2008 ML 320 CDI

Can the vehicle go into limp mode without the Check Engine Light coming on and therefore without a fault code?


Today it was -20 deg. C all day long. The vehicle was started this morning having been parked in the garage at -1 deg. C and then driven for 60km with no problems at all. After this, the vehicle was parked outside at -20 deg. C for 6 hours.


Starting the drive back home, I was careful not to race the engine until it reached near operating temperature. At the 18km mark the vehicle reached near full operating temperature and it was also at this time that I realised the vehicle had at some point in the first 18km gone into limp mode and would now only go up to 80km/h. No CEL lighting up.


At the 23km mark I pulled over, put the vehicle off for a few seconds, restarted and continued driving. The vehicle remained in limp mode with no CEL.


At the 30km mark the vehicle came out of limp mode and we were going 120km/h. By now the vehicle had reached full operating temperature.


At the 55km mark I had to slow down to a stop to take a turnoff. Straight after this, the vehicle was back into limp mode and now would only reach 60km/h all the way home. Still no CEL.


I can only suppose that the engine management system is protecting the engine against high RPM in really cold conditions but I would have expected this to be explained in the owner's manual if it were the case. Is this a known behaviour for the diesel ML/GL?

2008 ML 320 CDI

Came across a thread in the GL forum '08 GL 320 CDI Power Loss and Cold Weather' (see link below). This subject has already been discussed to some extent there. Seems the symptoms my vehicle is displaying may be related to a faulty DPF Differential Pressure Sensor.


https://mbworld.org/forums/gl-class-...d-weather.html

N4HHE

Happened to me on a Sunday in the middle of Pennsylvania in shirtsleeve temperatures. 700 miles from home. Afraid to turn the truck off before reaching a safe spot to spend the night so I limped about 100 miles.

Started and ran perfectly the next morning and 600+ miles home.

Dealer couldn't find anything wrong, that time. But after taking the car in about 7 times in 4 months with CELs, getting new AdBlue injector, NOx sensor, and a lot of "we can't find anything" they finally found a bad electrical connection "in the back" somehow related to the AdBlue system.

2008 ML 320 CDI

On my next trip the CEL did come on with the P1 402 OBDII code. After another 60km open road trip, the vehicle came out of limp mode but the CEL remained on for another 3 or 4 short trip cycles and then simply turned off.


With the little experience that I've had with this now and in combination with some other posts that I've read, it seems this vehicle doesn't like to do many short trips in succession when temperatures are very low. The vehicle needs a 60km open road run every now and again to regenerate the DPF by burning off the soot build-up in the diesel particulate filter in the exhaust system. If this problem occurs when most of your trips are long open road trips any way, then it is even more likely that the Differential Pressure Sensor could be faulty.

N_Jay

Have any of the short cold trips also been wet?


At first I attributed it to cold, but it only happened when cold and wet.

2008 ML 320 CDI

I wouldn't say wet but most of the time there was snow on the ground. At -20 deg. Celsius H2O is frozen but I suppose if snow lands on the Differential Pressure Sensor say, some of the snow turns to water because of its proximity to the exhaust system which makes it a wet condition for the DPS.


Something else I'd like to mention is that sometimes the limp mode limits the engine to 2000 rpm and at other times to 3000 rpm. When the engine is limited to 3000 rpm you can very easily get by and not even realise that you're in limp mode.


Just when I thought my troubles were over, my vehicle has gone back into limp mode now with the 3000 rpm limit. So I'll probably start looking for a DPS to see if that will resolve this issue.

N_Jay

They are only $50 to $75 and supposedly easy to change.


Just hearsay because I have not don't mine yet.


I wonder if it is actually the electrical connector?

2008 ML 320 CDI

I plan on having a close look at the Differential Pressure Sensor to see if I can find fault with it or with the electrical connector for it. For what I imagine it is doing, it should be a simple piece of equipment.


The dealership here does not have this part on hand but quoted me $124.00 (CAD) for part No. 642-905-01-00 which it says replaces part No. 006-153-95-28.


Today the temperature was much milder (-8.5 deg. C instead of -20 deg. C) and the roads were dry. All in all the vehicle did 250km today and all of it out of limp mode.


A year ago I read elsewhere in these forums, and I took it with a pinch of salt at the time, that these engines should NOT be nursed. Generally I tend to NOT drive my vehicles hard but in the last few days I have driven this vehicle harder once the engine reached operating temperature in case that is what is required for the system to re-generate the Diesel Particulate Filter. This driving style may have improved the limp mode situation I can't tell but on a side note, it almost seems to have improved the fuel consumption for these trips!


Wish there was a way to know what the status of the DPF was, when the DPF was being re-generated and how to initiate the re-generating process if ad hoc intervention was required.

N_Jay

Some companies require you know when you are regenerating because you are not supposed to shut down during the cycle. MB does not seem to have that requirement.
I have heard that regeneration occurs during normal high-speed driving over a fair distance. (Like highway driving).


I would bet the DPF differential pressure sensor is a piezo element an ad such the electrical signal would be very small. (making them susceptible to bad connections.On top of that the vapor being sampled is exhaust which is hot and corrosive. Then it is under the car where it not well protected from road salt and water.

N4HHE

I replaced a 2008 Ford F-250 Powerstroke with the 2009 ML320 Bluetec. Have fondled the pressure sensors on the Ford and can say they were nothing exotic. But they were on the dirty (upstream) side of the DPF. Can't think of anything one could do to keep them clean. But if yours works at warmer temperatures and not at colder temperatures then perhaps removing, spray with a cleaner, and reinstall is all that is needed? Can take them off the Ford without even jacking the truck up.

On a related note my ML320 didn't get driven much last year. When driven it was about 80 miles in the morning and 80 miles back in the afternoon. But over the year it started running rough. Not really running bad, but running diesel-loud like an old Dodge Cummins. Over Christmas it was driven 500 miles one day, 500 another. Noticed half way into the first day it was back to purring like a kitten. MPG was 25 that first day, the worst its ever done on that trip. Fuel gauge broke so I haven't filled since but guessing it was back to 27 MPG based on the Fool Computer's numbers (30 indicated).

2008 ML 320 CDI

My vehicle is back to running the way it should. I've been using it for 60km at-a-time trips over the last few days and that seems to have resolved this issue, for now anyway.


My best guess is that the vehicle needed the longer runs to regenerate (clean out) the Diesel Particulate Filter. Thank you N Jay and N4HHE. Your comments lessen any doubt that I might have had otherwise.

2008 ML 320 CDI

Some photos showing the location, part number, electrical connector and wiring of the existing Differential Pressure Sensor on my 2008 ML 320 CDI. A quick visual inspection didn't show any obvious defects or poor electrical connections. The part number of the existing DPS in the photos is 006-153-49-28. The new part number to replace this is 642-905-01-00.

N_Jay

Thanks

dpulling

I am having the same symptoms in milder weather. This began in the summer with two limp home mode episodes and no CEL, each ending with the next startup. After a three month time gap, including a long trip with no trouble, the limp home has returned intermittently, this time with the CEL showing the code "P1402" in my cheap analyzer. Sometimes the CEL is on without limp home, sometimes limp home without the CEL, sometimes both (limp home limits RPM to 3000 in 1st and 2nd, 2000 rpm in higher gears and no kickdown). I went to a local repair shop with a better analyzer which read "2078-8 - check component B28/8 (pressure differential sensor DPF)." The shop also found the P1402 code, which it appears is for the EGR valve. The shop monitored the DPF with a scan tool (I don't understand how) and found it "does not change volume with throttle." This shop concluded I needed a new diesel particulate filter, and referred me to MB. But from what I read here a likely suspect is the differential pressure sensor (which the shop quoted as "pressure differential sensor") and which it appears I could change myself. Before I go to a MB dealer for the final ($$$$) word, what does this look like to you? Is there a code to identify a fault in the pressure differential sensor separate from the diesel particulate filter?

brucelong

Thank you for this very useful post. I have a 2010 ML350 Blutec and is giving off the same trouble codes with all the same symptoms. According to the EPC, appears to use the same part. However, when I went looking for it in what appears to be the location described in your pictures, I can't find it.

Was wondering if you had to remove any underbody panels in order to access the DPS?

Thanks very much.

N_Jay

I'm still waiting for someone to post pictures.

On these cars I am surprised you don't have to remove panel to open a door.

brucelong

Haha, yes. Lots of panels down there. I think might have found it. In the 2010 W164.125 actually located in the passenger side engine compartment, just behind air filter box. Will pull it and confirm when new part arrives.

Thanks again. Having a picture of the actual part to hunt for was very useful.

2008 ML 320 CDI

To locate the Pressure Differential Sensor on the 2008 ML320 CDI you have to crawl underneath the vehicle to more or less the centre of the underside of the vehicle. The Pressure Differential Sensor is located at the front end of the rear propeller shaft. You can see some of the transfer case and prop shaft in the second photo (6 posts up from this post). No panels need to be removed to locate this sensor on the 2008 model but perhaps it is in a different location on the 2010 ML350 BlueTec.

dpulling, as to whether to replace the Diesel Particular Filter, I haven't ever done that but I understand it is a lot more expensive than replacing the Pressure Differential Sensor. I would first replace the sensor only and see if that solves the issue with the vehicle going into limp mode.

N_Jay

Thanks,
Funny thing is, I have not had a problem in over 2 years.
It will get replaced next time.

2008 ML 320 CDI

Replaced the Pressure Differential Sensor. New part No. A 642 905 01 00.
Opened up the old sealed unit. Seems like the 'sensor' inside is really a switch which gets activated by a pressure differential above or below a certain value.
The old unit had water inside, presumably condensate from exhaust gasses as the unit housing is sealed and was fully intact in this case. I suspect the moisture inside the old unit would freeze when the temperature outside dropped enough and that would cause the switch inside to get stuck in a certain position.
Next time, instead of buying a new replacement unit, I would disconnect the 2 rubber hoses from the unit and blow out the moisture with low pressure compressed air and then warm up the unit with a hair dryer to drive off any remaining moisture through the 2 open plastic tubes.




New replacement unit. CAD $101.22. Available for approximately USD $74.00.



Three terminals on the electrical connection.



Location of Pressure Differential Sensor on 2008 ML 320 CDI.



Was able to undo and reuse the old clamps on the 2 rubber hoses. If deciding to use new clamps, the larger hose takes a 13 mm clamp and the smaller hose an 11 mm clamp.



Not much inside the sensor. Looks like switch made up of a small square contact plate (not visible in the photo) sitting underneath the white cube made of some flexible, pliable, soft foam rubber like material.



Small contact plate visible on soft pliable cube.



Electrical connector to Pressure Differential Sensor. Three wires. Tabs on the side of connector to be pushed inwards in order to slide connector off of the Sensor.



Butterfly clamp holding rubber hoses further down from the sensor. It's easier to do the replacement with the hoses freed from this clamp. Nut on clamp takes 8 mm spanner.



Nut in photo takes a 10 mm socket.

N_Jay

Thanks!

Great write-up and pictures.

2008 ML 320 CDI

Thank you N_Jay.
For the record, I see elsewhere the small square plate is described as a chip! The inside parts of the old sensor pretty much broke to pieces when I opened up the old unit. A magnifying glass would have been useful to properly look at the pieces.
I don't know if I'm imagining things, but the vehicle seems to be driving even a little better and smoother since the replacement.
We're forecast to hit low temperatures again next week and I'm anxious to see if the new Pressure Differential Sensor is going to stop the vehicle from going into limp mode in the way it did before.

N_Jay

I was guessing that the sensor is a piezo element.
It would not like water in it. There may have been a membrane covering the actual sensor that failed.

My other guess as to failure mode has to do with the connector getting wet since piezo sensors put out very small signals.

2008 ML 320 CDI

It may be too early to say definitively, but so far it certainly seems that replacing the Pressure Differential Sensor recently has made a nice difference in our vehicle.
We drove in temperatures as low as -30.5 deg C in the week and over the course of 1200 km, the ML 320 CDI did not once go into limp mode.
Fuel economy now also seems to be improved compared to what we were getting over the last number of months.
Subjectively perhaps, but the drive generally feels smoother and more powerful compared to times when the vehicle wasn't in limp mode just before the replacement.

clvincent

thanks for the update

glad it was an easy fix and it stayed fixed

and thanks for that great write up