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I have a 05 Duramax 6.6. When I am pulling heavier loads my transmission sounds like it is shifting into over drive. However, my RPM do not increase and I am using Tow Haul. It does not have to be pulling up hill to do it. It can be flat terrain.
Is it some special feature of the transmission?
If you are below about 60 MPH, it won't shift into OD when in T/H. The TCC (torque converter lockup) will seem like a "gear" when under load. If you are applying more APP (skinny pedal, throttle) under load, these shift and lockup points will occur at higher RPM's. More info could be helpful to determine if you have a problem, or are experiencing normal operation: Engine RPM; road speed; load; grade; weather conditions; engine, tranny and ambient temperatures.
Thank you for the information. What is "torque converter lock up"? Also, the noise that sounds like the transmission shifting into Over Drive which would normally make the engine raise RPMs last for up to 1 to 5 minutes.
The transmission shouldn't make any "noise" itself. During some conditions, it can "whine", but you shouldn't be able to hear it under load or while cruising, with all the other noises going on. GM and Allison say this is normal, and even included a video describing this condition, among others, with early release trucks. The Allison transmission is very different from other automotive type transmissions we have experienced in years past, or on other brands. This is what makes it unique, and far superior to the competition. Ford's Torque-shift tranny is a bastardized knock-off of the design.
Torque converter lockup is an event. The torque converter isn't an "old" type, where it's always a fluid coupling. It has an actual clutch that engages (hydraulically, via electrical control), and locks the TC like a clutch on a manual tranny. This isn't new technology, GM has been using them for about 30 years in the lighter duty vehicles (cars and 1/2 ton trucks), and about 20 years in heavier duty vehicles. In T/H, this should occur at about 20 MPH in 2nd gear, and above about 50 MPH with T/H disengaged. The event isn't an actual gear shift, but can feel like one under many conditions.
I'm not sure what you mean about "shifting into Over Drive which would normally make the engine raise RPMs last for up to 1 to 5 minutes". The RPM's should drop, as higher gears are engaged, and the TC engagement should drop RPM's significantly (zero to 500 RPM's, depending on powertrain load at the time). That doesn't sound normal, in any case. 1 to 5 minutes is a long time, and no event should last more than a couple seconds. If it's taking that long for a gear to become fully engaged, you have clutch wear or fluid flow issues. This will usually be accompanied with fluid depletion symptoms (dark, grainy, stinky fluid) and high tranny temps.
Under heavy load during acceleration, the tranny can hesitate between shifts, but it defuels during this process (engine doesn't "run away", but may seem like you hit neutral for a second or two). This can be more exaggerated if a power adder (box, chip, programmer) is being used to increase power.
Do you have any DTC's (Diagnostic Trouble Codes), or have you seen the SES lamp come on during any of this? Tranny slip will set the SES, and normally placed the truck into Limp Mode (less power, limited RPM range, and sticks the tranny in one gear).
.............Or, you could be hearing the engine cooling fan engage, which will make the engine sound more noisy and faster, but the RPM's (according to the tachometer) shouldn't change because of it. The fan is quite noisy when fully engaged. It's a good thing.