I copied most of your message and texted it to the service rep to send to the tech. I mentioned this part to the service guy on the first day and he said "what's that?" Since they can't figure all this out at least this will give them a good first (at last) place to look. Thanks!

 

Oh dear - I hope you cut out my comments about the techs!

 

Lol, I edited. But you are right. When I picked up the car (on the day it ran) I spoke to the tech on the phone and he said it started the last 30 times, as if that means anything, and then went on to explain everything he had done. No mention made of checking the CPS. I'm sure he knew the car was not fixed, I'm surprised it started for a day. The CPS going out would logically explain all the issues. I don't care at this point that I might have needlessly paid for a fuel pump and SAM, I just want the car fixed. The car has never thrown a code and at this point their service department has lost all credibility with me. If they get their feelings hurt by my message I don't care, I just want them to put in a new sensor and see if that works. I'm assuming you can't tell if the sensor is bad or faulty by just looking at it. They had the car the first time for 20 days and gave it back and it still wasn't fixed. It is beyond frustrating.

 

Well, I wouldn't tick them off before they fix it - there will be plenty of time for that once it's fixed. You can then dispute the charges and threaten a crappy survey to MBUSA (these surveys directly affect their wallet).

Unfortunately, there is usually not a definitive method to detect a bad CPS except by replacing it. It's a Hall Effect Sensor, which is a magnet that passes current when it is magnetically attracted to a metal. The rim of the flywheel has alternating holes, and the CPS sits very close to, but not touching, that rim. As the flywheel turns, the CPS pulses the current as it passes over the solid parts of the flywheel. This allows the ECU to determine that the engine is still rotating. There is also one longer void on the flywheel that the ECU looks for to determine the angle of the crankshaft for timing purposes. No signal from the CPS tells the ECU the engine is not rotating, therefore it shuts off fuel supply.

I am not sure what causes the CPS to fail, but it seems that extreme temperatures can affect it. Most of the time we hear that it fails from heat, and then when the engine cools down, it works again. MB Roadside Assist techs have learned to pour cold water on the CPS to get a customer going again (and thus diagnose the problem). My guess is that the electromagnetic properties of the CPS are not failing (that would violate some laws of physics), but maybe it's a slight physical warping of the sensor or even the bell housing so that it moves just far enough away from the flywheel that it can't sense properly. It's also curious that some cars never have this problem, yet others have it multiple times. And that cheap knock-off CPSs also fail more often, or just don't work at all. To me, that just says it's a tolerance issue.

 

with all these issues, you might as well just replace the CPS, not expensive at all and pretty easy to replace

 

I mentioned the CPS when the car was first taken in. When it went back after starting for only one day I sent the service rep a text message containing part of the explanation regarding the CPS that Rodney wrote here. Yesterday the service rep text me again and said the car was starting every time but they were still trying to diagnose the problem. I text back "have you checked the CPS?" I got a text back that said he would instruct the tech to check it, which I assume will mean they will replace it. Why they have ignored this for almost 30 days now is beyond me. Maybe if it is the CPS could it mean I really didn't need a new fuel pump? Or is the tech just too lazy to tackle the job? I don't believe that the car is starting "every time" either. The last time they told me that it only started 5-6 times and then quit. A bad or failing CPS would logically explain the intermittent starting. So yes, I agree with you, it should have already been replaced on the first visit to the shop. After all it is a 10 year old part that has lived its life in the Texas heat.

 

If this turns out to be the CPS, I would absolutely DEMAND that the dealer refund all money paid! CPS failure is such a common problem, they should know to consider that first. If not, then they are just trying to pad their revenue number with unnecessary repairs. In a way, I hope it's not the CPS because I don't want to believe that the shop is either that ignorant or that untrustworthy.

 

Speaking from experience. The 272/273 crankshaft position sensors were extremely reliable in comparison with the 112/113. However, up until sometime in 2007 there were some that failed and there was a shop Foreman conference call that covered this topic about a crank no start with no codes and it mentioned a date range to look for stamped on the sensor. It's very possible you have one of these affected sensors.

 

Rodney - I agree.

 

Rocman8 - This could be very useful information, just want to clarify you are referring to the camshaft position sensor (CPS) that is accessed from the top of the engine bay? I remember reading about a crankshaft position sensor also, I believe it is referred to as a "CKP" sensor? I know there is confusion regarding the two sensors, myself included, so want to make sure we are talking about the same part. Thanks, Mike.

 

no, I am talking about the "crankshaft sensor". There are 4 cam sensors and I've never replaced one of those except for a 112/113 engine.

 

also, cps is another abbreviation for crank sensor. An abbreviation for a cam sensor would be cmp.

 

Crap, I've been calling a CPS a camshaft sensor when I meant the crankshaft position sensor...thanks for straightening me out. And thanks again for the original info!

 

Still waiting....had 4th conversation regarding CPS.

 

Useless dealership that is. Once you get the car back, if I were you, I'd tell them to hire new technicians or close the dreaded place. I am sure that in SDS, there must be a place to test the sensor in diagnostic mode like we do. It should be able to indicate whether the signal is low or high when its close or far from metal. Obviously they should conduct the test not once but several times for prolonged durations to inspect their observations and compare if they are the same or not in ruling out a faulty sensor. However I am unsure if this is possible but if my company equipment can conduct this test, MB surely can as well.

 

Hi Khan - I've been a MB customer for 30+ years and this is the first time I've had an issue with a car that couldn't be diagnosed and fixed. I've searched and read posts here going back several years and I haven't run across any diagnostic information for testing the CPS. Maybe there is a way but I haven't found it. I'm not taking the car back without a new sensor anyway. Well, there is the trick of pouring cold water on the sensor during high temps but mine started acting up in mild temps and with a cold engine. I'm not a mechanic but if the car cranks but won't start and it has a new fuel pump wouldn't the next step be to see if there is adequate pressure at the fuel rail? If the fuel rail has fuel the CPS should be working. However, from the first time it wouldn't start I was sure it was not getting fuel, so it made sense when they diagnosed a bad fuel pump. But did they bother to check fuel pressure or could it have been a bad CPS from the beginning? I agree with you that there should be a way to isolate and test the CPS. But if it is intermittently failing I'm not sure even a series of diagnostic checks would catch it. So best to replace it. I'm puzzled why the technician is so hesitant to replace a 10 year old CPS exposed to the Texas heat its entire life when the car intermittently fails to start?

 

Rodney, do these cars have a fuel pressure sensor that will trigger a code? I ask only because I changed the pressure sensor on my wife's XC90 when it triggered a generic OBDII code for low pressure at the rail. I was just wondering if even though the CPS throws no code, would a bad fuel pump, but good CPS, result in some code such that the absence of a code might be diagnostic of a bad CPS and not a bad fuel pump?

 

No fuel pressure sensor on this car.

 

That was my guess. I guess that's to be expected in an economy brand!

 

LOL! There is no fuel pressure sensor because the most excellent, exulted teutonic MBZ engineers deemed it was unnecessary!

 

Got a text from the service rep today saying the car has been starting every time and there is nothing to diagnose because the car is starting, but do I have a spare key? Replied yes I have a spare key and could drop it off and btw, when it wouldn't start here I tried the other key and it still wouldn't start. Within minutes I get a message back that the car is not starting after all and they will run diagnostics. Oh, and hang onto the other key for now since I had tried it already when the car wouldn't start here at home. At least the tech is thinking as far as the key and its associated electronics, now if I could just get him to the CPS...

 

this is your car right? why do you just demand that the replace the cps. Tell them if it still doesn't start you'll pay the $100 for the part.

 

Yes, it is my car. However, since I have already paid for the fuel pump and SAM my agreement with them is that if those parts didn't fix the car (which they guaranteed would fix it) then anything else is on them. So I'm not in a position to demand certain parts be replaced at this time.

The service rep asked for the spare key today which I dropped off. While I was there I brought up the CPS and that it could fail and not throw a code and there was no way to test it without replacing it. Another rep was there and politely told me I was wrong, that plenty of cars came in showing a bad CPS code. So that was the end of that conversation.

 

Sorry for your situation. Don't listen to these service reps. Half of them or more don't know what they are talking about and the turnover is so high on them. One month they are selling repairs on bmw then Audi then Lexus then Mercedes and on and on

 

Thanks Rocman8 - I do trust the advice I have been given here over anything the service reps or even techs there tell me. This has obviously turned into a much bigger situation than I expected when the car originally went in on December 8th. I will continue to document the progress and outcome here hoping this post might help others in the future.

I used to have a trusted service rep at this location but he moved to a different location that is across town and too far away. I don't know if this is just happening at my dealership but it seems my dealership does not want to actually speak to service customers anymore, all communication is done by text or email. If I call the phone is never answered but goes to voicemail, however the last few times I have been in the service rep area I noticed I never heard a phone ring. Even payments can now be made in advance online. Maybe this is how things work now but sometimes you need to be able to speak to an actual person.

The new service reps are not like the old ones. Less than a year ago I went in for new spark plugs and was advised I needed a new battery. My service rep that day was a young girl who was obviously new, I told her I always keep my old parts and I wanted them put in the trunk when they were removed. When she informed me about the battery I renminded her I wanted my old parts put in the trunk. She said in the trunk and sounded confused, I said yes in the trunk. I get a text the car is ready and go pick it up, check the trunk and there are the parts and head home. It is getting dark already when I go to the car to put the old parts in the garage. I'm not paying too close attention but I pick up the battery and think this looks really clean, I don't bother to check out the spark plugs but notice they were put back in the new box. That night I kept thinking how good my old battery looked and went to bed still puzzled by it. The next morning I was still thinking about that battery, and then the light bulb went on...could that really be the new battery? No, how could that be? Well, it was the new battery and the new spark plugs. I looked closer at the invoice and sure enough I was not charged for labor, just parts. It was easier to install them myself than go back and deal with that service rep again. How a simple request for the old parts be put in my trunk got translated to don't install the new parts but put them in the trunk is beyond me. Is it that unusual to ask for your old parts?