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I’m looking for help diagnosing a dead 2010 C300. It was working fine last week, but yesterday when I went to use it, it was totally dead. Battery is good, I actually had a new one I put in just to check, but still nothing.
The key fob does not work (Ie unlock, lock, or open trunk), when I open the door the dome light does not turn on, and the dash shows nothing. When I put the key in, there is no click, and no start, the dash continues to show nothing (no mileage, just blank). I tried unplugging the battery for 15minutes, changed it with the new one, but all the same...nothing.
I checked all the fuses under the hood, and in the trunk and all have continuity and are good, but I did notice the “E” relay under hood (Engine circuit 87 relay) is clicking, and after this when I open the driver or passenger door, the window slides down a small amount, and slides back up when I close it (it has never done this before). Also, when I try to turn the key now, the high beam and fog lights on the dash flash on for 5-8seconds, and then turn off, followed by the needles on the dash (like the fuel, temp, and tach gauge) just barley twitching. And sometime the lights under the dash turn on, but not the dome lights. I tried an OBDII scanner, but it fails to communicate with the car.
Weirdly the radio works, and will turn on, the hazard lights work (although the dash is blank while lights flashing), and no other button or light seems to work.
This seems like an electrical issue, maybe a short or bad ECU, but I have no idea where to start. Does anyone have any recommendation on where or how to start diagnosing this?
It won’t scan, so no DTCs to go by.
The symptoms seem to point more to an open circuit than to a short.
Has anything recently happened to the car? Hit a parking bumper rather hard? Bumped the wall in the parking garage? Jumped off a curb? Been under the hood doing work? Installed any kind of aftermarket devices?
First, use a multimeter to check the actual voltage of the battery at the terminals to make sure it’s around 12.6 volts. With a wiring diagram the voltage path to various non-functioning components can be traced. Since the starter relay is ticking, the DAS seems satisfied, but clicking sounds like low voltage at that point. It’s probably not the pyro fuse but maybe something else in the pre-fuse box or maybe the front SAM. Since the emergency flashers work, do the stop lights work when you press the brake pedal?
It won’t scan, so no DTCs to go by.
The symptoms seem to point more to an open circuit than to a short.
Has anything recently happened to the car? Hit a parking bumper rather hard? Bumped the wall in the parking garage? Jumped off a curb? Been under the hood doing work? Installed any kind of aftermarket devices?
First, use a multimeter to check the actual voltage of the battery at the terminals to make sure it’s around 12.6 volts. With a wiring diagram the voltage path to various non-functioning components can be traced. Since the starter relay is ticking, the DAS seems satisfied, but clicking sounds like low voltage at that point. It’s probably not the pyro fuse but maybe something else in the pre-fuse box or maybe the front SAM. Since the emergency flashers work, do the stop lights work when you press the brake pedal?
As far as driving incidences, nothing out of the ordinary that stands out. The last time work was done, it was almost a year ago, the rear subframe was replaced under warranty. The battery voltage is 12.6v (it’s brand new), and I even had the old one tested at the auto-parts store, and it tested good as well. Is the 'N' relay the starter relay? I thought that was 'M' (the green one), which is not ticking.
I’ll look around for a wiring diagram, and start checking.
Originally Posted by TimC300
Interesting issue.
I have a 2010 C300 4matic so im curious the cause of this.
First thing that comes to mind is issue with the front sam in the engine bay since all the things not functioning are linked to it.
When you put the key in the ignition are you able to turn it to on?
When you plug in your scan tool it powers on? Just wont communicate?
Has it rained since you last drove the car, or washed it?
I'll do some digging around for possible causes.
Is the SAM under the fuse box? How would you diagnose this?
When I put in the key nothing happens, the dash does no light up, I don’t see the mileage, I don’t hear the EIS or ESL. When I turn the key to start nothing happens, but I think it triggers the high beam and fog lights turning on and the dash needles twitching.
When I plug the scan tool in, the tool turns on (and gets power), but it never is able to establish a connection.
It did rain since it was last driven.
Originally Posted by streborx
Any chance of rodent damage?
Yes, rodents are possible. I park outside and do periodically get droppings and nesting material on top of the ECU heatsink. This has been happening for the past 10+ years since I’ve owned it, especially in the winter, but I drive it frequent enough that it has never been an issue, and I have never seen, heard, or smelled any other sign anywhere else.
As far as driving incidences, nothing out of the ordinary that stands out. The last time work was done, it was almost a year ago, the rear subframe was replaced under warranty. The battery voltage is 12.6v (it’s brand new), and I even had the old one tested at the auto-parts store, and it tested good as well. Is the 'N' relay the starter relay? I thought that was 'M' (the green one), which is not ticking.
I’ll look around for a wiring diagram, and start checking.
Is the SAM under the fuse box? How would you diagnose this?
When I put in the key nothing happens, the dash does no light up, I don’t see the mileage, I don’t hear the EIS or ESL. When I turn the key to start nothing happens, but I think it triggers the high beam and fog lights turning on and the dash needles twitching.
When I plug the scan tool in, the tool turns on (and gets power), but it never is able to establish a connection.
It did rain since it was last driven.
Yes, rodents are possible. I park outside and do periodically get droppings and nesting material on top of the ECU heatsink. This has been happening for the past 10+ years since I’ve owned it, especially in the winter, but I drive it frequent enough that it has never been an issue, and I have never seen, heard, or smelled any other sign anywhere else.
Rodents snack on wire insulation. They don't eat the wire itself, but sometimes chew through it, or expose it and it shorts to chassis, blowing a fuse.
I would hook up a battery charger so the battery maintains a full charge as you are trying to diagnose the issue. Connect charger to jump start locations in the engine bay. Or if you want to be able to leave the charger hooked up with the hood closed can always connect positive directly to the positive battery post, negative connect to negative jump start lug in engine bay.
Since you are getting power to the OBD diagnostic connector but the scanner is not connecting to the car it means there is an issue with the CAN. The OBD diagnostic port connector is wired to the front SAM.
You say it rained out. one reason for the issues you are having is water infiltration into the front SAM. I've seen where the cover on the front SAM was not installed right and water dripped down from the drain in front of the windshield. You mention checking all the fuses in the front SAM so you have removed that cover since it rained, probably cant tell if it was infact not clamped down right.
I would lift out the SAM and see if theres any signs of water underneath or on the circuit board. I dont see a need to disconnect any wires at this point unless you need a closer look at something.
Also feel the carpet in the drivers side footwell along the door sill. There is a CAN connector under the sill. I have seen a few people have to replace these due to water damage.
Disconnect battery ground while doing any work like this.
Would like to at least get the OBD port communicating with the car again. We know the connector power and ground are at least working. Pins 6 and 14 for the CAN do not seem to be working.
- Below says for up to Model year 08 but should be the same for 2010.
That info is much more useful than the PDF manual I have. It turns out it was water in the sill on the driver side floor.
I had already removed the SAM (and saw no water), tracing the ignition wires, and I was able to get continuity to the ignition, so I figured it was probably a problem elsewhere. I did check the carpet, and it was dry, but while I was under the dash tracing the ignition wire, I did see a drop of water on the A pillar near to the parking brake pedal. When I pulled out the carpet I found and inch of water in the sill. I don't think it was there long, or I'm really lucky, as I found no water damage on those CAN voltage bars, and the water was very clean.
I vacuumed out the water, and let it dry, and everything works again. The dome lights turn on when the door opens, I can lock/unlock using the key, and the car starts and no codes. It looks like the sunroof drain outlet was stuck closed, I agitated the outlet, and forced it open, and some water drained out.
Is there any good way to snake these? Or at least make sure the tube is clear?
Snake the drain, doubt it. The front drain hose takes a 90 degree turn so I dont think anything will fit down it. The rear drain hose maybe.
Looking at the diagram there are two connections that could be leaking. Then is just the hose going down to the wheel well. I had my wheel well liners off before and i looked for the ends of the sunroof drains and could not see them. Maybe they are behind metal panels.
I would test to see how water got in. Doubt you are driving around with the sunroof open. Is water getting past the seal along the glass? Spray or pour water all along the sunroof, dry all the water up then open the sunroof and see if theres any water in the track.
Looks like the headliner has to come down to see the drains. if it were my car i would probably want to fully inspect everything to make sure i fix the issue. It could be very costly if water is leaking in.
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