[Kane.vernon]

hello i have a 2009 bmw 335i xdrive i bought the car being told it needed a cam shaft sensor and water pump and was told the car wouldn’t start because of the cam sensor once i got the car i got 3 codes for it i can’t remember one of them but one had to do with the crank sensor and then one for no communication with the dme i replaced the crank sensor and cleared the codes and the only one coming back is u0100 for no communication i checked all the fuses i tested the realays and checked all the grounds i could find all seem to be in good condition i also check if dme is getting power and it is the guy told me it had a new battery in it and it turns out he didn’t register the battery to the car and im not sure if that would cause this or no but at this point im at a total loss this is my first foreign car so i dont know all too much but i dont know what to do from here help please any info would be greatly appreciated.

[RockCrusher]

Originally Posted by Kane.vernon

hello i have a 2009 bmw 335i xdrive i bought the car being told it needed a cam shaft sensor and water pump and was told the car wouldn’t start because of the cam sensor once i got the car i got 3 codes for it i can’t remember one of them but one had to do with the crank sensor and then one for no communication with the dme i replaced the crank sensor and cleared the codes and the only one coming back is u0100 for no communication i checked all the fuses i tested the realays and checked all the grounds i could find all seem to be in good condition i also check if dme is getting power and it is the guy told me it had a new battery in it and it turns out he didn’t register the battery to the car and im not sure if that would cause this or no but at this point im at a total loss this is my first foreign car so i dont know all too much but i dont know what to do from here help please any info would be greatly appreciated.

Not sure what problems can arise from not registering a new battery.

Well, check that. I seem to recall the charging rate is changed as the battery ages. A new battery needs a different charging rate and may not get it. For optimum life from the new battery the battery should be registered.

Next, you need to read all codes and freeze frame code/data which is the code which triggered the CEL and could point to problem.

Write this stuff down.

There is the chance the vehicle has rodent damage to its wiring.

Run your hand along all the accessible wiring harnesses in the engine bay. Any engine covers will need to be removed to get better access to the wiring harnesses.

At the same time look for rodent trash -- nesting litter or droppings-- on top of the engine. With the car elevated look on top of the underbody panels for trash. Any trash means rodents have been in the car and they gnaw to live and live to gnaw and wiring harnesses are very attractive. But I've seen plastic tubes that are part of the engine vapor recovery system gnawed through. Another sign is areas of the carpet that have had the nap removed -- for making a comfy nest...

Crank but no start could be a missing/bad crankshaft position sensor signal. A crankshaft sensor needs to be mounted so it is close, but not too close, to the metal tabs of the ring attached to the flywheel. And this ring, and its tabs, need to be free of any damage. Even one tab bent and out of position can be a problem.

But it could be lack of fuel. Do you hear the fuel pump running when you get in the car? A tech would disconnect the fuel pump from the car's wiring harness and connect the fuel pump to 12V (shop) power and confirm the pump runs. Or doesn't.

[Kane.vernon]

i wouldn’t think no fuel would be the issue considering the code i’m getting is for no communication but i can try doing that with the fuel pump and see how it goes i will run the codes again and see what else i can manage to find and i’ll check for damage as well

[RockCrusher]

Originally Posted by Kane.vernon

i wouldn’t think no fuel would be the issue considering the code i’m getting is for no communication but i can try doing that with the fuel pump and see how it goes i will run the codes again and see what else i can manage to find and i’ll check for damage as well

That the engine cranks is a sign the ECU has sufficient info to allow the engine to crank.

To start the engine needs air, fuel, spark.

To get fuel requires a working fuel pump and sufficient fuel in the tank and a suitable signal from the crankshaft position sensor so the ECU knows when to inject fuel.

And spark requires sufficient battery power. A coil can't generate a spark with too low a battery. Would the engine even crank if the battery was this low? Who knows? But ensuring the battery is good is fully charged, is a good idea.

But like for proper fuel injection the ECU needs to know when to trigger the spark and that's back to the crankshaft position sensor.

You don't have to deep dive into the fuel pump. If you are sure there's sufficient fuel in the tank and you can hear the pump run that's good enough.

But then you need to turn your attention to confirming the crankshaft position sensor is delivering a valid signal to the ECU.

One behind the steering wheel thing that can be done is observing the tach during cranking. If the tach needle or RPM indicator moves a bit that's a sign the crankshaft position sensor is (probably) ok. If there is no movement then more diagnosis is probably called for.

Or it may be possible to connect an OBD2 code reader/scan tool to the car's OBD2 port and config it to display engine RPMs and then during cranking one would set some RPMs. I can't say your car and its ECU would support this level of interaction with power on but the engine off. And then during cranking. You'd just have to try it.

(Over the years I have used various OBD2 tools with a number of cars/vehicles but I can't remember ever using one prior to the engine running.)

A tech would have a (factory) diagnostic computer or something aftermarket -- Snap On offers such tool, but for $$$$. 'course, a tech, a professional tech, would amortize this diagnostic tool cost over every car he services/works on.

But the tool provides graphing and the tech would be able to monitor the crankshaft position sensor signal and graph it and verify the signal is good.