Towing in cruise control

[Diesel Donnie]

We have an 05 GMC D/A- Towing in cruise control has presented a problem that tranny specialists have been unable to repair- When we encounter an incline that requires the Allison to downshift, the tranny downshifts into 4th gear and locks up- I must pull off the road, shut the engine off and wait a few minutes before I can restart the engine to continue towing- The "Check engine light" remains on but the tranny operates normally- Anyone out there "been there, done that"? Any info appreciated- THANX in advance- DD

[Jmooney]

Wow.... That doesn't sound good at all.... Are you watching temp?

[hammer]

Do you have that dmax chipped if you do it sounds like it's going into limp mode

[Diesel Donnie]

There has been no increase in tranny or coolant temp and, yes, we do have an after market program- THANX for your replies- I shall follow up with my tech- DD

[krsmitty]

There is an "Allison" expert that hangs out on the RVUSA.com forums. Just add your question to the ongoing Allison Transmission question thread. Might be able to help you.

http://www.rvusa.com/forum/vbulletin...hlight=allison

[Seann45]

There is also an Allison engineer that hangs out at the diesel place so maybe you can ask him..
www.dieselplace.com/forum/

[Tail_gunner]

I'm willing to bet most Allison knowledgeable people will tell you to go back to your original programming and try towing again. If it still happens after reverting back to OEM programing then you'll know it's a hardware issue and not software.

[Daytripper]

On some Allison's there is an rpm sensor that gets messed up and causes lock/unlock problems. Did you scan for codes?

[poohbear]

My 03 had a problem once and would only go 1st to 2nd and no further.Dealer replaced an external control module (pretty sure external,no bill for fluids/filter) and all was well.

[clutch]

Originally Posted by Tail_gunner

I'm willing to bet most Allison knowledgeable people will tell you to go back to your original programming and try towing again. If it still happens after reverting back to OEM programing then you'll know it's a hardware issue and not software.

This is the best idea yet.

[clubhouse]

What size wheels/tires are you running?

I can't find it at the moment, but I know I have seen a App Note from Allison regarding changing tire size from the standard 245/75r16 OEM tire size. The gist of note was the Allison 1000 not being able to get to 5th/6th under certain conditions.

If I come across the application note from Allison I'll post it.

[Firefighter78]

It's not a good idea to tow in OD. It will cook your tranny even if you run a bigger cooler. The low pressure side of an auto tranny runs a lower volume (GPM) in OD, the OD clutch pack will fry while the trans temp reads normal. Your better off running the motor harder, it will take it. I tried running OD with an F150 gasser few years ago thinking I was going to blow it up running the motor in passing gear for 100 miles, stopped for gas the floor boards were sizzling from the tranny being so hot.

[wwsmith111]

I usually don't tow in cruise as it causes the tranny to shift too often. Generally I use a constant foot pressure which seems to greatly improve my mpg. This allows the TV to slow down going uphill and speed up going downhill (within reason). The truck is working basically the same all the time with the variable being vehicle speed. Of course one has to vary this according to conditions.

[Diesel Donnie]

It is my understanding that it is proper to tow in tow haul in od and cruise- I will check with my mechanic and post his opinion- I have towed under these conditions as long as I've had this tv- DD

[wwsmith111]

What does tow haul mode do? All that my Tundra seems to do is shift at a higher rpm and it ruins my gas mileage.

[jamaddox]

On my F-150 tow/haul mode will cause the engine to downshift when the breaks are applied so you can use that to help braking. The only issue I found is that if I am going at a night rate of speed the truck wants to downshift to 4th gear and the transmission then sounds awful

[Camper_bob]

Originally Posted by wwsmith111

What does tow haul mode do? All that my Tundra seems to do is shift at a higher rpm and it ruins my gas mileage.

Reason for the higher shift points is that's where your power band is. Gas engines have their power and torque at the top of their RPM range, whereas diesel engines have theirs very low. It also holds gears longer so the transmission is not shifting so much. Tow/Haul also locks out your overdrive gear; towing in OD can burn up a tranny in a hurry, and it has very little torque so it's not mean to be used with any kind of load.

Some of these things are always happening while towing, some are only happening sometimes. I tried clicking my tow/haul mode off and on at different times last time we were out, and at some points I didn't really notice much difference because my transmission usually won't go into OD while towing whether I allow it to or not.

[Plowtoy]

As a former GM tech, I had a customer with a similar problem on their early D/A. It was fine until you hooked something behind it and drove it 20 or so miles. I was finally able to duplacate it by taking my folks Terry for a 20 mile test drive. At the time, GM/Allison would only allow for fluid and filter changes, if GM tech assist thought it was anything else, you got a new trans. Im not sure if its the same today, but may be worth while finding an Allison transmission specialist.
I also agree with the other post's about being brought back to factory tuning and tire size. Vehicles are very sensitive today and just a small change in things can really cause problems.
GOOD LUCK!

[clutch]

Tow/haul changes the shift points so the transmission doesn't shift as much. I always use tow/haul and cruise control when towing. If I have a big hill coming up I just step on the throttle and pick up a little more speed which helps to either delay a down shift or keeps a down shift from happening.