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Discussion Starter #1
So I have a 2000 ML320 and I got it with a check engine light. After working on the car, I am down to my last two codes, both are heater circuit problems in the post-cat oxygen sensors. After inspecting the plugs I noticed a lot of corrosion which all led me to new sensors wired directly to the car bypassing the original connector in the car which was too corroded to fix. Now, with fresh wiring i noticed on either side, the heater wires (the two thicker ones) one seems to have steady current while the other blinks on and off. I'm assuming this is why the sensor doesn't heat up because the power keeps getting cut off every second or so? If not please let me know as I am out of ideas and I've spend about a week trying to solve this! Thanks.
 

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2003 C320 coupe, 2000 ML320
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I've been battling the same problem for a year.. You're on the right track.. The heaters are monitored/controlled by (2) two FETs in the ECU.. I've been nursing a broken foot, so I haven't been able to get under the car, but my next attempt at a repair is going to run some heavy gauge wire from O2s heaters to the ECU.. If that doesn't fix it, .. it's the ECU and I'm not going to replace it... My O2s run just fine without the heaters,.. It just takes a few minutes for them to warm up..
 

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Discussion Starter #3
I've been battling the same problem for a year.. You're on the right track.. The heaters are monitored/controlled by (2) two FETs in the ECU.. I've been nursing a broken foot, so I haven't been able to get under the car, but my next attempt at a repair is going to run some heavy gauge wire from O2s heaters to the ECU.. If that doesn't fix it, .. it's the ECU and I'm not going to replace it... My O2s run just fine without the heaters,.. It just takes a few minutes for them to warm up..
Yeah, I pretty much tried everything hoping that it isn't the ecu. I have not seen exactly how much current runs through the wire and whatnot but I do know that there is current going to the sensors. I've heard of people wiring the sensors directly to the battery, essentially bypassing the ecu but I dont know if the ecu would accept it without turning on the CEL. Also, I've heard that the sensor's ground is the exhaust pipe? So where it screws in if there is no good contact, that may cause the circuit to be open? I don't see how that makes sense but maybe it's out of my reach. I always assumed the ground was one of the 4 wires of the sensor and that the ecu provides the current and ground.
 

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2004 Mercedes Benz CLK500 (Wifes), 2002 ML320 (Sons), 2005 ML500, 2002 ML500, 2000 ML320
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I built a W163 O2 sensor heater circuit test light from an old sensor connector after reading a extensive research paper on improper
testing resulting in good sensors being replaced way too often.

I have used this to troubleshoot a O2 sensor problem on a 2000 ML320 our family has, last year. The heater circuit current does flash on
a good W163 O2 circuit. I have verified this also on an 2002 ML320, 2004 ML500, and 2 2005 ML500's our family owns and all of these have no
CEL or O2 sensor issues.
 

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Discussion Starter #5
I built a W163 O2 sensor heater circuit test light from an old sensor connector after reading a extensive research paper on improper
testing resulting in good sensors being replaced way too often.

I have used this to troubleshoot a O2 sensor problem on a 2000 ML320 our family has, last year. The heater circuit current does flash on
a good W163 O2 circuit. I have verified this also on an 2002 ML320, 2004 ML500, and 2 2005 ML500's our family owns and all of these have no
CEL or O2 sensor issues.
Thank you. I did that already and both banks have a healthy circuit, this has led me to believe it has something to do with the ecu, it is pretty disappointing that it is something out of my reach as I've already done eveything I could think of. Anyways, I'll keep an eye out for any other suggestion that I have not thought of. Thank you again!
 

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2003 C320 coupe, 2000 ML320
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Here's what I've come up with... The heater is not chassis ground.. One side of the heater line is B+ (12v ) the other side of the line is run back to the ECU which grounds it with a signal switched FET.. The ECU monitors the FET's source (high/low parameters).. If the ECU see something out side of the high/low,it shuts down the FET and throws the heater code,... Not having tried the "new wire".. I can't say that's a fix, BUT.. FETs don't normal fail intermittently.. They're like a light bulb, they're good or they're blown..
2609557
PS.. On mine, I've checked and re-soldered the FETs.. for cold solder.. ..that's not it..
 

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Discussion Starter #7
Here's what I've come up with... The heater is not chassis ground.. One side of the heater line is B+ (12v ) the other side of the line is run back to the ECU which grounds it with a signal switched FET.. The ECU monitors the FET's source (high/low parameters).. If the ECU see something out side of the high/low,it shuts down the FET and throws the heater code,... Not having tried the "new wire".. I can't say that's a fix, BUT.. FETs don't normal fail intermittently.. They're like a light bulb, they're good or they're blown..
View attachment 2609557 PS.. On mine, I've checked and re-soldered the FETs.. for cold solder.. ..that's not it..
So from what I understood the switching of one wire on and off is the FET and the other is just 12v which is the other wire. Both of my banks do just that so the FETs are doing their job and as you said if they didnt it would be like a bulb and just be off. So assuming the ecu is fine and whatnot and the wiring works, I'm left with the sensor itself. If I wire it and hold it with my hand should it get hot? I tried it and it stayed the same temp. I'm thinking if maybe another factor makes that code go off such as an exhaust leak or a loose hose in the engine, I just recently cleaned out my egr valve since it was completely clogged and the hoses were all broken. After doing so my mpg went from 8-10 to 15-18 and it's been slowly declining again since, i am currently doing 12-14. My fuel trim values have gotten closer to 0 except one long term which is higher than the rest, I'm sure theres a leak somewhere I just dont know if they're related to the o2 sensors acting up. I'm sorry for my ignorance I just don't know exactly how eveything works. I thought it could even be the catalytic converters but that wouldn't be a heater circuit code. Thanks again guys!
 

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2003 C320 coupe, 2000 ML320
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It's the connectors, wire , sensor or ECU.. or might be a combo.. Let me go a little bit deeper on my trouble shooting so far.. Before I broke my foot, I was at the point of eliminating the ECU.. Right now I doing this from re-call, I didn't wright any of this down (dumb ass).. The heater codes are thrown when the ECU shuts down the FETs for whatever the reason.. To get the "check engine" light to go out, you have to clear the fault codes.. On the same vane, the ECU will not re-start the heaters once the fault has been tripped.. You can shut the car off and the heater/FETs will be working but you won't be able to see the code being tripped.. Now here's where it gets real sketchy.. There are only (2) two FETs to run all four heaters.. Two heater are paralleled into one FET, and I can't remember if it's Bank 1 into a FET, Bank 2 into the other FET or it's Front 1 and 2 into a FET / Back 1 and 2 into the other FET.. but it is two heaters on one FET.. Now lets do some math.. 9 to 14 ohms is within the high/ low single heater parameter .. Paralleled.. 4.5 to 7 ohms for a single FET.. ( E over I times R ) With a little fudge factor,.. 1.75 to 2.65 amps through the source on the FET.. Anything above or below.. The ECU shuts down the FET.. Now here's my reasoning why it might be the wire... Car shut off, you meter the heater lines at the ECU connector.. You get a reading of 10 to 12 ohms on each set of lines.. All looks good.. You start the car up, heaters start pulling some current, crappy connectors/wire resistance goes up, current drops below parameter, ECU shuts down the FET.. THIS IS JUST MY THEORY, this is not in any MB service manual that I'm aware of.. This comes from pulling apart the ECU and examining the heater circuit.. The ECU is a double sided print board and are bitch to signal trace,.. You can easily miss something when flipping them back and forth tracing print.. For me, I'm going to replace every possible cause for failure before I call it " it's the ECU"..
PS.. the way my foot is "F"ed up .. It's going to be spring before I can get back underneath the car.. So, please post the fix if you can solve the "no problem" heater codes.
 

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Discussion Starter #9
It's the connectors, wire , sensor or ECU.. or might be a combo.. Let me go a little bit deeper on my trouble shooting so far.. Before I broke my foot, I was at the point of eliminating the ECU.. Right now I doing this from re-call, I didn't wright any of this down (dumb ass).. The heater codes are thrown when the ECU shuts down the FETs for whatever the reason.. To get the "check engine" light to go out, you have to clear the fault codes.. On the same vane, the ECU will not re-start the heaters once the fault has been tripped.. You can shut the car off and the heater/FETs will be working but you won't be able to see the code being tripped.. Now here's where it gets real sketchy.. There are only (2) two FETs to run all four heaters.. Two heater are paralleled into one FET, and I can't remember if it's Bank 1 into a FET, Bank 2 into the other FET or it's Front 1 and 2 into a FET / Back 1 and 2 into the other FET.. but it is two heaters on one FET.. Now lets do some math.. 9 to 14 ohms is within the high/ low single heater parameter .. Paralleled.. 4.5 to 7 ohms for a single FET.. ( E over I times R ) With a little fudge factor,.. 1.75 to 2.65 amps through the source on the FET.. Anything above or below.. The ECU shuts down the FET.. Now here's my reasoning why it might be the wire... Car shut off, you meter the heater lines at the ECU connector.. You get a reading of 10 to 12 ohms on each set of lines.. All looks good.. You start the car up, heaters start pulling some current, crappy connectors/wire resistance goes up, current drops below parameter, ECU shuts down the FET.. THIS IS JUST MY THEORY, this is not in any MB service manual that I'm aware of.. This comes from pulling apart the ECU and examining the heater circuit.. The ECU is a double sided print board and are bitch to signal trace,.. You can easily miss something when flipping them back and forth tracing print.. For me, I'm going to replace every possible cause for failure before I call it " it's the ECU"..
PS.. the way my foot is "F"ed up .. It's going to be spring before I can get back underneath the car.. So, please post the fix if you can solve the "no problem" heater codes.
I'm going to get my hands on a multimeter and see the numbers in real time, when I bypassed the original corroded connectors, i noticed the wire under the insulation was black meaning moiusture has made it's way through all the wire, I sanded the ends for good contact before making my new connections. I'll keep you posted if I throw down some new wires I still have to figure out how to access all the wiring that leads from the bottom of the cabin out to the ecu, it's a bit messy with the abundance of wires but I'll take some time to figure it out.
 

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I would just run a new set from the sensors out over the top of the hood for a test first.. If it's a fix, then you can make a permanent run, You are aware of the w163 wiring diagrams at the top of this forum page ??.. It's good for locating the connector where the lines go into the ECU
 

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Discussion Starter #11
Oh I didnt know about the wiring diagrams, I'll take a look at them, maybe I can try the new wiring this weekend. I'll keep you posted. Thanks.
 

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Ya,.. You have to down load "Auto Desk".. And when you want to look at a folder it doesn't automatically load, it pops up as a "download".. Double click the file, then it comes up in Auto Desk.. The first time I used it, it didn't seem to be working but it does.. you just have to get the hang of it how it works... GOOD LUCK..
 

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Discussion Starter #13
So I attempted to find the wiring into the ecu and in that process I unplugged the ecu to have it out if the way and moved the fuses up a little. Anyways, after struggling to find the cables I plugged everything in to try again another day, I drove home fine and now my car didnt start this morning, didnt turn over, no click nothing. Doors dont unlock, cabin lights dont turn on the only life the car has is the first position of the key that turns on the airbag and srs light. Second position does nothing when usually it turns on all the dashboard lights. Does unplugging the ECU have anything to do with it? Because I drove fine last night after unplugging it
 

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I would first try unscrewing it again. Unseat and remove the AAM, be careful of the peg clip on the back, there are you tube videos, then reset it and put the fuse box back.There is always a chance all was fine then on the way home you hit a driveway bump and that messed it up.

Hope you did all the above with the battery disconnected?

If not there is a chance the transponder for the RFID chip got fried, especially if you had the key in and on during all of this it recognized the key for the start home but after that it can not be read. Or moving the fuse box you undid or loosened a positive battery cable.
 

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Discussion Starter #17
I would first try unscrewing it again. Unseat and remove the AAM, be careful of the peg clip on the back, there are you tube videos, then reset it and put the fuse box back.There is always a chance all was fine then on the way home you hit a driveway bump and that messed it up.

Hope you did all the above with the battery disconnected?

If not there is a chance the transponder for the RFID chip got fried, especially if you had the key in and on during all of this it recognized the key for the start home but after that it can not be read. Or moving the fuse box you undid or loosened a positive battery cable.
I did all of it with the battery unplugged, I'll undo the fuse box again and search for a video, thank you. I'll keep you updated. Hopefully it is just that and in the drive home it moved or something. Thanks again!
 

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Discussion Starter #18
I did all of it with the battery unplugged, I'll undo the fuse box again and search for a video, thank you. I'll keep you updated. Hopefully it is just that and in the drive home it moved or something. Thanks again!
Alright I did it and still the same result, the lights dont turn on inside, the locks dont work the only thing on is the radio and windshield wipers along with the headlights. None of the dashboard lights turn on in the on position.
 

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Some one else will need to step in if the transponder or the AAM is at issue, I can’t remember if you will have dash lights or not along with the others items not working?

And you have no start at all?

You have checked all the fuses none blown or not setting seating all the way in?
 

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Discussion Starter #20
Some one else will need to step in if the transponder or the AAM is at issue, I can’t remember if you will have dash lights or not along with the others items not working?

And you have no start at all?

You have checked all the fuses none blown or not setting seating all the way in?
Yeah no start, not even a click from what I can hear. This is pretty bad as I drive to school every morning. Hopefully I get help figuring it out.
 
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