I was 95% sure it was a fuel related issue. On the 5th time, it started just like normal, as if nothing happened earlier. I drove it around, and it felt like it was a bit down on power at first (a bit sluggish), but then it went back to normal with full power. No issues. I restarted the engine again and all was good. I blamed it on "bad gas" or some dirt in the fuel (my fuel filter is 1 yr old btw) and moved on. I parked it in the garage and did not investigate further.

Since we went on 2-week vacation, I completely disconnected the rear battery while we were gone. I connected the battery today, tried to start the car, and it hesitated to start the first time again !! On the second try - it was all good. Started with no obvious hesitation. I drove it around the neighborhood for 5-6 minutes, parked in the garage and turned it off. Decided to start it, and now I get just a single click and no start. The dashboard lights come on, i can lower the windows, change gears, stereo comes on, so it all gets power, but I can only here ONE click from under the hood. No engine cranking at all. I moved the gear selector to "N" from "P" - it made no difference. No movement or attempts from the engine to start at all. Just a 'click'.

So the big question now is - do I have a bad battery or is this something else, like a failed starter?
If a battery - which of the two batteries do I suspect?
If not a battery - is this how a starter normally fails? It gives me just one "click"? No other noises, errors, etc?
If not a starter or battery - what else could it be?

Thank you in advance for any tips and suggestions!

 

You can jump the starter relay in the front passenger Sam, I believe it’s the green one and it will tell you if the starter or battery cable to the starter is bad. Unfortunately on these Rube Goldberg contraptions anything could be the culprit.

 

Does not sound like a bad starter or a bad battery to me. Sounds more like some kind of sensor that's giving incorrect readings at start, causing fuel or ignition to be out of whack.

Could be the ignition switch itself, intermittently failing to pass signal to fuel, ignition, and/or the starter.

If you're going to DIY on any Mercedes, you really need to have some kind of OBDII code reader at bare minimum. It's very likely there's some information stored in your car's computer that'll tell you what's going on.

Check the stickys at the top of this forum for a thread on one of the better cheap solutions. Or if you're up to spend 600-700 you can get a fully functioning Xentry system, same as the MB dealers have, which has diagnostics, Workshop Information System (repair procedures), and Electronic Parts Catalog. If you want to go this route, I'd recommend you contact BenzNinja. There's cheaper Xentry clones you can get from China, but they often lock up and can't be recovered, and they have -0- support after the sale.

 

Update .... it looks like it's the battery under the hood.

First I plugged my old C3 STAR scanner and one of the few faults stored was under the BNS module, and it stated "B1832 - Component G1/4 (Starter Battery) is defective"

Since I was not convinced, I found my NocoGenius jump starter, connected it to the front battery, and after the second try, the car started perfectly normal. I let it run for 5 min. Disconnected the jump starter, turned off the engine - it could not start again. I am getting the single click again. Clearly, battery is bad. It's an OEM battery, not too old, but I guess it's time for a new one.

I will try to get a non-OEM battery tomorrow from a local parts store, and hopefully this will be the solution to my starting issue and the persistent red battery warning light I've been getting lately.



Thanks for all the input!

 

Just an FYI - starter batteries were discussed here a while back, and the conclusion was that it appears no matter where you buy a starter battery for these cars in the US, MB dealer or aftermarket, you're getting the exact same battery. MB doesn't ship replacement batteries over from Germany; no car manufacturer does, they're always sourced more locally by their regional distribution. There's only two companies in all of North America that actually manufacture car batteries, and the size/type for these cars ( group V4) applies to very few cars so there's no incentive for any real competition. You'll find that no matter where you go to buy, you're going to get a battery with a white case, 520CCA , 35AH reserve capacity.

 

New battery is in. Car starts just fine now. However, the red battery warning light was still on, even after I cleared the fault.

I checked with a voltmeter if the new battery was charging, and surprisingly it was not. It was showing 12.4V with the car running. Rear battery shows 14.5V.

This is very bizarre because about a month ago, I did check the front battery, and it was charging just fine. Something clearly changed.

So, I went on a mission to check the fuses under the passenger side foot well. With some moderate effort, I got to the fuses, (what a fun location) and sure enough, one of them, the smaller, 100amp fuse, was burned. How and why that happened, I have no idea? Did it happen because i "jumped" the car with my NOCO Genius car jumper 2 days ago? Or was it just old age? I mean, a 100amp fuse is pretty beefy, it can't just go bad for no reason.

Now I am waiting for the replacement fuse to arrive from Amazon and I hope that it will fix all my battery related issues.

 

New 100amp fuse installed successfully. Front and rear battery are not both charging properly.


The bad news - the Red Battery light still shows up on the dash when i start the car!! I reset the fault with my STAR and it still comes back, saying it's either a "faulty K57 relay or no circuit 61"

The relay was replaced, and the fuse is confirmed new now.



WHY does this keep happening? Where is this red light coming from? Could it be the rear battery being old (it's 5 yrs old) and causing the issue? My understanding is that the red battery warning light is related to the front battery, not the rear.

I am out of ideas at this point.

 

Time to pay for a professional diagnosis.

 

It sounds like a fried BCM then. I have identical issues and my front isn't charging.

Even reading your first post brought me down memory lane until the red light stayed there permanently.

I haven't got into the footwell yet for that fuse- but it sounds like you did and that didn't fix it

Only 1 thing left is the BCM.

 

I really hope you're wrong on the BCM, but I do see it as a possibility.

After the fuse replacement - both batteries are charging now, no issues with that. So it fixed the charging issue.
But the damn red battery warning light is still there, with the same error I have been getting for months now. The new fuse made no difference.

I took out the BCM out a couple of months ago to inspect it, since some people reported corrosion with these modules, fires, etc. My BCM looks like new inside. Everything is clean and shiny. No rust, no stains, no leaks, no visibly bad components.

I just ordered another brand new, OEM relay. I know that these things are pretty universal, but the last new one I got was colored black, and the old OEM I had was gray color. So, i am hoping, it was just the wrong relay and the new OEM relay will fix the issue.

Other than that .... what is really "circuit 61" ? Isn't that the same circuit with the blown 100amp fuse i just replaced?

 

Oh that's very good then..you didnt write that in your previous post(charging fixed)

 

I'm wondering if its not a marginal rear battery? I had a new AGM rear battery that turned out to be a bad one and I was getting all kinds of codes + the Batt warning light. It really had me chasing my tail until I replaced the rear battery. I am believe that it's the BCM that's triggering the Battery light on the dash. And believing that the BCM is comparing battery voltages and seeing that one or the other voltages is off, even though it is receiving the proper charge voltage. 5 years is quite a while for the rear battery. I'm thinking that on your STAR, you can watch the voltage and after you turn the engine off, but with the Ignition on, if the battery voltage drops fairly quickly, that might give you a clue.

 

I replaced the rear battery with a brand new one 3 days ago. It made no difference.

Now I have 2 new batteries, front and rear, they both charge fine when the car is running, yet the red battery warning comes on every time I start the car. I reset the code and it comes back.
The interesting part is that the K57 relay next to the BCM is always warm. If car is off...relay is warm. If car is on, relay is still warm. I replaced the relay - no change. This means the K57 is staying closed/energized, and the only time this should happen is IF the system is in 'emergency' mode. There is no reason for the charging system to be in 'emergency' mode given that both of my batteries are new, alternator is good, fuses are good, and both batteries charge fine. This leaves the BCM being bad. I ordered a new one (close to $700) so once it arrives, it better fix the problem.

 

Sounds reasonable to me. I also nearly bought a new BCM (the part number was a revise # so likly an improved part) until I replaced the "new" rear battery. Mine was pristine to the eye as yours apparently was too. Can't wait to hear the result with the new BCM and if the relay stays cool too.

 

sometimes, you need to use jump starter to help your car start. like lokithor, noco and more

 

Not advised on these cars, it leads to all sorts of electrical issues, and would bet leads to damaging the battery module.

 

Yes, the jump start definitely damaged my 100amp fuse in the foot well. Not expensive to fix, but a pain to get to.

 

I’m having the exact same symptoms but my car is a 99!!!

I was able to start it up after I had the modules reset in like 10 minutes time, but since then I have not been able to clear the system since I don’t have access. (And apparently there is something different that the STAR does that iCarSoft v.3.0 can’t perform)

 

Let me see if i got his correct. Both the consumer battery in the trunk and the starter battery in the engine bay both charge as they are supposed to.

You have used a maintance charger and you still have the red battery light lit up even with new batteries?

 

If his car is a 99, that's an R129, not an R230.