TRNW-2: 6t40 can bus diagnosing

[Justin Merritt]

I'm back at diagnosing this car this morning.  So all the replies so far
have been very helpful in making my mind understand what I am testing. Dan
wiring diagram that he posted this morning is the same that I was using
yesterday to try to diagnose.  His explanation of where to test at the tcm
connector and what I should be getting for ohm readings made the most sense
to me on how to test and which direction the harness I wanted to go.  First
thing I accidentally found by testing resistance at the techm connector is
that the wiring diagram is WRONG!  PIN 7&8 run toward the power steering
module and 6&14 go to the ECM.  I was looking for the terminating resistors
in each direction that Dan pointed out, and I ran upon that diagram
mistake.  Now that I have a better understanding of what I am looking for
by testing, I know which way to work.  From my Techm, I have the 120 ohms
to the ecm, but I have an open circuit on 7&8 to the power steering
module.  I go to the power steering module and test continuity and my
terminating resistors in both directions, and I have them.  So it seems my
problem is the data line running through the rack and pinion power steering
module.  I jumper wired the data lines at the power steering module and I
now have communication with my ecm and tcm.  Thanks for all the replies, I
have tried to keep up with reading and classes on the CAN Bus stuff, but I
never fully understood it until this real world experience.  Now to locate
a rack and pinion and get this car going.

Thanks Again
Justin

On Mon, Oct 20, 2025 at 7:01 AM Dan Hogan via trnw2 <trnw2 at trnw.net> wrote:

> Here's a possible HSCAN network schematic for a 14 cruze 1.4
>
>
> The HSCAN bus is terminated internally by the BCM on one end and the ECM
> on the other.  Highlighted in yellow.
>
> If you're reading 60 ohms at the DLC, one end of the bus is no longer
> terminated somewhere and I think you've found at least the likely end --->
> ECM.  Remember the MF3/MZ0 wiring is not present in your application.
> Ignore.
>
> *Battery negative disconnected.  No power to vehicle prior to test.*
>
> I think the fastest way to verify the bus wiring between the TCM and ECM
> would be to disconnect the TECHM connector, ECM connected normally and
> measure the resistance between pins 7 and 8 on X1.  If intact, this should
> be 120 ohms.  This would test the wiring and ecm termination.  You can also
> measure the bus going back to the BCM / DLC by measuring at pins 6 and 14
> here at the TECHM, this should also be 120 ohms.   While the TECHM
> connector is disconnected, good time to check terminals for pin fit or
> whether they look expanded or damaged.  Loose terminal fit can be real pain
> to diagnose.
>
>
>
>
> ---
>
>
> Dan Hogan
>
> Buster's Transmission Service
>
> https://busterstransmission.com
>
> 704.847.5321
> ------------------------------
> *From:* trnw2-bounces at trnw.net <trnw2-bounces at trnw.net> on behalf of
> Justin Merritt via trnw2 <trnw2 at trnw.net>
> *Sent:* Sunday, October 19, 2025 1:34 PM
> *To:* trnw2 at trnw.net <trnw2 at trnw.net>
> *Subject:* TRNW-2: 6t40 can bus diagnosing
>
> Hello all,
> I am working on a 14 Chevy Cruze 1.8 engine. I am chasing a loss of
> communication of both the transmission and engine control module. I
> struggle when it comes to testing wiring on the can bus lines, I just don’t
> have enough experience with it. I’m determine to figure this one of for a
> learning experience. I have included a diagram of some wiring I am testing,
> and I have continuity between pin 29 and 6 and 43 and 14, but I do not have
> continuity between 29 and 7 and 43 and 8. The diagram cuts off, but the
> mf3/mz0 squares just loop around the tcm and back into the wires 6 and 8 of
> the tcm.  Would I be thinking right that I have a break in the 2 bus wires
> 29 and 7, and 43 and 8 that is causing my no communication?
> Thanks for any help.
> Justin
>
>
> Sent from my iPhone
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