Towing Troubles
Part One
Q: All of our Duramax trucks have this disease called "limp mode." We just never know when an outbreak will happen. Does anyone have a cure for this virus that GM implanted in our vehicles to protect themselves from any kind of warranty repair? Without any programmers, I can pull a 20,000-pound trailer up a hill and floor it with no problems. It's nice to have the extra power and mileage I'm getting from a programmer, but I'm afraid it's going to break something next time it stumbles into limp mode. Does anyone have a cure?
Aaron Wiggins
Newport News, Virginia
If you're looking at adding...
If you're looking at adding more than 100 hp to an Allison transmission, or you'll be towing all the time, it's a good idea to upgrade the transmission with beefier clutches, a performance valvebody, and an aftermarket torque converter.
A: The Allison transmission found in '01 to '09 6.6L Duramax-powered trucks is one of the more complex transmissions on the market today, which also means it has its share of troubles. When the Allison transmission senses excess slippage, even for a couple of seconds, it will kick down into Third gear to save itself from burning up the clutch packs and will also unlock the torque converter in case the converter clutch is slipping. Towing is extremely hard on transmissions even in stock form, and when you add more power to the mix, you're asking for trouble. Remember that the transmission was designed from the factory to handle a 300 to 350hp engine, so modifying the engine's power output without modifying the transmission means you're exceeding the transmission's torque-handling capacity. Although it varies depending on usage, adding 60 to 80 hp is about the safe maximum for a stock Allison transmission to perform reliably. If you tow with that 60 to 80 hp, know that you're on the very edge of what the transmission can handle before going into limp mode. Over time, the transmission will start to slip and throw codes. If you're willing to drop down the power to stock levels, this will often keep the transmission alive for a few more years, but the reality is that 150,000 to 200,000 miles is not an uncommon time for a rebuild if you're using an aftermarket programmer.
Ready for the cure? It's not cheap or easy. If the truck keeps going into limp mode, that means it's transmission rebuild time. Expect at the very least to upgrade the torque converter, clutches, and valvebody. This will run anywhere from $3,000 to $5,000, depending on your upgrades and what your local shop charges for labor. While it's true that some Allison transmissions have lasted at high horsepower levels and haven't broken or limped, the vast majority will need help. The good news is, once your transmission is upgraded, it will be able to safely handle much more than 60 hp over stock, especially if it's not hooked up to a trailer. It seems like a mean trick to create a diesel engine that can make 200 hp more than stock with just a tune, and then hook it up to a transmission that can't take that type of power, but the simple fact is that in order to handle heavy towing and higher power levels, the Allison transmission needs some help.