Transmission Trouble ??

[EdHale]

I will definitely see you then. I will be helping work the north tunnel entrance to the infield starting Wednesday evening. I'll make sure I stop by. I will be just outside turn three, but I don't know my site number yet.

[Kennedy]

Locking out overdrive when pulling even a slight grade makes sense. Thinking back to the conditions when the fault first appeared, it was probably a 5% grade, but I had lugged 5th gear down to 55 mph (just before tow haul would have caused a downshift) while pulling the 10K lb travel trailer. I'm hoping the fault won't reappear when driving unloaded.........Thanks for all the assistance in working this thing out.

I would not expect it to happen unloaded, BUT some have reported this when pulling out to pass at around 60 mph.

[DmaxMaverick]

I would not expect it to happen unloaded, BUT some have reported this when pulling out to pass at around 60 mph.

I've seen this as well, but only with cruise engaged. The delayed downshift allows it to stay in OD long enough to get overrun when the boost comes up quickly, before ground speed increases. It happens very quickly. To prevent this, disable cruise and let it downshift (or downshift manually) for passing-type situations.

[Roy W]

Thanks for the warning /advice. Actually I drive like an old fart and can't remember the last time I hit passing gear. I have enough power that a downshift really isn't necessary when passing. I'm hoping that after the fault resolved itself and I continued to drive another 1000 miles under load without a repeat occurrence that maybe I had encountered "the perfect storm" when I lugged overdrive.

[DmaxMaverick]

Thanks for the warning /advice. Actually I drive like an old fart and can't remember the last time I hit passing gear. I have enough power that a downshift really isn't necessary when passing. I'm hoping that after the fault resolved itself and I continued to drive another 1000 miles under load without a repeat occurrence that maybe I had encountered "the perfect storm" when I lugged overdrive.

This is the primary contributor, even if once in a while. If you are loaded and need to speed up, for passing or whatever, force or let it downshift. Although the power may be greater at higher RPM's, and the shaft-torque is about the same, the torque-distance is less, as the same torque is spread over the higher RPM's. This, for the same reason Diesels use up gasser trannies. Even though they may have similar HP and TQ ratings, the Diesel is doing it with about 1/2 the shaft rotations. This results in near double the input torque, per revolution.

Add to that, with the TCM adaptive learn process, lazy old fart driving may make the TCM a little slow on the uptake when you do lay on it out of the blue. Perhaps the last piece of the perfect storm puzzle.

[Roy W]

When towing on the interstate, I set the cruise control on 60 mph and plug along. Question: At that towing speed do you think I'd be better off locking out overdrive and living with the increased rpm and reduced fuel mileage or just permanently remove the power adders ?? Tow haul of course.

[rapidoxidationman]

IIRC 65mph in overdrive puts my engine at its happy place for torque and efficiency of 1900rpm (best mileage too). If your gear ratios are similar, maybe bump up 5mph and see what happens?

Or just set your speed to whatever spins the engine at 1900....

[DmaxMaverick]

I do not recommend locking out OD with that load at that speed. I only suggest allowing it to downshift when you hit a grade, then engage OD once it's behind you. Nothing wrong with leaving the power adders in, but the additional power will require a little more attention to keep everything happy. I do the same, with 2-4K more in tow, quite a bit more power, and a taller gear ratio (33" tires), still on the (healthy) OEM tranny. I still get an occasional overboost, but that's my fault for letting it get there. Almost everywhere I go (camping) requires 7%+ grades, and none are a problem unless my attention wanders.

[Roy W]

Yes, we have traveled the Rocky Mountains occasionally (live in Florida) and have pulled some of those 7% 5 mile long grades with this truck as outfitted. It's hard not getting a small grin when passing 18 wheelers and motorhomes in the slow lane.

I think I now have a pretty good grasp on how to handle this situation in the future. Many thanks to you and John Kennedy for all your advice.

Roy Wires

[Ian7]

Question: At that towing speed do you think I'd be better off locking out overdrive and living with the increased rpm and reduced fuel mileage ...Tow haul of course.

please excuse the nube question, but with my '04 I thought being in tow/haul gave me overdrive lockout by default? if not, how would I do it, when, and why? many thanks.

[Roy W]

I can't speak for the '04 models, but on my '05, you push the tow haul button in and hold it in for a second or two, and it locks out overdrive. An additional icon will appear on the dash next to the tow haul icon indicating overdrive lockout is selected. Regarding when to use it........I would refer you to all the comments from Dmax Maverick and John Kennedy in this thread.

[Ian7]

thx.
so using tow/haul then only modifies shift points? and requires an additional action on the same button locks out OD?

(times like these I wish I could get a hold of the "diesel supplement" that didnt come with my used truck purchase)

[rapidoxidationman]

Didn't jbplock send you a copy of his? Unless you have a later '04(LLY, it should be virtually identical to the '03(LB7?)...

At any rate, I'm pretty sure the operation of the tow/haul and OD switch is covered in the plain old owner's manual - not the diesel supplement. The Allison transmission is also used on the 8.1L gasser.

[DmaxMaverick]

thx.
so using tow/haul then only modifies shift points? and requires an additional action on the same button locks out OD?

(times like these I wish I could get a hold of the "diesel supplement" that didnt come with my used truck purchase)

With 2003 and later models, the T/H and OD lockout are a function of the button at the end of the selector stalk (shifter). Previous models require a wired-in mod and a switch.

T/H only modifies the shift points and torque converter engagements. It does not inhibit OD, but it does delay OD engagement during normal driving.

[Ian7]

Didn't jbplock send you a copy of his?

crap, yes, found it filed in unintuitive location on computer
thx for the reminder :-)

and it turns out there wasnt anything there about tow/haul as you surmised, its all in basic manual; duh on me.

[a5150nut]

I sure like the TH mode on my 06. Use it to descend hills and additional slowing when needed. Or once in a while to confuse some idiot that has to ride my bumper.

Reminds me of the 7 speed autos used in CAT scrapers.

[EdHale]

I will be putting my '06 to work tomorrow morning. Heading to Talladega for a few days for some racing and crowd watching.

I'm close enough it it is only 95 miles, but still good to get off of this home base site for a few days.