There's many reasons why a late model mercedes won't start. Hopefully this list will help someone who's stranded on the side of the road with little mechanical experience, or perhaps troubleshooting the summer car after sitting in the garage all winter...if anybody has anything to add to this thread it is much welcomed. If this list can help only a single person then the list has succeeded in doing its job, maybe it can achieve sticky status if it proves helpful
Firstly, does your key unlock the car? this means your RF signal from the key is working and your key battery is good (not necessarily good, but good enough)
Secondly, does your key turn once in the ignition? if the key turns after you insert it into the ignition, it means the EIS has recognized the key as the proper key for the car, bypassing the anti theft preventatives. keep in mind it is possible for a key with a dead battery to start the car, but a low key battery is likely to cause issues with starting
i've also read elsewhere that wiping the infrared window on the key clean can help
now that the key has been eliminated as a source of issue, it's time to delve into the car side of the issue
Does the engine crank?
If the starter motor cranks for a few seconds and the engine doesn't start, then your issue is most likely a CRANKSHAFT positioning sensor. they can fail from exposure to heat (they are directly above the torque converter on the bellhousing). more seldom, this scenario can apply to a faulty CAMSHAFT positioning sensor, or very rarely a bad/slipped timing chain. I personally keep an extra crankshaft positioning sensor as this is a constantly failing item on our Mercedes
It's also possible the gear on the starter motor itself has worn down teeth, and it's not "catching" the flex plate on the engine. changing out the starter is a process that requires loosening two bolts on the bellhousing, and disconnecting the wire running to the starter. installation is the same just reversed
Does the engine NOT crank?a very likely cause is low battery voltage. a common scenario for a low battery would be hearing "clicking" noises from the relays in the engine bay when you try to start the car.
Does your battery continue to die after being charged? it's probably time for a new battery
If a new battery doesn't fix your issue, it's likely either your alternator, or your voltage regulator on your alternator is bad. there's also a large prefuse terminal box that runs between the battery and the alternator is often overlooked when dealing with electrical problems on our cars. it's underneath the passenger carpeting, labeled x4/10 in this thread outlining the fuses and relays
http://www.benzworld.org/forums/w215-cl-class/1625470-w215-hidden-fuses-revealed.html
If the engine still isn't cranking it would also be a good time to check any engine/starter related fuses for your car to see if they are blown. remember that sometimes a fuse can visually look okay but really, it is blown. having a fuse checker in your car toolbox is very helpful.
As is having a cheap code scanner for any codes the ECU might want to show you. because of my code scanner, i'm usually able to pick up codes while driving and then google the fault code for information on whatever is wrong with my car, while driving
Another very important lesson i've learned is DON'T DISCONNECT THE BATTERY WHILE THE KEY IS IN THE IGNITION, this is known as a reason for EIS's failing and also attributed to rear SAM failures
Some faults can be reset by disconnecting the battery and resetting the ECU, the proper method for solving issues is to fix the issue, and then reset the ECU. instead of just resetting the ECU hoping the problem doesn't come back, sometimes your ECU has fault codes that don't show up as CHECK ENGINE LIGHTS
Firstly, does your key unlock the car? this means your RF signal from the key is working and your key battery is good (not necessarily good, but good enough)
Secondly, does your key turn once in the ignition? if the key turns after you insert it into the ignition, it means the EIS has recognized the key as the proper key for the car, bypassing the anti theft preventatives. keep in mind it is possible for a key with a dead battery to start the car, but a low key battery is likely to cause issues with starting
i've also read elsewhere that wiping the infrared window on the key clean can help
now that the key has been eliminated as a source of issue, it's time to delve into the car side of the issue
Does the engine crank?
If the starter motor cranks for a few seconds and the engine doesn't start, then your issue is most likely a CRANKSHAFT positioning sensor. they can fail from exposure to heat (they are directly above the torque converter on the bellhousing). more seldom, this scenario can apply to a faulty CAMSHAFT positioning sensor, or very rarely a bad/slipped timing chain. I personally keep an extra crankshaft positioning sensor as this is a constantly failing item on our Mercedes
It's also possible the gear on the starter motor itself has worn down teeth, and it's not "catching" the flex plate on the engine. changing out the starter is a process that requires loosening two bolts on the bellhousing, and disconnecting the wire running to the starter. installation is the same just reversed
Does the engine NOT crank?a very likely cause is low battery voltage. a common scenario for a low battery would be hearing "clicking" noises from the relays in the engine bay when you try to start the car.
Does your battery continue to die after being charged? it's probably time for a new battery
If a new battery doesn't fix your issue, it's likely either your alternator, or your voltage regulator on your alternator is bad. there's also a large prefuse terminal box that runs between the battery and the alternator is often overlooked when dealing with electrical problems on our cars. it's underneath the passenger carpeting, labeled x4/10 in this thread outlining the fuses and relays
http://www.benzworld.org/forums/w215-cl-class/1625470-w215-hidden-fuses-revealed.html
If the engine still isn't cranking it would also be a good time to check any engine/starter related fuses for your car to see if they are blown. remember that sometimes a fuse can visually look okay but really, it is blown. having a fuse checker in your car toolbox is very helpful.
As is having a cheap code scanner for any codes the ECU might want to show you. because of my code scanner, i'm usually able to pick up codes while driving and then google the fault code for information on whatever is wrong with my car, while driving
Another very important lesson i've learned is DON'T DISCONNECT THE BATTERY WHILE THE KEY IS IN THE IGNITION, this is known as a reason for EIS's failing and also attributed to rear SAM failures
Some faults can be reset by disconnecting the battery and resetting the ECU, the proper method for solving issues is to fix the issue, and then reset the ECU. instead of just resetting the ECU hoping the problem doesn't come back, sometimes your ECU has fault codes that don't show up as CHECK ENGINE LIGHTS
