Oldcardriver

Hi there,

I have a 2000 C200 Kompressor, W203, 180,000km, M111 engine, which stopped dead whilst driving. The engine cranks but doesn't start. Using a cheap scanner I get a P0340 code, camshaft position sensor. I have replaced the sensor with a new, but not OEM, item but the engine still won't start. After clearing codes, it throws the same P0340 code after an attempt to start. I've checked the wiring by looking into the harness connector with a voltmeter, with the key on I get 12V, 0V, and something in between on the third pin, so that looks good to me. I've checked all the fuses that sound to me like they are engine related, in the engine bay, in the boot (fuel pump fuse) and next to the drivers door.

The car has had a suite of codes for the past few years; P0101, P0410, P0221 (fixed with a second hand throttle pedal), but apart from the throttle pedal code these have not stopped the car from driving.

Any ideas on what to look for next?

Thanks,
Oldcardriver.

junkyarddawg

If you are sure that the replacement sensor is "known to be good" then you need to take closer look at the wiring from the sensor to the ecu.
That sensor is a hall effect sensor so it should have battery power on the Red/Green wire(socket#3) with the key on, the Brown/Green wire (socket#1) should have ground all the time, and the other wire Yellow/Gray (socket#2) is the signal wire, likely has 5 volts with key on and switches from 0-5V while cranking the engine. But to test the signal you have to plug the sensor in and "Back Probe" the #2 signal wire with a paper clip.
Further wiring harness testing involves going "Pin to Pin", with the sensor and ecu unplugged, to verify total circuit continuity with a ohm meter.
Pin 1 at sensor to pin 41 of ecu plug connector #3
Pin 2 at sensor to pin 28 of ecu plug connector #3
After you test continuity from pin to pin you need to test for cross continuity between the two wires and then test for short to ground on both wires.
Be careful not to force probes into the delicate plug connector sockets as you can easily "spread the sockets" making them too loose to make proper electrical contact.
Loose sockets, in many cases, can be carefully re-tensioned with a fine tipped pick.

Oldcardriver

Thank you for those suggestions. My multimeter test has demonstrated that the 12 V and ground are on the sensor connector, but I will back probe the signal wire as you suggest and look for the changing voltage from the sensor.

Thanks too for the pinouts on the ECU. With that I can do the continuity checks. Do you have a link to a wiring diagram or to a physical diagram so I can find the ECU and connector #3?

Oldcardriver.

junkyarddawg

The ecu should be in the fuse box under the hood, if you follow the engine harness from the firewall to the fusebox you will see.
Once you find the ecu you should see the engine side harness coming into the fuse box and connecting to the ecu at conn #3
Also wanted to let you know that I have seen the retaining clip break where it holds the connector onto the cam sensor, when this happens the
connection will be too loose and cause no start or poor engine performance. Have also seen the wires crushed under the valve cover after a gasket replacement.

Exorpmtech

You say you replace the camshaft sensor. Usually a cam sensor wont cause a no-start, but will cause codes. The crank shaft position sensor absolutely will cause a no start and is an extremely common failure. Short of checking continuity from the sensor to the ME(computer) you need a basic scope to check the wave form if you want to be super technical, but I'd start with a crank sensor.

Oldcardriver

Hi there,

Thanks to the information from junkyarddawg I was able to check all the wiring between sensor and ECU for continuity, which checked out OK. I then put a scope on the signal wire, cranked the engine and got no signal. So I put the old sensor back in and still got no signal. I then removed the sensor and looked down the hole with a torch to look at the cam lobe. I cranked the engine and guess what - the cam is not turning!

I took the cam cover off to find a loose cam chain, and when I pulled one side of the chain it came up from the depths of the engine to expose broken side plates (see photos below).





Both cams look OK, and turn freely. I only moved them a small amount to avoid driving valves into the pistons (see below).

So, before I spend any more time pulling the front of the engine off and replacing the chain, does anyone now how likely it is that I've bent valves when the chain first broke or when I've been cranking the engine over trying to diagnose the sensor problem? This is a Kompressor engine, so presumably has a relaxed compression ratio, but I'm assuming that the valves still interfere with the top of the pistons?

When cranking it over it did sound just like normal, with normal compression pulses. Is there a chance I've dodged a bullet here, or is the engine a write-off?

Any thoughts?

Oldcardriver.

Oldcardriver

How can the engine start without a cam sensor? Without this sensor the ECU can't resolve the ambiguity between compression and exhaust strokes, so I'd think that a cam sensor fault would have to disable the engine.

See also my post above.

Oldcardriver.

Exorpmtech

The ME knows where the crank is at based on the Crank sensor. Cam sensor is not required. The cam sensors help the computer know if there is a cam timing issue and give it data on what its doing with the variable cam timing. Obviously moot point considering you popped a timing chain.

Oldcardriver

Yes, but for a four stroke engine doesn't the ECU need to know when to fire sparks, and also when to fire injectors. So it needs to know camshaft position.

In the early days of electronic management some engines just fired sparks at the top of every stroke. Maybe that's what these engines do?

If my engine still worked I could do some tests to determine some answers, but alas, it's dead

Oldcardriver.

Exorpmtech

YES, it needs to know when to fire, based off the CRANK. Test it when you get it running. I don't know if that engine is interference. I believe its an M271. You may be able to slap a chain in it and let it live again. I've never seen one break.

Oldcardriver

How can the engine know when to fire from the crank angle only? It needs to know cam position to know the difference between a compression stroke and an exhaust stroke. Each crank throw only needs to fire every two revolutions.

Oldcardriver.

Exorpmtech

If you ever run into a later model benz with a bad cam sensor, it cranks and cranks for like 10 seconds, then will finally start. My best guess is it tries to start on a default timing map and if it doesn't detect a start, it changes 360 degrees and then it finally starts. I don't know for sure. Just telling you what I've seen over the years. For all I know, you could be correct, it might not run without a cam sensor, or the ones I've seen fail ran two cam sensors and it figured it out based on the second cam sensor and crank.

Oldcardriver

Maybe the V6 engines can do this because they have two sensors, but this is a M111.955 4 cylinder 2 litre engine with only one cam sensor.

Has anyone got any advise on the overlap question? Obviously, I don't want to spend money on a new chain only to find out the valves, pistons or head are damaged. The car is 19 years old.

Oldcardriver.