OM642 DEF heater - PeachParts Mercedes-Benz Forum

psaboic · #1 08-26-2019, 01:07 AM

Posted this in the ML subgroup but no response, so I will post it here for help.

Wife's 2012 ML350 Bluetec popped a check engine light. Code is P20BD which is which is for "Redundant heater "B" (DEF) circuit low or no voltage. In this case it is for the DEF heater. I can clear the code and religiously about 30 miles later it will come on, so it is a hard failure.

Sooooo. it appears that the DEF tank heater took a dump. Before I throw $400+ on a new heater unit, does anybody have any good troubleshooting tips or even better, a schematic of the heater circuit that I could use to narrow it down?

Many thanks

psaboic · #2 08-26-2019, 10:51 PM

Ok, no answers here, or in the ML sub forum, so I guess I am on my own..........

TimFreeh · #3 08-27-2019, 11:03 AM

There are not too many people here that have experience with the later diesels, Benzwhirled is kind of a better bet for the newer stuff.

The description for the error code you provided would tend to lead me to think that the issue was a power supply issue for the heater not necessarily an issue with the heater itself. Have you verified the circuit that is supposed to provide feed power for the heater is providing power?

I don't have access to the MB diagnostics associated with the error code, maybe somebody here that has access will post them.

pawoSD · #4 08-28-2019, 11:58 PM

I would do some basic checking of things like the connector to the heater and associated wiring....

Graham · #5 08-29-2019, 12:17 PM

Quote:

Originally Posted by psaboic

Ok, no answers here, or in the ML sub forum, so I guess I am on my own..........

You might try the Diesel forum on MBWorld. https://mbworld.org/forums/diesel-forum-76/

The emission system on a Bluetec can be VERY expensive to maintain. If the heater only costs you $400, you are lucky. Even luckier if you find it is covered by the Federal Warranty: https://mbworld.org/forums/diesel-forum/615996-adblue-heater-failure-covered-under-emissions-warranty.html

ROLLGUY · #6 08-30-2019, 12:23 AM

I had the DEF heater in an '09 R320 rebuilt from a place in Canada. I think it was $350 total, and only took about a week. The advantage of getting a new one is that your car can be fixed right away, not having to wait for parts. However, the R drove just fine with the DEF tank out. It just had a CEL illuminated the entire time.

Graham · #7 08-30-2019, 09:11 AM

Quote:

Originally Posted by ROLLGUY

However, the R drove just fine with the DEF tank out. It just had a CEL illuminated the entire time.

On the newer Bluetecs, they only allow ~10 starts after an emission related CE light comes on. You have to get it corrected quickly.

THis is for Sprinters, but similar applies to other Bluetecs:

Quote:

* Start count down. '2010 & up "Bluetec" Sprinters will start counting down how many more times the van can be started if there is an issue with the DEF Adblue system. If DEF is not added and it runs out, or Nox sensors fail or any other components for DEF/Adblue system fail-it will enter "Start countdown" It is imperative that the Sprinter is brought in before it gets down to it's last few counts as it gets more difficult & expensive to diagnose what was wrong/fix/verify if not many starts left. Depending on the year/software in engine computer it will start at 16 or 9 starts remaining.

psaboic · #8 08-30-2019, 09:56 AM

Well, I managed to score a new Bosch DEF heater for $382 plus free returns (if that turns out to not be the problem and it is wiring or something else). Gonna drop the DEF tank this weekend and do the job, if the wife lets the thing stay parked long enough for me to work on it....... Pretty sure the heater is toast as it is original to the car, and the same code (P20BD) keeps popping up whenever it is cleared.

TimFreeh · #9 08-30-2019, 07:37 PM

Quote:

Originally Posted by Graham

On the newer Bluetecs, they only allow ~10 starts after an emission related CE light comes on. You have to get it corrected quickly.

THis is for Sprinters, but similar applies to other Bluetecs:

But apparently this piece of information doesn't apply to a 2012 ML350? The original poster never mentioned anything about about 'after 10 starts you're dead' kind of issues?

I personally know of an ML350 that has a CE light on for an implausible NoX sensor reading for either the upstream or downstream sensor that has been ignored by the owner for at least 10K miles. There has never been an edict issued by the car that it will not run after 10 starts. I rode in the offending vehicle 3 days ago with the CE light illuminated and it ran just fine, maybe the ECU simply injects a default amount of ADblue based on some other parameter of operation when it can't rely on the O2 sensor to fine-tune the ADblue delivery?

I've never had an issue with the Adblue hardware on any of my cars but my understanding is that the '10 starts' messages are issued if the ADblue tank is empty and the user ignores the warning messages which are issued for hundereds of miles before the countdown starts. This is not a fault of the ADblue sustem, its a requirement of the EPA that the car can't be operated without ADblue.

Don't know anything about Sprinter world but its NOT true that all CE emissions related codes on Mercedes Bluetec result in the '10 starts and you're done' message.

Graham · #10 08-30-2019, 08:41 PM

Quote:

Originally Posted by TimFreeh

Don't know anything about Sprinter world but its NOT true that all CE emissions related codes on Mercedes Bluetec result in the '10 starts and you're done' message.

It was certainly true for the problematic 2014 GLK250s NOx sensors when they first came out. Many reports over on MBWorld GLK forum.

LiamsmithEz · #11 08-31-2019, 07:45 AM

97 SL320 · #12 08-31-2019, 10:30 AM

Quote:

Originally Posted by psaboic

Wife's 2012 ML350 Bluetec popped a check engine light. Code is P20BD which is which is for "Redundant heater "B" (DEF) circuit low or no voltage. In this case it is for the DEF heater. I can clear the code and religiously about 30 miles later it will come on, so it is a hard failure.

Thanks for posting a detailed description of the fault code, too many people post a code number along with " tell me what part to change."

While I don't have specific knowledge of this car, much can be discerned from the code details.

" Redundant Heater " This could be a miss spelling of " Reductant Heater " however the " B " complicates things. We either have a backup / spare heater in the system or there are heaters in two locations ( A and B )

The " circuit low / no voltage " portion. Sometimes descriptions become generic and this is where a service manual comes in handy for a expanded description / conditions that set the code.

Circuit low means something is pulled to ground or possibly the computer isn't seeing current flow due to a broken wire / open heater element.

No voltage means that heater element isn't receiving power.

We would need a diagram to determine where the power comes from and what controls it. If the part you are replacing contains only the heater element and the no voltage portion of the code is accurate, replacing the heater won't fix the problem.

If the part you are replacing contains come control circuity, this might fix the problem.

97 SL320 · #13 08-31-2019, 10:35 AM

Quote:

Originally Posted by TimFreeh

But apparently this piece of information doesn't apply to a 2012 ML350? The original poster never mentioned anything about about 'after 10 starts you're dead' kind of issues?

I know for most / all diesels that if DEF isn't added for an extended period of time, there is a step down of speed to the point the engine won't go much above idle.

It does bring up an interesting point if a emission related failure will cause the same step down. I'd also wonder if there was a non step down emission failure and empty DEF if the step down would still occur.

psaboic · #14 09-01-2019, 11:55 AM

Ok, here's an update, and it is not bad, but not good either. I managed to get the DEF tank out. Six 10mm nuts for the heat shield, and four 13mm bolts and a strap that hold the tank up. HOWEVER...there is a filler tube, vent tube, pump outlet line, and 5 different wiring connectors you have to remove before you can fully drop the tank. To get to all of these you need to pull off the right rear wheel and remove the plastic fender liner. Once that is all removed and the tank is out, take off three 4mm hex bolts, and lift off the DEF pump off the top of the heater. Once the pump is off, remove the big locking ring, and pull out the heater unit. BIG TIP, either make sure the DEF tank is nearly empty or use a floor jack under the tank to support it (ours was damn near full and I drained about 7+ gallons out of it)

Now for the bad news. The parts supply house sent me the wrong damn heater unit. My old one is about 7 inches in length from top to bottom. The one they sent me (correct part number too) was 14.5 inches in length. Apparently two different types of DEF tank were used. One takes the small heater (Code U42) and one takes the large (Code U77) Both seem to have the same part number. SOoooooo, The wife's car is down for about 5 more days until I get the correct part (Yeah, she is NOT happy).

TimFreeh · #15 09-01-2019, 12:32 PM

Quote:

Originally Posted by psaboic

Now for the bad news. The parts supply house sent me the wrong damn heater unit. My old one is about 7 inches in length from top to bottom. The one they sent me (correct part number too) was 14.5 inches in length. Apparently two different types of DEF tank were used. One takes the small heater (Code U42) and one takes the large (Code U77) Both seem to have the same part number. SOoooooo, The wife's car is down for about 5 more days until I get the correct part (Yeah, she is NOT happy).

Now that everything is apart I think you'd be well served to actually verify that the heater pump is getting power. If the issue is actually a wiring problem associated with power delivery to the pump you're going to look awfully silly to your wife 5 days from now when the new pump arrives.