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Odd! When the ABS is disconnected the ECM and TCM are disconnected from CAN bus so it should not have changed the values at all when ECM was disconnected. Are you sure you hit the right pins? I cannot verify the actual value the resistance should be since I do have shorts to ground in my system. I have about 8k ohms to ground which lets the system work OK, but I have to fudge things if I want to program ECM etc.
From the first set of results with around 2+ volts that sounds OK. The 8k ohms is like mine where I actually have two modules partly shorting internally to ground. Seems ABS is prone to internal shorts. My TCM also has a partial short. Car runs fine without codes. The buses are fairly robust and can function with a few anomalies.
Going for dinner now will look at numbers more after.
It would appear to me that the issue is in the TCM connection or wiring leading to the connection as the change in readings for 6&14 to ground are significant when you connect and disconnect the plug. I tested the pins in the transmission and there appears to be negligible resistance but it is definitely not zero resistance.
The TCM pins on the transmission read:
Pin 2 to ground = 22k ohms
PIN 6 to ground = 23k ohms
The reason why the ECM is still reading after the ABS is disconnected is because I replaced the CAN bus with brand new twisted pair. As much as possible; I attempted to avoid chaining the connections in and out of each module as well as avoid as many wiring harness connection plugs as possible to eliminate any resistance or broken connections they might cause. The ABS to ECM is wired to the input of the CAN ABS module… the YAW is wired to the output. My understanding of the CAN spec is that this should not make any difference.
It is my understanding that the Yaw is a local CAN bus and is not part of the main system. It has its own separate connections on the ABS module. It could make things very unhappy if it is on the main CAN bus.
Correct on the yaw sensor…. It is indeed “local” and MUST be wired to the ABS correctly, I spoke without considering the whole bus. Thank you for correcting that supposition on my part.
I wanted to update everyone that might be following this thread.
It turns out that it is indeed the ABS system that is the originator of the ground fault to CAN. But it is NOT the module but rather the plug. Apparently at some point a past owner must have had the plug replaced with a junkyard plug. The work was relatively professional as they pulled the pins from the original module and placed them into the replacement. This preserved the original wiring and avoided a few dozen splices in the wiring harness leading to the plug. Unfortunately, the mechanic must have put the CAN low pin in the wrong position as Pin 15 turns out to be some other wire which I will have to trace as it is most definitely NOT part of the CAN bus twisted pair. Currently pin 15 goes to ground, which is what I have been reading on the meter all along. I will update this thread as I resolve this issue and let everybody know if i finally have a running car.
Correct on the yaw sensor…. It is indeed “local” and MUST be wired to the ABS correctly, I spoke without considering the whole bus. Thank you for correcting that supposition on my part.
I wanted to update everyone that might be following this thread.
It turns out that it is indeed the ABS system that is the originator of the ground fault to CAN. But it is NOT the module but rather the plug. Apparently at some point a past owner must have had the plug replaced with a junkyard plug. The work was relatively professional as they pulled the pins from the original module and placed them into the replacement. This preserved the original wiring and avoided a few dozen splices in the wiring harness leading to the plug. Unfortunately, the mechanic must have put the CAN low pin in the wrong position as Pin 15 turns out to be some other wire which I will have to trace as it is most definitely NOT part of the CAN bus twisted pair. Currently pin 15 goes to ground, which is what I have been reading on the meter all along. I will update this thread as I resolve this issue and let everybody know if i finally have a running car.
Thank You All!
Update:
Rewired the ABS plug. It was wired upside down.
The wheels were in the wrong place, the CAN Bus was in the wrong place… etc.
To ensure accuracy I traced every wire to its plug or connection, which took all day.
After I put the whole system back together, minus the TCM which I still need to wire to the CAN bus… I still had a short to ground on the CAN Bus.
Also the ABS motor is running on key on and it sounds rough.
Next I removed the CAN bus pins from the ABS plug and tied them together to complete the bus without including the ABS module… success. No short to ground and the 6&14 CAN bus wires are now 60 ohms😃. Which is a Hugh improvement from where I was. Unfortunately, I still can read the ECM.
My question, among many, is how much damage would have been done by reversing the the wires in the ABS plug? Is it toast? Would it have also damaged the ECM?
The mind boggles at the idea that the car would have run at all… I am at a lose for what the next step should be.
Thoughts?
the photo of the plug is before I rewired it. Wiring diagram ABS plug Original wiring before I rewired
Last edited by Bluefish001; 06-08-2023 at 04:34 PM.
Gosh, are you sure it had been rewired in reverse....if it was its a heck of a find.
Just been going over that photo to map to the pin outs of the plug and it seemed to mostly track if in fact the two black wires are to the (****Correction) "top" of the plug (pins 16 and 47).
This matches the numbering visible in the photo of the socket on the ABS unit.
Not sure but you might want to verify a couple (could be obscured or optical illusions on my part, but hey).
Pin 2 - shows as vacant but you have a white wire connected there.
Pin 4 - supposed to have a white wire (B+ supply in) but photo obscured and looks like it could be vacant.
Pin 13 - supposed to be green but looks a lighter colour. (goes off to sliding roof module so possibly not initially important).
Pin 25 - supposed to be blue but looks green
Pin 28 - supposed to be White/green stripe, but appears solid white (green stripe could just be obscured in photo).
******Taken a second look now I'm sitting down in the weekend and realized I got top and bottom out of whack when I looked at the ABS socket and the shaping of the keyway.
Last edited by h2o2steam; 06-09-2023 at 08:11 PM.
Reason: Correcting my top and bottom interpretation.
Ok so I think I am losing it. Only a few wires were wrong on the ABS.
The CAN bus test perfectly now.
Now I have to chase down the SCP issue.
2 - 5 is 120 ohms
ODB port 2 - 16 is 9volts
ODB port 10 - 16 is 6volts
Apparently they are to be less than 3volts.
the only way I can remove the voltage is to remove all of the SCP modules. Even one module returns the volt state to the recorded values above. It is absolutely NOT the SCP wiring as this has been completely replaced with new wiring, the same as the CAN bus. Triple checked the module plug pins to ensure that the new wire pair is in the correct position. My guess is that a common ground or signal wire is shorted to power… but I am at a lose at the moment of how to approach this issue.
Of interest is pages 42 (ref: 3.7) and 43 (ref: 3.7) which covers diagnostics of SCP and typical voltages on various modules.
Separately you mention in prior posts that you have run all new wiring.
I did note when I read the SCP standard that the wire pairs require a minimum twist rate of 1 twist per inch and no more than 10 inches of untwisted wire going to any connection to ensure interference rejection.
Also that the wiring is in star formation to reduce relative distance between any module.
I too am puzzled by the voltages you are getting at pin 2 of the DLC.
The SCP system by all technical accounts runs a 0-5v signal sent in mirror image across SCP- and SCP+ wires which seems to be supported by the information in the document I have linked.
However, in the pinpoint test G239843t29 in your prior post it is referring to you testing across pins 2(SCP+) and 16 of DLC and a voltage reading above 3V indicates a short to battery. But pin 16 is B+, so a short to that I would have thought would give you a low voltage differential to SCP+. I would have thought logically you would be testing from ground to pin 2 to identify if SCP+ is being pulled high by a short to battery, not to 16. I'm reluctant to say or think there is an error in that pinpoint test G239843t29, but it doesn't seem to make sense to me.
Based on your actual voltage readings, assuming B+ is say 12.5v, your readings of ODB port 2 - 16 is 9 volts (referencing to 12.5V) is actually 3.5V to ground ODB port 10 - 16 is 6 volts (referencing to 12.5V) is actually 6.5V to ground
These are higher than the "SCP Network Integrity Check without WDS" information in what I have linked you to (page 42), but as you are so deep into it already, I suggest you follow the "one module at a time" testing procedure to check you are getting the appropriate voltages on SCP- and SCP+ when ignition off and on. Hopefully you will find the culprit module or connector mis-wired.
Of interest is pages 42 (ref: 3.7) and 43 (ref: 3.7) which covers diagnostics of SCP and typical voltages on various modules.
Separately you mention in prior posts that you have run all new wiring.
I did note when I read the SCP standard that the wire pairs require a minimum twist rate of 1 twist per inch and no more than 10 inches of untwisted wire going to any connection to ensure interference rejection.
Also that the wiring is in star formation to reduce relative distance between any module.
I too am puzzled by the voltages you are getting at pin 2 of the DLC.
The SCP system by all technical accounts runs a 0-5v signal sent in mirror image across SCP- and SCP+ wires which seems to be supported by the information in the document I have linked.
However, in the pinpoint test G239843t29 in your prior post it is referring to you testing across pins 2(SCP+) and 16 of DLC and a voltage reading above 3V indicates a short to battery. But pin 16 is B+, so a short to that I would have thought would give you a low voltage differential to SCP+. I would have thought logically you would be testing from ground to pin 2 to identify if SCP+ is being pulled high by a short to battery, not to 16. I'm reluctant to say or think there is an error in that pinpoint test G239843t29, but it doesn't seem to make sense to me.
Based on your actual voltage readings, assuming B+ is say 12.5v, your readings of ODB port 2 - 16 is 9 volts (referencing to 12.5V) is actually 3.5V to ground ODB port 10 - 16 is 6 volts (referencing to 12.5V) is actually 6.5V to ground
These are higher than the "SCP Network Integrity Check without WDS" information in what I have linked you to (page 42), but as you are so deep into it already, I suggest you follow the "one module at a time" testing procedure to check you are getting the appropriate voltages on SCP- and SCP+ when ignition off and on. Hopefully you will find the culprit module or connector mis-wired.
Good luck with the hunt.
Quick, hopefully, question:
What is a {common logic ground] and how is it different from a ground on a Jaguar. I ask because I am suspicious that it might be related to my problem, but it could also be that I don’t understand what it should be accomplishing. BTW, thank you for your response and the article link… I will be working with it later this week. I am using twisted pair designed for CAN. I also added a protective casing to ensure that it remains damage free over the coming years.
Last edited by Bluefish001; 06-13-2023 at 09:17 AM.
Hi,
Yes there is a power ground and a logic ground.
Sometimes in various cars the logic ground is partially isolated from the power (chassis) ground as additional protection for the sensitive computer systems.
However looking at this schematic, logic ground (pin 12 of CR9 of the front electronics module) is a separate wire grounding to chassis ground point G9 (upper left A pillar). Also at the data link connector (IP39) pin 4 is the power ground and pin 5 is the signal/data ground, both terminate at the common chassis grounding point G31. Further proof the power and data systems are earth strapped in these cars.
We do have ground issues in these cars with corroded lugs or failing grounding posts that fracture, so you might wan to take careful look at that point.
The more common grounding lugs that are problematic seem to be in the engine bay area or boot.
Back to your measurements; the diagnostic procedure document stipulated measuring from SCP- to chassis and SCP+ to chassis as confirmed below.
Let's just see if all the individual modules pass the above isolation test procedure for correct voltages and resistance measurements on SCP- and SCP+ at the DLC.
If they do, then it might be a case of trying to work out if any SCP commands are being relayed to other modules, or if none at all are operating.
In the Appendix of the schematic, there is a SCP message matrix that denotes which modules are sending particular messages and commands out and which module is to receive them from the SCP Bus.
There is a long list there, but if all modules prove to be electrically present from the prior testing, you might be able to test some specific functions to see which modules might be responding and which aren't.
Embarrassed to ask this, but where is the General Electronics control module?
I can finally read the SCP bus and I am seeing this error:
Since there does not appear to be a discrete module with that name, I am guessing it is part of another physical module… like the FEM?
06-06-2023, 04:24 PM
