Crank no start

Hello. I am new here and having a crank no start issue as well on a 2012 Mercedes Benz C300. I tried the suggestions above for spark and can verify that I do have spark. I pulled one of the ignition coils and then pulled the spark plug from the cylinder. I hooked my test light to battery negative and had someone crank the engine while shifting the test light away to see whether the spark can jump onto my test light and it did.

I also checked all the fuses in what Mercedes calls their front SAM and rear SAM and they are all working. I’ve also checked the pre-fuses that are close to the battery and the pyrofuse, and they are all working fine.

Next, I started checking for fuel. There are 6 different fuses associated with the fuel system on this car, so says the fuse diagram I found in my trunk, and I’ve checked them all. They are all good. I’ve pulled them all and checked for corrosion and then tested them for continuity and they all passed.

I’ve also tested the fuel pump relay and it tested good. First, I switched the relay — there are a total of three that are the same — then I actually tested for resistance on the control side, tested for a click on the control side, and then for continuity on the loaded circuit. I also tested to see if the loaded circuit pulled a load enough to light the test light and it did, so I am not concerned that the relay is bad.

I tried jumping the load side of the fuel pump on the power distribution box or front SAM but the car still didn’t start. I then went directly to the fuel pump under my back seat and tested for power and ground there while someone cranked the car. I got battery power and ground with the connector unplugged and back probed, but as soon as I plugged the connector while it’s still back probed, the reading on my multimeter drops to under 8 volts. So I have a huge voltage drop somewhere.

I’ve put 12 volts directly to the fuel pump and can hear it run. In fact, I can even hear the auxiliary fuel pump also run on the other side of my back seat, so it sounds like the issue isn’t with the fuel pump unless the fuel pump is pulling the power to ground.

I don’t have a wiring diagram for the fuel pump system., but I started thinking that the ECM is likely grounding the fuel pump, and plugged in my diagnostic scanner to see what direction it might send me in. The scanner pulled 8 codes in the ECM alone: P0521, P1304 (stored), P1085, P1437, P0866, P1106, and P2225 (stored). Only the second and last codes are stored, the rest are current. I started focusing on P1106 (the power at the input of the combustion ECM is faulty), but since I don’t have a pin out for the ECM connector, I found the MAP sensor and checked to see whether I had at least my 5 volt reference. I found that I had 5.02V on one pin, 4.86V on another pin, and 0V on the last pin with the key on and in the position just before cranking and the MAP sensor unplugged. Once I plug the sensor in, I still get my 5.02V, but the 4.86V drops to 3.84V, so it confirms I have a voltage drop in at least in that circuit. When I unplug one end of the ECM, I get battery voltage in the area that was 3.84V at the MAP sensor, and this is where I’m stuck. I am not sure where to start tracing wires. Initially, I thought that the MAP sensor might have been pulling the power to ground, but I think unplugging the ECM and getting battery voltage confirmed that the problem isn’t with the sensor.

Question: Do I have a break in a wire from the ECM to the power distribution box or front SAM?

Any help is greatly appreciated. Thank you so much.

flascie97, I moved your post into its own thread. You'll get more visibility this way.

I then went directly to the fuel pump under my back seat and tested for power and ground there while someone cranked the car. I got battery power and ground with the connector unplugged and back probed, but as soon as I plugged the connector while it’s still back probed, the reading on my multimeter drops to under 8 volts. So I have a huge voltage drop somewhere.

That isn't actually a huge voltage drop, considering that the engine was cranking at the time your 8.0V measurement was taken. The engine cranking will bring system voltage down, especially if the battery is weak from multiple starting attempts (which I'm assuming you've been doing during your testing).

I found the MAP sensor and checked to see whether I had at least my 5 volt reference. I found that I had 5.02V on one pin, 4.86V on another pin, and 0V on the last pin with the key on and in the position just before cranking and the MAP sensor unplugged. Once I plug the sensor in, I still get my 5.02V, but the 4.86V drops to 3.84V, so it confirms I have a voltage drop in at least in that circuit.

Again, your reading of 4.86V changing to 3.84V with the MAP sensor plugged in isn't indicative of a voltage drop, but normal operation. You were very likely measuring the MAP signal circuit, which the MAP sensor is going to change once the sensor is plugged in and working. Your readings of 5.0V and 0V on the other two wires are also a good sign.

The scanner pulled 8 codes in the ECM alone: P0521, P1304 (stored), P1085, P1437, P0866, P1106, and P2225 (stored). Only the second and last codes are stored, the rest are current.

For testing purposes, I'd suggest clearing the codes and see who comes back after a key cycle and starting attempt. I know that most of those are shown as Current, but I've found that MB sometimes flags a code as Current when it isn't.

Please include code definitions along with the numbers so everyone doesn't have to Google all the codes that comes back
Would the car run when you manually energized the pump, will it try to run on alternative fuel?

Hello, and thank you for responding. Apologies for not writing out what all the codes meant. I've done so below now, although I've since followed the advice here and cleared all the codes to observe the codes that came back.

The initial codes were:
- P0521 (The charge movement flap [cylinder bank 1] is faulty: current and stored)
- P1304 (The pressure sensor/pressure switch of the evaporative emission [EVAP] control system is faulty: stored)
- P1088 (A great leakage in the evaporative emission (EVAP) control system was identified: current and stored)
- P1437 (Communication with the A/C is faulty: current and stored [event])
- P0866 (The on-board power supply voltage is too low (current and stored)
- P1106 (The power supply at the input of the combustion ECM is faulty: current and stored)
- P2225 (The left fuel level sensor has a short circuit to battery [+]: stored)

I stopped chasing the seemingly nonexistent voltage drop issue and refocused on the fuel system. I first took off the intake and the MAF sensor (but left it plugged in) and pushed some starter fluid directly into the throttle body while someone was cranking the car. The car started and ran for about 3 seconds, then died. That test confirmed for me that it is more likely a fuel issue, so I stayed the course.

Next, I used my scan tool to re-verify that I was getting pressure at the rail and I was getting over 4 bars. To be absolutely sure, I went and rented a pressure tester from AutoZone and static pressure read around 38 psi with key in the position just before cranking. Once I cranked the vehicle, the pressure went as high as 58 psi while the car ran again for about 3 seconds before it died. The pressure then dropped to around 55 psi.

Next, I got a Noid light, unplugged one of the fuel injector connectors and plugged it into the connector. Upon cranking, I did see the Noid light blink. I was able to get to another fuel infector connecter easily, so I repeated the test on the same bank and the Noid light again blinked. So, this test verified that I was getting signal from the ECM to fire the fuel injector. All I needed to test now was whether fuel was actually getting into the cylinder.

Since I was doing all that cranking, I grabbed my holiday borescope gift from last year and lowered the camera into one of the combustion chamber after removing the spark plug to see whether there was any evidence of gasoline in the chamber. It was completely dry. I checked the spark plug and it was also dry with no smell of fresh gasoline. I pulled at least 4 spark plugs and checked for gasoline and they were all dry. I tried one last thing: I tucked a piece of paper down into one of the spark plug holes, while putting the other spark plugs back in, I put the ignition coil back in, then had the car cranked again, but no fuel could be found or smelled on the paper. This test confirmed to me that either I have a number of fuel injectors completely clogged causing no fuel to enter the combustion chamber or that the P1088 (A great leakage in the evaporative emission (EVAP) control system was identified: current and stored) and P0521(The charge movement flap [cylinder bank 1] is faulty: current and stored) is causing the ECM not to turn on fuel, because it recognizes that massive leak "potentially" from the faulty flap.

Am I moving in the right direction in terms of my diagnosis?

Hello. I wanted to follow up and check on whether someone has any advice on a direction to follow after reading my latest posting above. Thank you.

Doing a bit more digging and testing, I'm now leaning towards the fuel pump control module being the culprit, as it seems to control the fuel injectors. It's unlikely to me that all the injectors would simply be clogged at the same time, which might mean that something is causing them not to be turned on. I would very much like to run some tests at that control module found on the passenger side rear of the car. Would anyone have a wiring diagram for a 2012 Mercedes C300 (W204) that I can use to start running the tests?

Of the codes you listed, the only one I would think that could be related to this issue at hand is the low voltage to ECM.
The large leak is to do with evaporative emissions. You could throw the entire evaporative system in the trash and not loose injector control. Same with the "charge movement flap". That's likely an intake manifold tuning device, which has no say in start up injector control.
If the power supply to ECM code is a hard fault,(meaning it comes back right after clearing codes), then that is where I would focus my efforts.
I know the noid light didn't flash on the injector circuit. That could be a control issue or a power supply issue. I wonder if the injectors have good voltage while cranking the engine.

Thank you for responding. The noid light did flash, which is what is puzzling to me on why none of the injectors seem to be firing. You made a good point, however, on checking whether there is good voltage coming through on the injectors. I think my scan tool can read injector voltage, so I would check into it. If you have a pinout of the ECM for this 2012 Benz C300, I would appreciate having it, although I'll do a google search for it.

I apologize, I must have misread your post. If the noid light flashes, and the engine fires on carb cleaner or starting fluid, then it may be worth your time to collect a fuel sample.
I like to fill a goodish sized glass jar and give it time to settle, allowing to to see if there's water or sediment or something else in there.
I have seen all the injectors plugged up in a car before. Usually from sitting a very long time, or from crap in the tank (turpentine, sand, sugar...)

Thanks for the advice around the fuel sample. I had taken out the fuel pump and noticed some debris, but no enough to be concerning. I took your earlier advice and concentrated around the code for low power supply at the ECM.

There is no power on pin #3 at the ECM connecter with ECM unplugged and vehicle on, although there is power at fuse 25 and relay on the front SAM, when ECM is plugged in. Also, there is no power on pin 94 and 95, which is powered by fuse 24, but fuse is okay. I tested the circuit at the SAM and I do have good ground and both powers there. I've also tested the three grounds on pins 2, 4, and 6 at the ECM, and they all tested good.

Are there other tests I should consider doing before condemning the ECM?

I wanted to report back with some good news that I’ve finally gotten around to fixing this crank no start issue on the Mercedes C300. Replacing the fuel pump under the right rear seat solved the no start problem. I went ahead and changed the fuel filter assembly under left rear seat as well. My main issue with noticing that the fuel pump was the culprit was two-fold. First, the Autel Ultra Lite scan tool was telling me that I was getting 5.4 bars of pressure to the fuel rail, and when I pulled the fuel pump completely out and put 12 volts directly, it ran and sprayed fuel with enough pressure to convince me to start looking elsewhere. Well, once I had the vehicle checked with the original scan tool, it showed only 15psi getting to the rails, which was not within specs. As soon as I replaced the fuel pump, the pressure jumped within specs and before even cranking the vehicle, I knew it was fixed, and it was. I just want to say a big thank you for engaging me and helping to steer me in the right direction. This issue has annoyed me for last 7 months, but it’s not a daily driver, and I certainly didn’t just want to throw parts at it.

That's good to hear! Thanks for following up with the fix