Developed an issue with my 08 ML350, I replaced the 7 year old main battery over this past winter, everything has been fine up until this point. My wife was out running errands the other day, came home the car sat for about an hour, we went to leave again and the car wouldn't start. It cranks really strong for like 2 seconds and then just stops right before the car would normally start. Put a battery charger on it and the light was showing green like it was fully charged, switched it over to the 50 amp engine start and it fired right up. Went to AutoZone to have the battery tested and they tested it at the connection points under the hood and it failed on the cold cranking amps. I got the battery from Sam's Club so I went there and they tested as under the hood and confirmed the battery failed and gave me a new battery under a warranty swap. I had them test the new battery before I left the store and it tested good and had over 1100 cca. I swapped the battery out in the parking lot because they won't install batteries if they are under the passenger seat, new battery in and car did the same thing, it cranks strong for a few seconds and then just falls flat on its face. Had them come out and test again and under the hood it only has 550 cca and directly at battery it was testing at 940 cca, I then jumped it and went to Smyth auto parts and had them test everything, battery fails under hood but good at the battery, alternator was charging at 14v and of course the car started 2-3 times in the parking lot while doing tests so the starter showed good. There arent any signs of corrosion on the cables or anything and jump starting the car gets it running. Are the starters easy to get to on these cars? Easy way to test? Any other suggestions on what could be going on?

 

Any codes?

 

Nope, nothing. I even scanned it to make sure and no codes.

 

I would check the starter, but I'm not aware of any way to do this without removing it. I once had a starter with a worn sleeve bearing (not a MB) that exhibited this same behavior.

 

Have the output of your alternator checked. If the regulator is faulty it might be cooking your battery. Start with a clean slate and check all the grounds to make sure they aren't corroded. Maybe even a parasitic drain somewhere? See video below

 

Ive considered just replacing the starter since its not very expensive.

 

Very informative video thank you. I guess I cant fully rule out a parasitic draw, although putting a brand new and tested battery in and having the same result I feel kind of rules out this being the main culprit. And the alternator was charging at 14v when tested at the battery by the auto parts store. I was leaning more towards a bad cable or ground (even though I dont see any signs of corrosion), starter going out (even though I've never heard of jump starting a car with a bad starter), or some kind of fuse or relay.

Is there additional testing on the alternator that you are suggesting?

 

https://www.omegaowners.com/forum/in...?topic=90619.0

Follow this guide to the letter and report back.