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Hi All,

Have done some searching and not found anything that relates to this issue specifically so here goes...

The other day I was driving the car, all running fine. Came back about an hour later, car started but took a little longer than normal followed by the engine management light. The light stayed on for a good 25 miles until i stopped and got out. Went back about 6 hours later, fired up first time and no engine manegement light.

So today, I went to work again with car acting fine again, no light, no loss of power, no additional cranking required. Drove the 35 miles home, pulled up on the drive and left the car for 2 minutes before attemptng to go back out. Would not start for the life of me! Kept on cranking but nothing to start it. I reset the ECU and it fired up again straight away...there are also some bulb blown warnings on the dash for the rear lights but actually all the bulbs are fine...then out of nowhere it cut out! Left it for about 3 hours and it fired up straight away again but with the EML on.

I know people say the Crank Sensors fail but if this was the case the car wouldnt even crank would it? Im guessing its an ECU fault, anyone else had any similar experiences?

Any help appreciated!

Thanks
James
 

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2008 CLK550 Cab
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It will crank all day long with a bad crank postion sensor. The sensor just sends certain information to the ECU, but the starter will still spin the engne. But more likely you have a bad cam position sensor, not crank position sensor. Search that.
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550, 2011 C300 FORMER: ML350, CLK550 Cabriolet, C240, ML320, 300TD
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Hi, James. First of all, you should fill in your profile so we know what model and year CLK you have. This does sound like classic CPS failure, but it also could be a weak battery. The car has a built-in voltmeter - with the engine off, turn the key to position 1, make sure the MFD shows 0mph, then press the odometer reset stalk three times "1-2-3". You should get a beep and then the voltage will be displayed. Now crank the engine. If voltage drops below 10, ten your battery is weak. If the engine rotates, but never "fires", then I suspect a bad CPS. You could have both problems - they are very common in hot weather.
 

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Wrong!!!!!

It will crank all day long with a bad crank postion sensor. The sensor just sends certain information to the ECU, but the starter will still spin the engne. But more likely you have a bad cam position sensor, not crank position sensor. Search that.

The engine will spin over, but not start with a bad crankshaft position sensor, it will not start until it see engine RPM signal from CPS, this is most likely cause of the OP's complaint, I've changed enough of them to last me a lifetime.

Yidney, I don't know where you got your info, but it's wrong. I happen to be MB Master Mechanic btw.




Bam Bam
 

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Coupe/Convertible Forums Moderator
CURRENT: 2011 SL550, 2011 C300 FORMER: ML350, CLK550 Cabriolet, C240, ML320, 300TD
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19,441 Posts
I guess the issue here is one of terminology - a common problem when customer's talk about engine starting. There are basically two things that (should) happen when you activate the ignition switch: the starter should activate to rotate the engine (customers often call this "cranking"), and the ECU should activate to supply a fuel/air mixture and spark causing the engine to fire (customers often call this "catching").

If the engine "rotates" but does not fire, that is quite often a bad CPS, but it can also be a fuel delivery problem. If it "fires" and runs a bit and then dies, then it's probably not the CPS, but something else. With bad cam sensors, the engine will usually run, but throw DTCs and illuminate the MIL.
 
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