Allison Transmission Shift Issue

toored · July 19, 2011

I hope this is the right forum. I have a 2001 GMC 2500HD with 8.1 Liter, Allison transmission, and 4X4. On the highway, when climbing hills (even small grades), the transmission down-shifts too quickly to maintain vehicle speed (it down-shifts almost immediately upon entering the hill). Then it remains in the lower gear for the entire climb. It seems the transmission shift profile is too sensitive to the RPM drop and resultant throttle position change. It even does it with little payload and no trailer. This is very annoying and is worse with the cruise control on. I'm sure it's not helping my fuel mileage either.

I've been told this is normal for these trucks and I should just get used to it. However, this engine has an enormous amount of torque and doesn't need the tranny to downshift on most hills, except when towing heavy loads. I've tried it with the "Tow" mode on and off and it makes no difference.

Does anyone know if someone offers a transmission software re-tune that would change the sensitivity to down-shifting, or perhaps some other fix for this issue?

Thanks a million for any advise!!

8.1HD/ALLISON · July 21, 2011

I hope this is the right forum. I have a 2001 GMC 2500HD with 8.1 Liter, Allison transmission, and 4X4. On the highway, when climbing hills (even small grades), the transmission down-shifts too quickly to maintain vehicle speed (it down-shifts almost immediately upon entering the hill). Then it remains in the lower gear for the entire climb. It seems the transmission shift profile is too sensitive to the RPM drop and resultant throttle position change. It even does it with little payload and no trailer. This is very annoying and is worse with the cruise control on. I'm sure it's not helping my fuel mileage either.

I've been told this is normal for these trucks and I should just get used to it. However, this engine has an enormous amount of torque and doesn't need the tranny to downshift on most hills, except when towing heavy loads. I've tried it with the "Tow" mode on and off and it makes no difference.

Does anyone know if someone offers a transmission software re-tune that would change the sensitivity to down-shifting, or perhaps some other fix for this issue?

Thanks a million for any advise!!

Talk to Justin or Jenna at Blackbear Performance. They did a tune for my 8.1 and I love it. Changed some of the shift points for me and I saw a slight increase in fuel milage.

toored · July 24, 2011

Thanks for the reply 8.1/Allison,

I asked about a PCM retune a while back to correct this problem but was told that the PCM (computer) for the Allison in the 2001 is separate from the engine PCM. I was also told there are no retunes available for the Allison. I will contact Justin and see if things have changed.

Thanks again!

8.1HD/ALLISON · July 24, 2011

Yes the computers are seperate, ecm and trans. You will have to get the autocal to program them with Justin. Get in touch with them, they can answer your questions.