e38 t42 PRNDL issue

[Swampmonster]

Good afternoon,

I am working a 2007 corvette engine in a 1955 chevy bel air swap. customer swapped engine and installed a truck 4l80E trans.
issue is car shifts quickly to 4th . in scanning live data the PRNDL gear selection data states data is invalid. also line pressure shows to be a diagnosis mode max pressure.

car has a 12611832 e38 ecm with a 12611833 OS and a 24235340 t42 TCM with a 24235341 OS. after reading a enormous amount of info on the . compatibility of the e38 t42 . i download a 2008 express van file from the repository and wrote entire OS for TCM .
I now have 12611832 ecm with 1261833 OS and 24235340 TCM with 24239927 OS. still have issue with invalid PRNDL info and max line pressure.

The only DTC are P0102 MAF code and a U100 loss of communication with Ecm code .

My main question is . is the 12611832ecm with a 1261833 OS compatible with the 24235340 TCM. or do I need to locate a TCM 24252114 ( express van TCM) and install?

Also on a side note there is a urban legend about a PDF file floating around that is a E38 t42 compatibility chart. is that true?

[Shnraines]

No tcm codes? What harness are you using?

[Swampmonster]

HPI harness. i have done some more digging . and followed the gm trouble tree for the u0100 code. at the aldl connector resistance between pin 14 and 6 with ignition off is 120 ohm. the trouble tree tells me it is supposed to be 55-65 ohms. if i understand the flow of the high speed lan schematic correctly. pin 14 on the aldl which is hi speed lan negative connects to pin 37 on my tcm i have and travels out pin 6 of tcm to pin 27 of PCM . on the positive side of the lan the flow is from pin 6 on aldl into pin 38 on tcm then out of tcm from pin 7 and into pin 28 on pcm. pcm makes up the terminating resistance for the hi speed lan circuit correct?

i can unplug tcm from circuit and i no longer have resistance between pin 14 and 6 on aldl. which seems to indicate i have a module to module comm error. the resistance value being 120ohm instead of 55-65 ohm forced me to chase a rabbit trail to make sure it wasnt a wiring issue.

[Shnraines]

I would call or email hpi and see what they suggest. Can try adding a 120 ohm resistor across PCM or aldl CAN terminals to see if that fixes it. I would also expect you should still have CAN from ALDL to ECM with TCM unplugged so that is suspicious.

[blindsquirrel]

120 ohms with everything plugged in indicates that one of the resistors is missing. 120 ohm in ECM in parallel with 120 ohm in harness = 60 ohm at DLC.

There are two different topologies for the CAN wiring. The ECM of course has a built-in resistor as it's always at the end of the run (and has to be, because it only has one pair of CAN pins). But a T42 also has a resistor inside, it's just not used in most applications (TCM has two pairs of CAN+/CAN-, it's usually used as a pass-thru and connects the ECM to DLC). Since you have 120 ohms at DLC 6/14, and then open circuit with TCM unplugged, that tells us two things: one, the harness is built in the inline arrangement, and two, that they left out the second 120 ohm resistor in the harness, located between TCM and DLC. If that resistor were there it would measure 60 ohms normally, and 120 ohms with TCM unplugged.


GMLAN inline simplified.png

GMLAN 'Y' simplified.png

[blindsquirrel]

The other 4L80 OS that gets used for swaps is the 2007 Suburban 2500. If it's not in the repository I can post it again.