1991 C2500 4L80e to 1995 4l80e swap (7427 ECU swap)

[TheScaryOne]

I've got this 1991 Chevy that's 4l80e has gone south. I'm lead to believe that the 91-93 style transmission with the Bosch force motor and cleaning cycle is inferior to the 94-96 style, and would like to install a Hughes 4l80e, which they list as fitting 1991-96, which I'm assuming means the later style transmission. TCI is in the same boat, lists one part number for that whole year range. After doing some research it looks like my easiest solution would be to install a later 7427 ECU with $31 code in place of the stock ECU and run the later transmission. I found this whole thread which made me decide to tackle the project at all. After a bunch of reading it looks like all I really need is the ECU, I've got a spare Moates adapter, programmer, and datalogging cable.

Other than swapping the ECU harness over, what else am I looking at changing on the 1991? I read that the VSS circuitry changed but is forwards compatible, but not backwards compatible, but that I should probably swap to a later style DRAC and that I'll probably need to change the knock sensor as well? Are there any other big gotchas that I'm not seeing on this project? Thanks for any help.

[dave w]

I'm thinking the 4L80E connector is different 91 vs. 94/95 . There are aftermarket 94/95 4L80E pigtails that can be spliced on the 91 connector. https://www.eficonnection.com/home/p...nector-pigtail

dave w

[TheScaryOne]

The truck already has the 94+ trans harness, I'm guessing it was swapped out because of the notorious 91-93 in-pan harness problems.

[THEFERMANATOR]

The harness change occurred mid year 93 because of all the leaks of the early style harness. The harness is compatible either way. I know you'll have to do some rewiring and swap your drac to a vssb since the vssb outputs the 40 tooth per revolution signal(don't think the drac does). It's a doable swap, but I don't understand your reasoning for doing it. The early design works fine, it just requires a cleaning stroke of the force motor every 10 seconds for which there is an accumulator built into 91-93 valve bodies that slows the pressure change enough that you barely see a blip in the pressure reading during the cleaning cycle.