Never had a start issue in the 2 1/2 years I've owned this S550. The battery is fine I believe. Everytime I start the car, the starter always spins nice and fast. I have the push button start and when I press the start button on the dash, there is like a quarter second lag before the starter spins. It's always been that way, so I considered it normal, just thought I'd add that. This morning at 10am I started my car and drove to work, no issues. 5pm comes I start the car normally and go home. An hour later I try to start it to go to dinner and while all the dash lights come on I only get one faint click. I turn the car completely off and try again, same thing. Starter I'm guessing? I just ordered a diagnostic tool on Amazon and I should have it by 8am tomorrow.. What do y'all experts think?

 

Well, when working on these cars I've learned you can never assume anything and start with the basics. You said you believe the battery is fine but you need to find out for sure for sure if you have a charger put a charger on it for a couple hours and then try it or take it out and take it to your local auto zone, etc and have them load test it. Its already a small tiny battery under that hood and I only get about 2 1/2 to 3 years out of mine down in hot Florida.

 

Well I feel like a fool. I was convinced it was the starter so I called AAA for a tow. I had previously hooked up my trickle charger to the battery and it said the battery was 100%. The AAA guy said let's at least give his jumper pack a try. It started right up! Guess I'm ordering a new auxiliary battery tonight!!

 

I recommend either buying the correct one from Mercedes or I know that Interstate Battery makes a exact replica of the exact same size it is a small tight fit others here have reported I forget what group size they say will fit but I don't think you can use the Mercedes mounting bracket

 

Based on the info you give in this post I think your starter/solenoid is bad. The thing in your post that points that way is that it starts when the engine is cool but then It won't when it is hot. Failing starters can do exactly that.

But, let's see how it goes after the small AUX battery change.

 

I did end up going to the dealer and got the OEM battery, $253 plus tax. I put it in and all is fine now. I don't think I had any starter or solenoid issues as the not start condition just occurred once and the new battery took care of that problem. For whatever reason the engine seems to run more smoothly that it did before. Honestly it ran great all the time but now it seems to run silky smooth.

That leads to a question. Does the engine while running get electricity for the spark plugs from the auxiliary battery I just replaced or from the main battery? If from the auxiliary battery, that would explain the engine running better.

 

no

maybe they gave it an italian tune up
you got some good petrol
or it made you think time to upgrade - most vehicles seem to behave better when you look at something else / get back in yours after a road test - which I guess means most modern stuff isn't as clever as the marketing

 

Just wait until your new battery charge goes down with the smart charging and you may run back to the issues. Depending on how much you drive. It took about a month form my starting trouble to start again after battery change.

When problems return don’t get another battery. Give the new one you now have a good charge and it will be ok for another month.

Or fix the problem..,

Just saying…

 

The old battery I replaced had a build date of April 2016, so it was 8 years old. I'm guessing I was overdue for a new battery. This S550 is my year round daily driver and I put on about 10,000 miles annually; it will stay charged.

 

In the car the charge will drop to about 80% from full charge.

But if it is ok now then very good..!

 

Wow. 8 years is an eternity for any battery that is awesome that it worked out. Like I said when working on these cars never assume anything and start with the easiest things first like the battery... a hell of a lot easier to change than a starter let me tell you. The next thing you should check just on principle not that you have any problems now but you should check the date of your consumer battery which is located in the trunk behind the vertical bulkhead panel it's a little tricky removing that panel but it can be done without removing the fender Wells. That battery you can just go to Walmart and get an AGM h8 quite a bit cheaper than MBZ battery. When the consumer battery starts to fail all kinds of crazy things happen locks don't work automatic trunk does not work abs will come in and out of functionality all kinds of strange things start happening. It's unlikely that the starter battery has caused the car to run better I usually chalk it up to it being in your head because you are so stoked and excited that it's fixed..... in your mind you think it's running better. I've had this happen a lot to me when I perform a significant repair on the car!

 

the OEM battery (back when made well) will do 12 years no problem at all - my original 18 years old in the garage is still better than the two replacements - they only do 4 years now - to meet the economical age of keeping any new cars running

 

The faint click you hear could indicate that the starter solenoid is engaging, but the starter motor isn't receiving enough power to crank the engine. This could be due to a weak connection, corrosion on the battery terminals, or even a failing starter motor. Although you mentioned the battery seems fine, it's worth double-checking the battery voltage and connections, as sometimes a battery can have enough power for lights but not enough to start the engine. Additionally, the lag you mentioned before might hint at an underlying issue with the ignition switch or starter relay that has now worsened. To get to the bottom of this, I recommend checking fault codes from the Engine Control Module (ECM) and the Body Control Module (BCM), as they might store codes related to electrical issues affecting the starting system. A full system scanner like the youcanic will allow you to read these codes across all modules, providing a comprehensive overview. If you're using a basic OBD2 scanner like the Foxwell NT301, keep in mind it may only access engine-related codes and not provide insights into BCM-related issues. Have you inspected the battery terminals and ground connections to ensure they are clean and securely fastened?