6L90 Transmission—All You Need to know

The 6L90 transmission from General Motors is commonly known for its performance and fuel-saving capabilities. It was fitted into many GM cars and trucks and has evolved tremendously. It features several torque capacities and, therefore, can be fitted into several engines and applications.

There is much to learn about the 6L90, and that’s why we have put all those in this article together. We will talk about some of the common problems of the 6L90 and possible ways to fix them. We will also highlight vehicles outfitted with the transmission. But first, let’s see detailed information about the 6L90.

6L90 Transmission Explained

The 6L90 transmission is a six-speed hydraulically operated automatic transmission designed for longitudinal engines and particularly for rear-wheel and rear-focused four-wheel drive vehicles. Built-in 2007, this tranny consists of six forward gears and one reverse gear. The first gear ratio is 4.03, second:2.36, third: 1.53, fourth:1.15, fifth: 0.85, sixth:0.67, and the reverse at 3.07.

Instead of the usual planetary gear set configuration, wider gear ratios were used, allowing for better performance and fuel economy. The 6L90 transmission features multiple torque ratings, which makes it suitable for different engines and applications. And also allows users to upgrade by building one with even higher torque to withstand higher engines.

Its maximum torque rating for gasoline trucks is 531 lb-ft, and it can withstand gasoline trucks outputting 452 HP.

Similarly, it has a torque rating of 520 lb-ft for diesel trucks and puts up with a diesel truck outputting 300 HP. With a 500 lb-ft torque rating for passenger cars, it can withstand passenger cars outputting 555 HP with its gearbox torque of 855 lb-ft.

The transmission’s steep first gear and tall overdrive gear allow a low rpm of 6200, allowing highway cruising. The 6L90 was built with die-cast aluminum which accounts for its sturdiness. And has a vehicle weight rating of 15,000 lbs.  The GM 6L90 fluid capacity lurks at 13 quartz for a 30mm torque converter with the DEXRON VI as the recommended fluid.

Before General Motors phased in the 6L90, there was the 6L80 which was designed in 2005. But unlike the usual outfacing of a former, the 6L80 was still in production. Hence, 6L80 became a contemporary to the 6L90. So in many cases, people often talk about them as though they are one.  But they are not the same.

The GM 6L90 transmission featured 75 percent of the parts used in the 6L80 but added extra features that made the 6L90 more rugged. While the 6L80 used four pinion gears, the 6L90 utilized six, invariably strengthening the input gear set; how did GM actualize this?

GM used a longer transmission case, 35mm longer than the 6L80 case. Thus, allowing more room to accommodate two extra pinion gears, making it a total of 6. The output gear set of the 6L90 was also wider than that of the 6L80. Both actions of which strengthened the input and output gears

The 6L90 longer transmission case also allows extra fasteners between the transmission and transfer case for decreased noise and vibration. The 6L90 also had more clutch plates, as an extra clutch plate was added to each clutch. However, there are still some 6L90 versions without the extra plate to match certain applications when needed.

These 6L90 transmission upgrades made the 6L90 stronger, allowing it to be called a heavy-duty 6L80. The transmission was outfitted to several GM rear-wheel and 4-wheel drive heavy-duty and diesel trucks, Cargo Vans, and cars. Here is a detailed list of vehicles that have the 6L90 transmission.

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6L90 transmission problems and fixes

Like other components, there 6L90 transmission was plagued with several issues, which is almost the same as the 6L90e transmission problems. So here are common 6L90 transmission problems and ways to fix them.

Failure of the reverse gear to engage

A common 6L90 problem is the inability of the reverse gear to engage when selected. This is mainly caused by a faulty Park straw—the metal piece preventing a vehicle from rolling. In other cases, it could be due to a bad clutch. To fix, check the condition of the park straw and clutches and replace them if needed.

Slippage between gears

The slippage between gears is another common GM transmission problem. Drivers have reportedly experienced slippage in the reverse gears or between the second and third gears. This issue most often stems from a crack 2-3-3/5-R drum. Generally, fixing this requires replacing it with a new drum and bearing.

Slippage in first gear

Drivers have also complained of their first gear slipping. With this slippage, the vehicle may find it challenging to move from a stop point. This often stems from a solenoid stuck in the off position. It could also result from issues with the valve body, preventing fluid movement to valves.

This problem can be fixed by replacing the valve body so fluid can enter the right valves to activate the right clutch or band for proper gear switching. Ensuring the solenoid moves freely could also save this situation.

Transmission overheating

There have also been reported cases of the 6L90 overheating. Overheating most times results from dirty or low fluid levels. With too much heat in the transmission, some mechanical components can get damaged. Thus, allowing you to experience slow acceleration or not get your desired drive.

The best approach to fix this is to top up your transmission fluid or carry out a 6L90 transmission fluid change. The 6L90 recommended transmission fluid is the Dextron VI ATF.

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Is the 6L90 transmission good?

Having a good return on investment makes spending worth it. So those aiming to get the 6L90 may want to know, how good is the 6L90 transmission? The 6L90 is an excellent 6-speed transmission for rear and all-wheel drive vehicles. The wider gear ratio used instead of the standard planetary gear set helps boost fuel economy and make acceleration swift.

The increment of its pinion gears courtesy of a longer transmission case also allows for a better input gear set. The longer transmission case also allows for extra fasteners within the tranny and transfer case, helping to minimize noise and vibration.

The transmission also features different torque ratings to meet several engines and applications though the highest it can withstand is 555 horsepower. However, it allows you to upgrade for better performance. So if you want more power, you can have your stock 6L90 rebuilt or even buy an already-rebuilt 6L90.

If rebuilding from scratch seems expensive and you don’t have a stock 6L90, look for a used 6L90 and rebuild from there. There are many shops offering a used 6L90  transmission for sale. To cut the cost even further, do this yourself if you know your way around and have the 6L90 transmission rebuild kit. If, however, you can’t, contact a professional to have it done.

With its fuel-saving capacity, better acceleration, minimized noise, and ability to withstand different engine needs, the transmission can be termed good. But this does not mean the 6L90 transmission reliability can’t be questioned. Because while they perform excellently, they are also plagued with several issues.

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Frequently Asked Questions—FAQs

What cars have a 6L90 transmission?

So for those asking, what vehicles have 6L90 transmission? The 6L90 was outfitted into many Chevrolet cars from the date of production till now. It was featured in the Cadillac CTS-V Sedan, Wagon, and Coupes fitted with the 6.2L V8 Supercharged engines.

The Chevrolet Camaro ZL1, with a 6.2L V8 supercharged engine, also featured the 6L90 transmission. You would also find the 6L90 in Chevrolet Suburban 2500 produced between 2008-2013 and many other Chevys.

What is the difference between a 6L80 and 6L90

Most people often refer to both transmissions interchangeably. However, while the 6L90 transmission specs are close to that of a 6L80, they are not the same. So the 6L90 transmission vs 6L80, what is the difference? Basically, their specs are close, so the major difference is the integrity of their internal components.

While the 6L80 transmission case allows for only four pinion gears, the 6L90 features a longer transmission case allowing for additional two pinion gears, making it a total of six, making the input gearset stronger. The 6L90 also features gear ratios wider than the 6L80, allowing for better fuel economy and performance.

The longer transmission case also housed more fasteners between the trannny and transfer case, reducing noise and vibration. Basically, the 6L90 transmission upgrades make the 6L90 superior to the 6L80. You can call it the 6L90, a big brother of the 6L80.

What does 6L90 stand for?

The 6 stands for the number of forward gears, the L means longitudinal, and 90 is the maximum torque rating. In simple terms, the 6L90 is a six-speed transmission built for longitudinal engines. The automatic transmission, operated hydraulically, has six forward gears and one reverse gear. It has a gross vehicle weight of 15,000 lbs and a combined gross vehicle weight of 21,000 lbs.

What is the strongest GM transmission?

The strongest GM transmission ever built is the 10L90E. It is big and heavy and performs amazingly while boosting fuel economy. The 10L90E is a ten-speed electronically controlled automatic transmission with ten forward gears and one reverse gear.

Built for longitudinal engines, the transmission weighs 230 lbs and is available only for Chevrolet performance for the Gen V engines like the LT1, LT4, and LT5. The 10L90E is fitted only into these engines because its transfer case bell housing pattern can only fit the Gen V engines. The 10L90E uses the Dexron Hp automatic transmission fluid.

Which Chevy transmission is the best?

Many Chevys have seen good transmissions made by GM. But the best Chevy transmission is the TH350.  The Turbo Hydramatic 350 is a three-speed automatic transmission with three forward gears. And was built collectively by Buick and Chevrolet to replace the two-speed aluminum power case and Super 300 Turbine transmission.

Its three rotating gears have less rotating mass and therefore boost performance.

The stock TH350 may not be able to withstand much engine power, but it allows users to build it to whatever capacity they want at a relatively low cost. This TH350 can be tweaked into having extreme power capabilities.

Another perk of the TH350 is that most shops can build it—at least a level 1 or 2 TH350. It is also very reliable and simple to repair. The TH350 was first outfitted to the 1969 model cars. And was fitted to Chevrolet and GMC Vans and trucks until 1986. But was replaced in 1984 with the 700R4.

What truck has the 6L90?

The 6L90 transmission was outfitted into several trucks, but the most popular are the GMC Silverado and GMC Sierra. It was fitted to the GMC Silverado and GMC Sierra 2500HD and 3500HD with 6.0L engines between 2007-2014 and 2015-2019. The GMC Sierra and Silverado 2500HD and 3500HD 6.6 LT8 engines featured with the 6L90 between 2020 and now.

What’s better, a 6 or 8-speed transmission?

An eight-speed transmission is better than a 6-speed tranny. Usually, the higher the gears, the better the performance and fuel economy. More gears allow the gear ratio spread widely with minor ratio differences between gears. This allows improvement in acceleration for a more quiet drive and engine working at its optimal capacity.

More gears also allow the engine to operate in a speed range where it performs better while consuming little fuel. Typically, an eight-speed transmission can output about 11% more fuel efficiency than a six-speed transmission. However, which transmission is best for you depends on your car.

Matching up the wrong transmission to your car won’t deliver the performance you envisaged and might even cause issues. Smaller cars, for example, are stuck with fewer-speed transmissions, so anything higher may be doom for these cars. Here, using a five-speed transmission rated for three-speed transmission may cause many car issues.

How do I reset my 6L90 transmission?

To reset your 6L90 transmission, follow this: with the engine at idle, change from neutral to reverse gear, leaving the shift lever in reverse for five seconds. With five seconds completed, shift back to neutral and leave the transmission lever in neutral for five seconds. Repeat these procedures ten times.

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Final Words

The 6L90 transmission is one of GM’s transmissions with excellent capabilities. The six-speed automatic transmission utilizes a broader gear ratio which accounts for its improved performance and fuel economy. While the 6L80 six-speed automatic transmission did excellently, the 6L90 upgrade made the 6L90 a better transmission.

Fortunately, the 6L80 was replaced but existed together with the 6L90. So if the 6L80 doesn’t match your engine, you could upgrade to the 6L90. The 6L90 features several torque ratings, making it fit for different engines and applications, but the highest it can withstand is 555 hp. If you want more power, you can have your stock 6L90 rebuilt or buy an already-built 6L90.