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Top Tech Questions - You've Got Questions? We've Got Answers!

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

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.

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5 comments
chrisscalf2
chrisscalf2

i have a 1995 chevy 3500 6.5 deisel it runs fine but it stalls going down the road and starts back up when i turn the key off and turn the key back to start  it does it several times a day and sometimes it doesnt do it at all wat can i check

dantambo
dantambo

I have a 1999 chevy i ton pu w/the 6.5 tubo diesel motor and auto trans,it only heats up towing 10,000 lbs or so over hills and loose power when hot??

gerry822
gerry822

i have a 1998 6.5 turbo, at times at start up, the engine revs to a high rpm on it's own.  Is there anyone out there that has had this problem?

beehive
beehive

I have a 1993 6.5 and when I get to a corner it also quits , I think it has something to do  with engine oil pressure , when it is too low it cuts out the fuel pressure. The only way it will start again is , I installed a fuel pump manual switch in the truck and I turn it on after it cuts out then it restarts, when I am going again I switch it back off. I changed the fuel pump relay and a new oil pressure switch , and it still does the same thing at corners.

Lawrence Najolia
Lawrence Najolia

I HAVE A 1990 CHEVY  TRUCK WITH A 6.5  WHEN I PUT THE A/C ON THE TRUCK KILLS SOMETIMES IT TAKES A LONG TIME & SOMETIMES IT KILLS RIGHT AWAY WHAT CAN I CHEECK TO STOP IT FROM KILLING ON ME BUT IT ONLY KILLS WHEN I COME TO A STOP  THANKS FOR YOUR HELP ON THIS

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