Duramax and Allison Limp Mode

Let's explore some of the tuning limits and possible problems you may experience with the basic, stock Duramax-equipped truck.

What is Limp Mode?

There are mainly two limp modes that the Duramax performance crowd may experience…fuel and transmission limp.  The way to tell the difference without checking codes is a fuel, or engine limp, will limit rpm to usually 2000rpms allowing shifting, and a transmission limp will limit shifting without limiting rpm.  Transmission limp will usually give a bang/jolt and be stuck in one gear.  This is a fail safe mode.  Usually pulling over and turning the truck off and cycling the key a few times will take it out of limp mode and you will be back on the road.  Checking, recording, and then clearing codes with an EFI unit.  Edge CS/CTS will also do the trick.

Stock Transmission Limitations

The biggest limiting factor as it relates to tuning and horse power/torque is the stock transmission, so we’ll cover that next.  The cool thing about an Allison (well, one of many) is that it will protect itself if overpowered.   So I recommend tuning to try and please the customer’s hunger for horse power but still have the safety and longevity of the truck in mind.  The tough part is every truck is a bit different in its set-up, mileage, tire size, history, and the person behind the wheel.  So these are broad averages gathered over many years from personal experience as well as the feedback of thousands of customers.

2001-2005 5 speed Allison equipped LB7/LLY in good condition should handle around 80-90hp.  This decreases with large tires and higher mileage trucks because the clutches in the Allison and the torque converter are wear items.  More miles equals more wear.  Adding bigger tires creates more load causing that HP limit to decease.  The most stress comes from 4-5 or 5-4 full power shifts and this is where 95% of a transmission limps will take place.  So if you’re running more than the recommended HP or have a tired transmission, only going full power in the lower gears may save some pain.

2006-2010 6 speed Allison LBZ/LMM equipped trucks can be freaks in the added HP they can handle without limping.  I recommend a max of 120hp if you never have plans of upgrading the transmission.  If you like living a bit on the edge, then we have seen 150hp tunes live for a long time in the right hands.  Allison designers strengthened the C1/C2 drum assembly as well as upgraded the torque converter.  I also think a more relaxed algorithm in the limp mode setting is a factor in why they do not limp like the earlier models.  I have seen 6 speeds that never limped but were totally destroyed, and I have seen some that have limped easily on smaller tunes.  So this is a theory of mine only and not confirmed.

Fuel System Limitations

The second biggest tuning limiter is the fuel system.  Maintenance and mileage are the two biggest battles the fuel system faces.  Trucks that have the fuel filters changed on a regular basis and have below 150,000 miles can usually handle about what the tranny can handle.  So with the upgraded LBZ/LMM fuel systems they will be about 150hp and the LB7/LLY will be in that 100hp range.  Purchasing a lift pump early in a trucks life will help in saving the fuel system and also increase what HP the truck can handle.  Just because a truck doesn’t throw a fuel limp code does not mean it is working correctly.  Data logs tell the tale on how well the fuel system on your truck is keeping up for the job needed.

This is a basic outline on what an average truck can handle.  Of course your truck is not average, nor is our customer service, so if you have questions on how we can help with your needs, please give us a call. 208-685-1000

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