Obd2 K-line circuit

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ATXOffRover

ATXOffRover

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26
Location
Texas
Having some communication errors with my obd2 port. Was wondering if anyone new the voltages that are supposed to be coming from the ecus I seem to be getting 0 from bcu when it’s disconnected from header and a solid battery voltage from ecu and various voltages from the other ecus. With them plugged into header I receive battery voltage up to the obd2 socket pin 7. Checked continuity from obd2 socket through header to ecu/ bcu plug and get .5 so my guess is I’m supposed to receiving some sort of voltage from the bcu Kline wire but cannot find voltage range.
 
Describe the symptom with details if you want help, checking voltage on the K-line with multimeter is not the way to troubleshoot a communication error, eventually with oscilloscope.
 
JUKE179r said:
Welcome to the forum Hoss. :cool:
What type of Disco do you have?
What is your Disco doing or not doing?
Click to expand...
Hey man! Thanks for the welcome. I’ve got a 2004 Discovery 2 with a 3 inch terrafirma lift, HD steering and drag link, Adjustable panhard, extended brake and ABS lines, JKS sway bar quick disconnects, Sound proofed/ insulated interior.
 
ATXOffRover said:
Hey man! Thanks for the welcome. I’ve got a 2004 Discovery 2 with a 3 inch terrafirma lift, HD steering and drag link, Adjustable panhard, extended brake and ABS lines, JKS sway bar quick disconnects, Sound proofed/ insulated interior.
Click to expand...
OK as you're in Merka, do we assume it runs on gasoline?;)
 
sierrafery said:
Describe the symptom with details if you want help, checking voltage on the K-line with multimeter is not the way to troubleshoot a communication error, eventually with oscilloscope.
Click to expand...
Hey buddy! I have a multimeter as well as a power probe. So let’s see I’ve done so many different tests at this point. I just left a rover shop where they were able to let me run comparison diagnostics and with the header connected on the working vehicle I was seeing roughly 6.2 V with my power probe and multimeter. With the header disconnected I saw several different voltages as well as a ground changing to power, odd to say the least. So I think I answered the question about the bcu voltage there at least and I think I may have narrowed it down to a 12V or battery short somewhere in the k-line to my ecu.
On my vehicle I get battery voltage from the k-line coming off the ecu and matching when on all k-lines with the header connected. Even after running a continuity check from the port all the way to the ecu plug and seeing .5 or.05 ohms I can’t even remember anymore haha but it seemed in reasonable spec gave a resistance check against ground and nothing so my guess is either my k-line is receiving 12V from somewhere in the wiring or my ecu plug may be faulty. This is 1 of 2 ecus I have tried and they both were showing battery voltage on the k-line.
 
I’ve back probed the k-line to see if there were any changes to the voltage when trying to make a connection and it didn’t budge
 
Not sure how to get my photos small enough but in short the k-lines on head and obd2 are 6.2 V on the working vehicle and roughly 12.1 V on mine
 
sierrafery said:
Describe the symptom with details if you want help, checking voltage on the K-line with multimeter is not the way to troubleshoot a communication error, eventually with oscilloscope.
Click to expand...
Only part that doesent match the working vehicle voltage is the ecu I just spliced in a new k-line and am still receiving 12.4 V now I’m wondering if the can data bus between ecu and transmission might be coming into play or something I have a new ecu c0637 I could split in if that could potentially have a issue.
 
ATXOffRover said:
Only part that doesent match the working vehicle voltage is the ecu I just spliced in a new k-line and am still receiving 12.4 V now I’m wondering if the can data bus between ecu and transmission might be coming into play or something I have a new ecu c0637 I could split in if that could potentially have a issue.
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I still dont understand what's the problem, are you trying to start the vehicle with a replacement ECU or what? You can't compare voltage on the K line on a running vehicle with one which doesnt start if you see what i mean. Did you plug in a scanner and there's no communication with any ECU or why are you insisting on that K line? You have to explain what's going on not what you are measuring there
 
sierrafery said:
I still dont understand what's the problem, are you trying to start the vehicle with a replacement ECU or what? You can't compare voltage on the K line on a running vehicle with one which doesnt start if you see what i mean. Did you plug in a scanner and there's no communication with any ECU or why are you insisting on that K line?
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Sorry I wasnt very clear haha I cannot connect my obd2 scanner of any kind to my vehicle. Link fails
 
Feel free to ask any testing specific questions. I have seen several of these threads with your responses and feel like I have gone through each and every test. And unfortunately dont own a Oscilloscope. The OBD2 port was working was just a little finnicky with live data prior to diving into a o2 sensor diagnosis. After undoing battery, disconnecting ecu, and reconnecting about 10 times now right at the end it stopped communicating all together. The OBD2 Scanner receives power just wont link.
 
Last edited:
First of all you'll have to try with a dedicated scanner which has the ability to select each ECU at a time to see if only the engine ECU is not communicatng or none of them. The D2's diagnostic protocol is not entirely OBD2 compliant. you'll have to unplug one ECU at a time and and try so to communicate with the ECM. If there is no comms at all with any ECU.. it's possisble that one of the ECUs is at fault and mixes up the K line (SLABS, ACE, Transmission, except the BCU which manages the diagnostic link) as the BCU acts like a gateway and disables the data link if one ECU is gone. I've seen cases when only the ACE ECU was at fault and there was no comms on the data link.
 
sierrafery said:
First of all you'll have to try with a dedicated scanner which has the ability to select each ECU at a time to see if only the engine ECU is not communicatng or none of them. The D2's diagnostic protocol is not entirely OBD2 compliant. you'll have to unplug one ECU at a time and and try so to communicate with the ECM. If there is no comms at all with any ECU.. it's possisble that one of the ECUs is at fault and mixes up the K line (SLABS, ACE, Transmission, except the BCU which manages the diagnostic link) as the BCU acts like a gateway and disables the data link if one ECU is gone. I've seen cases when only the ACE ECU was at fault and there was no comms on the data link.
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I have tried disconnecting each ecu from the header to no Avail. Do I need to physically disconnect each ECU or is the header being disconnected ok? I think I will invest into a Nanocom just didnt want to drop that kind of money if I wasnt able to figure this out first haha. Its just really wierd that it was working and out of nowhere it stopped.
 
In comparison to a running/communicating rover my voltages to each ecu are the same except for the main engine which pretty much makes sense as that was the one that I was disturbing when trying to fix a previous owner wire splice job to o2 Sensors. I get Battery voltage from the ECM ECU K line so im thinking there may be a cuplrit in 1 or even 2 of the harness connections. Just nothing clear on how to test. I have also purchased a Used ECU/BCU/Gauge Cluster and swapped out but still nothing.
 
So you can unpin only the K line for each ECU, EXCEPT THE BCU and and leave just one for example the engine ECU with the BCU then you'll have to get communication with it if the problem is with one of the other ECUs or if you suspect the ECM unpin the K line for that and see if you get comms that way cos even if the others are not OBD2 at least you should get some false codes or something... as i said voltage measurements on the K line are irrelevant ... in USA you can buy Foxwell Nt 530 cheaper which would work for your vehicle.
 
Hmmm I wonder if the one
sierrafery said:
So you can unpin only the K line for each ECU, EXCEPT THE BCU and and leave just one for example the engine ECU with the BCU then you'll have to get communication with it if the problem is with one of the other ECUs or if you suspect the ECM unpin the K line for that and see if you get comms that way cos even if the others are not OBD2 at least you should get some false codes or something... as i said voltage measurements on the K line are irrelevant ... in USA you can buy Foxwell Nt 530 cheaper which would work for yor vehicle.
Click to expand...
I wonder if the 1 I own would work seeing as sometimes when it was working before I was unable to select Live data or read codes but could still connect I will go attempt now and will return shortly to let you know
 
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