Hi all,
My 2009 s550 has been running flawlessly for over a year now with no issues to report. Last month I went outside to run an errand and I opened the car, put the key in the ignition, everything lit up as usual and turned the key. Nothing... The lights on the instrument cluster did not dim, no audible clicking, just no cranking. I looked at the battery terminals and everything was clean and tight. I started doing diagnostics and removed the starter relay and manually tried to get the starter to engage. Nothing... I opened the positive power distribution box, PDB, (before cleaning image attached) the one next to the battery that connects the battery to the alternator and starter as well as the rest of the car. I did nothing more than pop the over off the PDB, looked at the connections, probably poked at them and tried to crank the car again. Success! Great. So I cleaned up the connections on the distribution box and started my day. Later that day I was going to take a ride to see my daughter about 200 miles away. Long painful story short, I made it about 90 miles into the trip and I began to see my instruments dim then one system fail after another, I felt my car dying around me! I thought the new alternator I had put in a few months ago had died. I got her towed home and charged the battery. The car started fine and I drove it down to my local Advance Auto Parts and had them do a battery and alternator test. No problems... I used my code reader/scanner and watched the charging voltage fluctuate with engine RPM and it looks good. I also had to clear over 200 codes thrown by the systems failing due to no/low power. After I cleared the codes no codes resurfaced after a 20 minute drive.

After than I went home and took the power distribution box, PDB, apart and carefully removed all signs of corrosion from the terminals in the box (there was very very minimal corrosion), and checked all of the fuses as well. I split the box in two and removed the very clever metal fuse between all of the terminals. Again, no real corrosion anywhere. I hit the fuse/plate with a fine grit sandpaper to make sure it was clean and corrosion free. I reassembled everything and cranked the engine and monitored the charging voltage and everything looks perfect.

I'm never happy when I can't find a cause for an issue that forced me to break down on the side of the road about 90 miles from home. Again, I found nothing wrong with the car! There has to be a reason, but I don't see it. My only other guess is that there may be corrosion inside the alternator cable. If so, that would explain the no charge while driving issue, but not the initial no crank.

Has anyone else has this issue happen to them? Any suggestions or possible causes? My total trust in my beautiful s550 has been shaken. I'm trying to find an Exorcist to help...


My power distribution box, sans cover.

 

that box of tricks shows you have at least the mini facelift away from the twin battery madness of the early cars - however it likely means behind the ignition switch is a smaller motorcycle battery that also needs changing now and then

I very much doubt it will help your interesting cause but may as well start from a good place

next to your real issue - it seems after a certain age interesting weirdness becomes many a merc - why and how remain an anathema

its interesting you don't mention a red battery symbol on the speedo cluster - that one usually gets owners of the non twin battery model confused for 18 months before they ditch the car

 

Botus,

Thanks for your reply. Before I replaced my battery, (one of the cells had reversed polarity), and my alternator, I did get a red battery symbol popping up now and then. But, since I replaced those parts I hadn't seen it in over a thousand miles.

I never heard about the small battery behind the ignition. I'll see if I can squeeze under there and take a look.

Any more thoughts? I just seems the the car didn't want to go for a ride that and day it was pissed I forced it to... until it didn't.

 

usually that red battery light on the later cars is a nightmare no one ever discusses how they fixed (if they ever did) - many pop up for months chasing their tail then vanish without trace - I suspect there is some software update dealers do to get them to behave (once all other issues are resolved)

the little battery on the w221 is designed to help you get out of park when stuck with a flat main battery
only on later cars does it do stop start logic control - and on earlier mad cars with SBC aka the Maybach and E class 211s does it provide brake boost back up

failure to start comes up frequently on later cars with duff starter motors....

you could chase earth point and canbus junction stuff under the carpets to see if you have any soggy fun Merc so love to create on ageing cars

 

one other thought that box of tricks you have photo'd, on the earlier cars inside its full of fusible links (big boy fuses) - double check they all behave after your alternator fun

 

I would recommend whenever you scan the car you take photos of all the fault codes before you clear them. I'm sure I can store and view them on my Foxwell scanner I have but its just easier to snap photos on my phone then view them on my pc. Could of had a fault code telling you what was wrong at the time it occurred. I make a new folder every time I scan the car with the date so i can look back and see if theres a pattern.

I also find it interesting that the red battery warning did not pop up while you were driving and the car seemed to stop charging the battery.

I had an issue with the red battery warning popping up, I would start the car 1st thing in the morning and as im pulling out the driveay it would pop up. I would drive to the end of the street, shut off and restart the car and the battery warning stayed off and did not come back the rest of the day. Upon scanning the car I noticed a fault for the battery sensor. Long story short that issue just went away on its own after a few times of the warning popping up.

You mention watching the charging voltage. What readings were you seeing? When I had battery issues I bought an Ancel BM200 bluetooth battery monitor which is very handy for seeing battery voltage while driving. Basically just shows the voltage at the battery posts. It records and graphs it out so I can go back and see whats going on while the cars park. I recently installed a new battery and have the monitor hooked up to get an understanding of how the voltage should look in case theres issues down the road. This feature would have come in handy the day of your long trip to see what the voltage was.

I'm not familair with the two batteries in the W221.

 

that coincidentally is exactly what happens on mine since a recent bit of stupidity turned up on my twin battery car

in mileage terms it suddenly started that silliness inside the last 200 miles and will show on first start of the day - shut-off and its gone for the rest of the day

 

I definitely did continuity checks across the terminals as well as voltage checks as well when the car was running. Everything looks good... unfortunately.

 

The only other thought that I can add here on my single battery 550 is that just sits for weeks at a time since it's a "spare" car at my lady's house. I was wondering if I'd be advised to buy a battery tender/trickle charger. I do put a charge on it once in a while when I've been away for a longer time. Maybe the act of charging the battery with one of the those battery repair, desulfating chargers might be doing the electronics harm and adding to it's insanity?

 

TimC, wow, thank you for the full reply! I deleted codes from the car because those codes were thrown as the electrical system was dying and I really couldn't trust any of the codes stored in it's last moments of sanity. I also take pictures of the code on my phone. I have a fairly good quality TopDon reader and I have been monitoring the battery voltage while driving, graphing the output as you show, and all looks about same as your picture, the voltages wiggles a bit with engine speed but staying usually around 14.5V while normal driving.

Since I've cleared the codes upon the car's resurrection, I've gotten no codes after about 200 miles. I'll definitely keep my scanner attached while driving until I hopefully regain my confidence in the car. I'll also continue my wiring checks while under the car as soon as the weather gets tolerable in the north east...

Thank you!