Hi all:

swapped a 4l80e into a GMT900 in place of a 6l80e, using a PCM of NC conversion harness, and a T42 GM controller. Runs and drives great.

I am trying to get tow haul mode to work for an alternate shift pattern. I first tried modifying pattern A and B with no success (raise 3-4 shift MPH to 200 to prevent shifting to OD). The light on the dash comes on with the push of the button, but the TCM wont respond to the commanded change. I know not a ton of people have done the 6l to 4l swap on the GMT900's, but I thought I would throw this out there.

Any thoughts?

Do the GMT900's use a canbus data line to switch the modes?

 

Doing some more digging, it was suggested to me to try swapping to a different OS on the TCM. anyone on here with a GMT900 with a successful 6l80e to 4l80e swap with tow haul working?

 

You ever get this to work? I think the vans ran a e38 ECM and t42 to operate the 4l80 in them and would imagine it's just a operating system issue you are having.

 

Yeah we did get it to work, and without an OS swap as well. Alvin just had to change a whole bunch of settings around, and it works great, pattern a is normal shift, pattern b is tow haul.

 

I have a "drag setting" in tow haul mode also. I have an 04 so I didn't have the issues you did. I'm glad you got it to work. It is a nice feature to have.

 

Do you remember what had to be changed for tow haul to work I just swapped a 4l80 in place of a 6l80 and my tuner can’t seem to get it to work

 

dang, 5 years later! Sold the truck ages ago and I don't remember what he changed to get it to work, but I can do you one better. I happened to save the tune, so if you have HP Tuners go ahead and take a look at the file and see if you can compare what is different. good luck!

-LT

 

Thank you so much I’ll send it over to my tuner and I’ll jus have him look

 

do you happen to have a log of it working so my tuner can see what its doing