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Thread: 80E/T42 Canbus confusion

  1. #1
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    80E/T42 Canbus confusion

    OK, I don't know if this is an error in the GM diagram or something else. Example vehicle is a 2007 Suburban 2500 w/4L80E.

    Suburban 4L80E TCM.png

    All GMLAN connections are daisy-chained except for the start and end, there are no 'tees' anywhere in the middle. So how can a module have three pairs of CAN wires?

    The 4L60E version of the same vehicle shows only the normal CAN wires at 6/7 & 37/38, 15/19 are 'not used'.

    The 80E 2008 Express shows the same weirdness.

    Nowhere in the other diagrams can I find the other end of where the wires from pins 15/19 might be going. It's hard to find them by search as the ckt ID is the same for all the CAN wires no matter where they are in the vehicle, but I have checked individual diagrams for everything I can think of and can't find any destination for these extra wires. Please tell me this is just a printing error and I can ignore it.

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    Advanced Tuner Shrek's Avatar
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    No sir - not a printing error. GM's wiring varied between platforms. All variations are functional, but some are overly complicated (for no consistent reason).

    If you are building a wiring harness for an LS conversion in a Hot Rod, then the CanBus wiring can be very simple. You can build a twisted pair of wires in the "tee" formation that you mentioned - others may call this a "Y" shape.

    Two wires go to the T42, two wires go to the E38 (or E67), and two wires go to the DLC (OBD-II) Port.

    To build the CanBus "Y", create 2 short sections of twisted pair wires - one for the TCM and one for the ECM. Create a longer section for the DLC Port. Then solder the 3 "+" wires together, and then solder the 3 "-" wires together.
    Last edited by Shrek; 04-13-2021 at 07:32 PM. Reason: typo

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    What component is at the other end of the wires going to TCM 15/19? I can't find it in the manual. I also can't see how the parts that are shown would work with an extra pair of wires going to something.

    Suburban GMLAN 1of2.png Suburban GMLAN 2of2.png

    There are two 'legs' of the circuit, but the split happens at IP junction block C2. BCM, TCM, ECM (and its internal terminating resistor) are on one leg. VCIM, TCCM, EBCM, ESC, TR are the other leg.

    Why would a standalone harness with only an ECM & TCM use a 'tee'? Where would it go? That would leave only one pair of wires going to the TCM.

    I'm not building anything, just trying to comperstand what's going on. (I have a lot of free time, can you tell? )

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    OK, confirmed it's a printing error. I happen to have spare T42s here and checked one. Service Number 24234503, shows as valid interchange for 2007 Suburban 2500. So I bench flashed it with 3GNGK26K77G211058.

    CAN+, pins 7 & 38 have continuity. Pins 7 & 38 have no continuity to any other pins. 7/38 have no continuity to pin 15.
    CAN-, pins 6 & 37 have continuity. Pins 6 & 37 have no continuity to any other pins. 6/37 have no continuity to pin 19.
    47.2k ohms between 6 & 7 and between 37 & 38.
    123 ohms between 15 & 19.

    15 & 19 are not valid CAN pins.

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    Advanced Tuner Shrek's Avatar
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    Quote Originally Posted by blindsquirrel View Post
    ... Why would a standalone harness with only an ECM & TCM use a 'tee'? Where would it go? ...

    My description may have failed to "paint" an appropriate picture.

    The point is that in a conversion project, the CanBus wires do not need to be separate and distinct for each connection point.

    Draw a "Y" on a piece of paper. Place the ECM at the top left branch of the "Y". Place the TCM at the top right branch of the "Y". Place the DLC at the bottom branch of the "Y".

    Where the branches of the "Y" intersect, is where the soldered joint would be.

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    Advanced Tuner Shrek's Avatar
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    Quote Originally Posted by blindsquirrel View Post
    ... OK, confirmed it's a printing error ... 15 & 19 are not valid CAN pins ...

    Once again, this is not a printing error. This is how GM wired some CanBus communications.

    Most people know that a Gen 4 ECM contains an internal 120 ohm resistor, which is required by the communications protocols.

    When most people build a "bench" harness to program modules, the ECM harness does not need anything "extra" to function correctly.

    When building a "bench" harness for a T42 TCM, there two common options. One option is to require that the TCM harness be connected to an ECM harness, and that an ECM is present to bench flash the TCM. The other options is to add a 120 ohm resistor to the TCM bench harness wiring, such that it then "terminates" and is programmable in isolation.

    What most people do not know, is that just like the Gen 4 ECMs, the T42 TCM has its own internal 120 ohm resistor (for CanBus termination). That is what T42 pins 15 and 19 are used for.

    The answer to your inquiry is that sometimes GM wired the T42 TCMs to be Can terminated, and other times they did not. Either way works just fine.

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    Advanced Tuner TheMechanic's Avatar
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    Quote Originally Posted by blindsquirrel View Post
    OK, confirmed it's a printing error. I happen to have spare T42s here and checked one. Service Number 24234503, shows as valid interchange for 2007 Suburban 2500. So I bench flashed it with 3GNGK26K77G211058.

    CAN+, pins 7 & 38 have continuity. Pins 7 & 38 have no continuity to any other pins. 7/38 have no continuity to pin 15.
    CAN-, pins 6 & 37 have continuity. Pins 6 & 37 have no continuity to any other pins. 6/37 have no continuity to pin 19.
    47.2k ohms between 6 & 7 and between 37 & 38.
    123 ohms between 15 & 19.

    15 & 19 are not valid CAN pins.
    This gave me a flashback of a CAN issue I had with a Malibu I think. It used the terminating resistor in the TCM. Looking at the CAN diagram I see that 15 19 could be valid in this situation. Generic computers to work with all kinds of product lines. I'll have to go back and check out the diagram. I think it was a 2008 with a 2.4. I'll have to check SI when I get home

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    Quote Originally Posted by TheMechanic View Post
    This gave me a flashback of a CAN issue I had with a Malibu I think. It used the terminating resistor in the TCM. Looking at the CAN diagram I see that 15 19 could be valid in this situation. Generic computers to work with all kinds of product lines. I'll have to go back and check out the diagram. I think it was a 2008 with a 2.4. I'll have to check SI when I get home
    Yes, but all CAN+ and CAN- pins should be common if they are really CAN pins.

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    Quote Originally Posted by Shrek View Post
    My description may have failed to "paint" an appropriate picture.

    The point is that in a conversion project, the CanBus wires do not need to be separate and distinct for each connection point.

    Draw a "Y" on a piece of paper. Place the ECM at the top left branch of the "Y". Place the TCM at the top right branch of the "Y". Place the DLC at the bottom branch of the "Y".

    Where the branches of the "Y" intersect, is where the soldered joint would be.
    That would only make sense if somehow you had the ECM in the front, and the TCM in the trunk. If ECM & TCM are relatively near each other why in the world would you not simply go:
    DLC <--> TR <--> TCM <--> ECM

    If the TCM were at the end of its own leg of a Y it would need a terminating resistor at the unused pair of CAN pins - if you were interested in doing it properly. It might 'work' without it but there's no reason to do it that way. The pins 15/19 in question are isolated internally from the other CAN pins.

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    The 123 ohms between pins 15 & 19 would only function as a terminating resistor if two of the CAN pins were looped over to it, they are not connected internally to any of the other known CAN pins.

    Here's a simplified version of the leg in question from the Suburban with the distractions removed.

    Suburban GMLAN ECM-TCM-BCM.png

    If the TCM pinout labels are correct, where is the circuit that uses pins 15/19?

  11. #11
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    Quote Originally Posted by blindsquirrel View Post
    That would only make sense if somehow you had the ECM in the front, and the TCM in the trunk. If ECM & TCM are relatively near each other why in the world would you not simply go:
    DLC <--> TR <--> TCM <--> ECM

    If the TCM were at the end of its own leg of a Y it would need a terminating resistor at the unused pair of CAN pins - if you were interested in doing it properly. It might 'work' without it but there's no reason to do it that way. The pins 15/19 in question are isolated internally from the other CAN pins.

    You seem to be having difficulty conceptualizing what a "Y" could look like - but placing the ECM and TCM at the top branches puts them directly beside each other.

    Regarding your comment about "doing it properly" ...

    Doing it properly means establishing proper communications between the ECM and TCM, and ensuring that the DLC Port communicates with both - for data logging or reprogramming.

    GM did this so many different ways, that there is no universal "right".

    We have been producing Gen 4 LS conversion wiring harnesses, wired as I described, for nearly 15 years. All of our customer's vehicles operate as they should, and the DLC Ports function as they should.

    I guess we did it properly ...


    Regarding pins 15 and 19 - as previously stated, these are an internal 120 ohm resistor.

    If you want to Can terminate the T42 using its own internal resistor:
    - connect pin 6 to pin 15
    - connect pin 7 to pin 19

    The above are essentially jumper wires.

    Then use pins 37 and 38 for Can Bus communication with the ECM and DLC Port.

    You generally help people on this Forum, which is why I entertained your question in the first place.

    Presenting as condescending - when you don't know what you are talking about - is not a great way to make friends.

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    I apologize. I am not asking about CAN general practices. I am asking specifically about the 2007 Suburban document that lists CAN wires at 6/7, 15/19, and 37/38. Where is the diagram that shows where 15/19 go? Nothing more than that please.

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    Settle down there Shrek. You've been hanging out with Donkey too much! You explained what you were trying to convey very well. Blindquirrel was looking for other info.

    Blindsquirrel-I looked into your conundrum and also found the same discrepancies. This is what I found-The connector end view that you posted is Document ID: 1735742. This does in fact show the 3 sets of Can lines in the TCM. If you go under master electrical list-under TCM there are 2 choices. The other one is Document ID: 1746637 and this shows "not used"

    If you look at the wiring diagrams the wires do not show in any document. I also looked under the 2008 express you mentioned and it has similar pin outs with the 3 Can line set up Document ID: 2495424.

    Looking at the wiring diagram of the express shows the 3rd can bus and it just loops back into the module- Document ID: 1879008 This is to loop back into itself for a terminating resistor for some reason. So on the express the tcm is last in line and silverado it was not. Maybe there were some odd ball suburbans that used this configuration that SI doesn't show very well.

    Cannot get image to load for some reason-the line goes out 6/7 and enters 15/19 and through a 120 ohm.

    express Can bus.jpg

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    Thank you! I hadn't thought to look at the '08 Express GMLAN diagrams. I understood 15/19 could be used that way, but the diagrams said it wasn't.

    What's weird is there were no '07 4L80/T42 platforms that used that wiring arrangement with the loopback yet they all got the incorrect '08 Express TCM pinout copied in. '07 Express was still on the P59. The only one that got both the pinout and diagram correct is the '08 Express...

    I mean, I'm assuming that the error is in the pinout and not the diagrams. At least now I know I wasn't just overlooking the diagram that showed how 15/19 were used. I've found other pinouts that have errors - for example 2006 GTO only has 7 ignition coils. C2 54 is listed as ckt 1310 EVAP vent solenoid instead of ckt 2121 IC1 control. In my offline version of eSI, anyway.

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    I work with these diagrams almost every day at the dealership. I've proven many incorrect schematics- usually wasting my flat rate time...grrrrrr. I just send the info in to engineering and they pass it along to somebody else.

    I remember a few years ago I was following a diag for a seat module/motor. It said to use a test light to check a circuit. I read the diagram and that circuit is pulse width modulated and will never cause the light to illuminate. The guys at gm got a good laugh at that one.