Cruise control?

Hi,
i have 3 peterbilt 387 trucks that i purchased a while back. All 3 are identical and from the same company. They all came with ZF Merritor freedomline auto 12 speed trans, C15 435hp, and all broke down at about 1.2mil miles.
I changed the trans to eaton 13 on 2 trucks and one has the 10 eaton. Now, none of them have a working cruise control.... My mechanic is telling me he doesn't really know how to fix this issue. I am in Chicago,IL and i have no idea where i could go and have them fixed. I spoke with Trent from diesel dr. and he told me i have to flush the transmission ecm, not the engine's so i am at a total loss. The dealer will not even see my truck without an appointment and i would like to hear your thoughts and suggestions.
Please excuse me if i didn't post in the in the right section as i am new to this forum.
Thanks!
Andre

 

I put the true manual trans. Installed a clutch pedal, replaced harnesses, the whole 9 yrads

 

Yes, it does, and so is everything else. The one that was finished today, has the check engine light on, so i thought i'll need the ecm reflashed, however i don't know anyone trustworthy here in IL. All i've heard was Diesel dr.
Thank you NM

 

I thought of it, but i wanted to see if there are any other options, other places that specialize in this. I've been down for the past 3 weeks and i was hoping to get started asap