Duramax LB7 is known as a reliable and powerful engine. They were produced between 2001 and 2004 but are still very popular. In those years, they were paired with a 5 speeds Allison automatic transmission. They worked well, but when the old transmissions were ready for replacement, people started using 6-speed transmissions. And they worked great!
So, is it a good idea to put a 6-speed Allison with the LB7? Yes, it sure is. You will get an extra speed that will allow higher cruising speed and lower road noise coming from the engine. Also, you get a little higher fuel mileage. The 6th gear would work as a double overdrive and add a lot to highway speeds. Of course, the main benefit is lower engine RPM, which brings everything else to the table.
But you should know that not every transmission works with every engine. There are some prerequisites that the car needs to follow through on. The most important ones are your truck’s application, engine displacement, rear axle ratio, and choice between a manual and an automatic.
The era of Duramax engines started with the LB7. And it couldn’t have started better.
LB7 Duramax Engine Overview
This 6.6L diesel engine has a block of cast gray iron and a deep skirt design. Its robust design is made for heavy-duty work. Two upper inches of each cylinder bore were hardened to resist wall wear.
The rest of the internal components were created to be lightweight but, at the same time, durable and robust. The crankshaft’s durability is increased so it can withstand high temperatures. Connecting rods are made from forged steel and can endure high horsepower loads. Another innovative feature is using a high-power Bosch CP3 injection pump that uses Common Rail technology.
This engine has its downsides, like problems with the fuel injectors. Internal elements of the injectors often wear out, causing a leak. The head gasket design was flawed, leading to failure at some moments.
Allison Transmission Overview
The LB7 was paired with the Allison 1000 5-gear transmission. This transmission is the smallest coming from this manufacturer. It is rated to handle up to 620 lb per ft.
Check Out How Allison 3000 Series Automatic Transmission Works:
Compatibility of 6-Speed Allison Transmission with LB7
When the work life of an old 5-gear transmission came to an end, mechanics disassembled it. They saw that most of the internal parts were the same as in a 6-gear transmission. The main differences are in the valve body and the transmission control module (TCM).
The process of upgrading the 5-speed to 6-speeds is to replace the valve body and reprogram the TCM.
Benefits of Using 6-speed Allison Transmission with LB7
This 6-speed conversion brings some instant improvements to the table. Sixth gear acts as a second overdrive, lowering overall RPM by around 14%. With that comes lower engine noise at cruising speed. Engine speed will also increase, and average cruising can go from 62 MPH to 72 MPH, leading to a more enjoyable ride. The bump in fuel economy is there, but don’t expect too much.
Things to Consider Before Installing a 6-speed Allison Transmission in an LB7
If you have a well working 5 speed Allison, you may keep it. The conversion itself is not cheap. The conversion kits can cost well above $3, 000 and that is a nice sum. The upgrade with the conversion kit is visible and clear, but it will take its toll on your wallet.
Steps to Put a 6-Speed Allison Transmission in an LB7
As we mentioned, the core of this process is replacing the valve body. This is a big job; if you are not a mechanic, we recommend leaving this job to one.
Preparation for Installation
Before starting this job, drain all the fluid out of the 5-speed Allison. Dispose of the old fluid properly. After you remove the oil pan, the transmission valve body is accessible. The valve body is responsible for controlling the gear the car is in.
Removal of Old Transmission
The valve body is connected to the transmission case with several bolts. You will need appropriate power tools to remove them. The bolts that hold the valve body in place are similar to those that hold the two halves of the body together.
Installation of 6-speed Allison Transmission
Now comes the tricky part of the installation. The orifices on the valve body need to be perfectly aligned with those in the transmission case. You need to wiggle everything in place when you line everything in order. An additional press should do the trick. Keep a couple of bolts near so you can secure everything.
In the conversion kit are 4 bolts used for securing the transmission. They are a little shorter when compared to the factory bolts. Don’t use factory bolts! Tighten the bolts using a 108 in-lbs torque.
After securing the valve body you can place the transmission filter and the pan. The pan is reusable, so do not damage it during removal. Refill the transmission with fluid. A deep pan can hold between 18 and 24 quarts of fluid.
Electrical Connections and Programming
Now it is time to replace the TCM. It is under a black cover bolted to the driver’s side of the fan shroud. The TCM is held in place with two upper bolts. There are also lower mounting tabs that need to be aligned upon reinstallation.
The TCM is held inside the cover with four plastic tabs. Be careful not to break any of them. The TCM tuner is programmed based on your engine specification and tire size. Plug the new TCM with two connectors into the colored ports on the TCM.
The new TCM will take a few days to learn your driving style.
Maintenance and Upkeep of 6-Speed Allison Transmission in an LB7
It is not enough to upgrade the transmission. You now need to maintain it properly. There is the transmission fluid and internal and external filters to think about.
Importance of Regular Maintenance and Upkeep
Maintaining the 6-speed Allison 1000 transmission is similar to any other transmission. It is important to keep the transmission working properly. Shifting into gears can become a problem if the transmission is not well maintained.
Tips for Maintaining and Extending the Lifespan of Your Transmission
The internal filter is not often replaced so you don’t need to consider it. It is usually replaced when the whole transmission is going through an overhaul. The external filter replacement is easy. That filter is screwed on and can be replaced in a matter of minutes. Regarding fluids, Allison recommends using TES 295 synthetic fluids.
Common Issues and Troubleshooting
The Allison is generally a good transmission but it has its problems:
Limp mode. Sometimes the truck can go into Limp mode. A limp mode resembles a truck shutting down. That can happen when a truck switches to a 5 or 6-gear. When it happens, the shift goes into third gear. This happens because the truck detects a problem and starts pulling power. The best way to resolve this is to check the error codes with the OBD II scanner and resolve the problem
Delayed shifts. If the transmission takes 2 or 3 seconds to switch from park to drive, then you have a problem. Usually, the problem is in the valve body. This problem can be associated with low air pressure in the valve body or an oil leak
Intermittent connection. Sometimes there is not only a delay in shifting gears but erratic transmission behavior. It can switch into random gears or stay in the same gear. This problem is often attributed to a wiring issue.
Can You Put a 6-Speed Allison in an LB7: FAQs
What Year Did Allison Go to 6-Speed?
Allison introduced a 6-speed transmission in 2006.
What is the Difference Between a 5 and 6-speed Allison?
The main differences are in the valve body and the TCM. The newly programmed TCM lowers the RPM by 14% with the help of the sixth gear.
Is An Allison Transmission Good?
Allison Transmission is dependable and durable for heavy-duty applications like commercial and military vehicles. They can handle large torque and horsepower outputs, providing fluid shifting, good fuel efficiency, and low maintenance requirements. They are a better option overall because they are made to increase vehicle performance and driver comfort.
Final Considerations
Upgrading an aging 5-speed Allison in an LB7 engine is an overall good story. These transmissions are already 20 years old; re-hauling them will bring great good to these trucks. They are completely compatible with the new TCM and will bring better driving characteristics to these trucks.
You will have a higher cruising speed, less noise in the cabin, and the truck will consume a little less fuel. Just prepare yourself financially because this overhaul is not cheap!
Pedro is a skilled automotive expert whose roots trace back to his apprenticeship with the AYES program at PCTI Automotive Technology. There, he distinguished himself by securing state-level accolades in the Ford competition designed for AYES-certified mechanics. In addition to his hands-on technical prowess,
Pedro boasts extensive experience in sales, notably within the electrical supply sector.
