About the GM 6L80 Automatic Transmission

Nov 8th 2022

About the GM 6L80 Automatic Transmission

The 6L80/6L90E automatic transmission fills a much needed duty gap between the versatility of the 4.0L series and the Allison variety. All previous GM transmissions had been based on hydraulic controls with more electrical aspects with each upgrade, but this unit was developed as an electro/hydraulic controlled transmission. This transmission can be used in vehicles weighing up to 8,600 lbs. or a gross weight of 14,000 lbs. Go Powertrain provides a selection of different performance ratings for the 6L80, including heavy duty and super sport/super duty.

This is a completely remanufactured automatic transmission assembled to exacting tolerances and live-run dynamometer tested for reliability.

This transmission is used in everything from pickup trucks, to SUV's, to audacious muscle cars, to family sedans. The 6L80E transmission is widely used in popular GM applications such as the Silverado and Sierra trucks, Tahoe, Suburban, Denali, Hummer H2, Cadillac Escalade, STS, and CTS, Camaro, and G8.

The 6L80 has had a load of transmission problems, some of which directly inhibit drivability.

What Are Some of the Common Issues with 6L80 Transmission?

Cracks in the 1-2-3-4 pistons
Slipping 2nd and 3rd gears
First Gear Slipping
Cracked drum
Reverse gear engagement failure
Clutch Failure
Stator Support Issues (Failed Seals and Cracking)
Low Sprag Problems
Torque Converter Problems
Intermediate Shaft Problems
Valve Body Problems
The OEM Oil Pan
The High Pressure Oil Pump
The Transmission Electro-Hydraulic Control Module (TEHCM)
The 4-5-6 (Overdrive) Clutch Pack

Cracks in the 1-2-3-4 Pistons

Issues with the 1-2-3-4 pistons are common in 6L80 transmissions. The piston typically gets cracks that result in clutch failure. However, GM updated the piston. Therefore, it will be wise to update the piston to the latest designs. The piston housing may also crack. This is, however, not as common as the cracking of the piston itself. The housing’s inner diameter develops a crack and may break away from the accommodation. This will result in 1-2-3-4 application problems.

The 6L80E transmission is prone to issues that include: 2-3 harsh or delayed shifts, harsh or delayed shifts from park or reverse, burnt 1-2-3-4, or 3-5 reverse clutch, check engine light (DTC P0751 Code), converter shudder due to flaking of the converter lining, shudder on low/reverse apply, TEHCM and/or CAN software malfunction.

Slipping 2nd-3rd Gears

Slipping from 2nd to 3rd with a 6L80 transmission is unfortunately a common experience among Chevrolet and GMC pickup truck owners. This slipping problem results in a difficult time utilizing any of the gears. As a result, the vehicle will consume a lot of fuel.

In some instances of slipping, you will hear a sound indicating there is a slip. This will be a good indication of a slip. If you use a diagnostic tool, an error code will be displayed. The most common error code for this problem is P1870 which indicates a slipping transmission component.

To curb this problem, the solenoids of the two gears will have to be repaired or replaced. This is because the problem primarily stems from worn-out solenoids. Though fairly rare, there are instances of total mechanical failure with this problem.

First Gear Slipping

Slipping in the first gear can result in poor acceleration and torque. Typically, you will feel a sense of loss of acceleration on pressing the throttle pedal in first gear. Also, the diagnostic tool will indicate the P1870 code when you connect it with the transmission control unit.

To diagnose and possibly fix this issue, you will need to check the condition of the transmission fluid in the system as well as the solenoids. As non-synthetic fluid and worn-out solenoids are often the reason for the problem.

Cracked Drum

Another significant problem is a crack that affects the drum weld. The result of the crack is usually a 3-5-R clutch issue and failure. Typically, rebuilders weld these cracks. This becomes a short-term solution for a long-term problem as more problems will arise later on when the welds slowly weaken and fail. If that doesn’t happen first, another crack somewhere else on the clutch typically does. 6L80 transmission rebuilders will now install an updated version of the 6L80 clutch drum to resolve the problem accordingly.

Stator Support Issues (Failed Seals and Cracking)

You should pay close attention to your stator tube cracks and sealing rings, but that’s pretty hard to do considering they’re on the inside of your transmission. On the back of the stator support, there are seal rings. These cover the feed oil circuits for the 3-5-R and 1-2-3-4 clutches. The units that came out earlier had issues with the ring’s design, resulting in leaks mostly during cold operation. The result was mostly a slipping or delayed engagement that went into reverse.

General Motors came up with an update for the seal rings on the support to curb the problem. However, the updated design does not fit the earlier design pumps. Therefore, you may be required to replace the pump entirely. Alternatively, you could have the pump machined, which would allow you to fix the updated rings.

Depending on where the crack develops, the crack may lead to clutch failure, mainly to the 4-6-5 clutches. Also, there are problems with the stator support tube or the liner developing cracks. These issues are not common, but they may occur. Therefore, it will be good to take a minute to examine your stator support ID if you’re having trouble with your transmission and are mechanically savvy.

Reverse Gear Engagement Failure

The reverse gear is critical for the drivability of any vehicle. It is not uncommon to find the reverse gear of a 6L80 failing to engage, which points to a possible problem with the transmission.

The signs of a reverse gear engagement failure can be; difficulty in engaging the gear or no gear is not engagement. This issue is best solved by heading to a trained mechanic because there are a few mechanical issues that could cause this problem.

Clutch Failure

Clutch failure is relatively easy to diagnose. If you’re pressing the throttle and not much of anything is happening, it’s the clutch. When there is a complete failure, the car will not move entirely.

The 6L80 has a friction type of clutch. Therefore, you must examine the friction plate in the clutch assembly of the 6L80 transmission. Necessary repairs should be conducted if there are any problems detected. Most of the time an updated clutch assembly installation will clear this problem but not always so it is imperative to always let a professional take a look.

Low Sprag Problems

6L80s with high mileage will have problems akin to the 1-2-3-4 clutch operation. However, these problems will have no relation to the operation of the clutch.

A sprag with issues will have the L/R clutch holding the output carrier at first when driving. After the vehicle gets to 2 MPH, the L/R clutch releases. When the sprag doesn’t hold, the car drops to neutral. This creates the impression that the 1-2-3-4 clutch has failed.

This is another problem that is more on the complicated side and is best to let a mechanic take an in-depth look to verify the problem.

Torque Converter Problem
Much of the concern with this units torque converter is that they made countless vehicles of different masses, power levels, tire sizes and more with a common converter, offering few variabilities in stall speed and stator dimensions. This makes it an imperfect match for nearly any application; sacrificing power, efficiency and shift quality for just about all users.

One of the least reliable parts of the factory torque converter is the cover. Unfortunately, the cover is made like most torque converter covers; a low quality casting designed to be incredibly cheap to manufacture with minimal regard for the longevity or operable quality of the torque converter itself. A quality solution is the use of a billet steel cover, precision machined to perfection. This is largely a reliability upgrade and is foundational to all of our 6L80-E and 6L90-E torque converters, regardless of build specifications.

Further, the stator is often inadequate for any vehicle that is even slightly modified. At Next Gen, we are proud to offer custom stators of virtually any stall speed as they are precision machined right here. This allows us the flexibility of making the most precisely mated stators possible for each application. Because of this, we ask many questions about application, power and build for all customers prior to shipping their built transmission. We seek to ensure every 6L80-E or 6L90-E has superior drive quality and feel.

Lastly about the converter, is the lockup clutch assembly. For applications with added weight, power or the expectation of long-term use, the lockup clutch can quickly lose friction material contributing to a softer and softer lockup over time, eventually resulting in an unusable torque converter. Although we offer a plethora of solutions for this, all of them revolve around enhancing the material and volume of the torque converter clutch.

This serves the purpose of optimizing for longevity and reliability, but also enhances lockup-quality. This is a very noticeable upgrade for performance vehicles as well as heavy vehicles who both suffer from slow TCC synchronization times.

Intermediate Shaft Problems
As with many 6 speeds, the intermediate shaft withstands the brunt of the activation of overdrive. Overdrive has the highest torque multiplication factor of the transmission due to its long gear ratios, placing an undue stress on the intermediate shaft and hub assembly in this transmission. Fortunately, and as usual, the nerds of the world have catered to this with a variety of solutions.

Primarily, we begin by having this component cloned from a high-grade 300M billet steel, permitting about a 40% increase in tensility and yield. This is important for any application where the transmission may experience additional load in higher gears, or loaded up-shifts are to be expected.

As an additional measure of reliability, these shafts are then heat treated to further enhance their durability. The heat treating process enables these shafts to be severely abused without fatigue, stress or wear. This is a must for anyone who fears breaking the intermediate shaft of the transmission.

The Valve Body
As many 6L80-E and 6L90-E owners already know, the valve body is the home of the majority of this transmissions functional issues. As a mecca for all oil traveling through the transmission to its designated locations, prolific issues such as bore wear, valve seizure, flexion of castings and poor sealing quality all contribute to premature failure as well as horrible shift quality compared to an upgraded transmission.

Although there are calibratory variations between different applications exist in our 6L80-E and 6L90-E valve bodies, the mechanical and hydraulic platform is common. These valve bodies benefit from virtually all internal components being modified. These enhancements range from steel valves in place of lower quality aluminum valves that are linked to accelerated bore wear, to our durable and air-tight laser-cut crushing style separator plate.

Inside the valve body is also a variety of shift valves that are rather complicated and suffer from a pre-existing disposition for valve seizure. When these valves stick, the transmission will lose its ability to perform predictable tasks such as simply shifting into the oncoming gear while accelerating. This can produce a variety of codes and other conditions.

Further, there are a variety of areas where sealed end plugs can be used in place of failure-prone end plugs that do not feature any sort of O-Ring to protect them from venting. This is a common practice as it would require a large increase in labor time during the OEM's assembly of the product at the factory. The product is lack of reliability for us, the consumer. Our resolution of sealed end plugs is a simple, yet massive upgrade.

The OEM Oil Pan

This is a brief inferiority, but it is worth discussing. These pans, much like the rest of the transmission industry, are made out of stamped metal that is heated and then cooled naturally to retain its shape.

This is a problem for many reasons:

It does not allow for a proper mating surface where the pan mates to the case because there is no precision flattened surface on the pans side.

The pan is incredibly thin, and cannot safely withstand any rugged use safely. Off-Roaders should beware of factory pans.

The pan is nowhere near large enough, this transmission needs more fluid to operate at peak performance and cool temperatures, the factory pan inhibits this by negating a potential safe increase in oil. This forces the transmission to run hotter than it should, drastically effecting the lifespan of the friction elements inside the transmission and torque converter.

The pan does not have sufficient surface area to exhaust heat properly, heat goes to where there is a lack of heat. This is because heat is a form of energy. Increases in surface area enhance a metal’s ability to release heat into the environment around it.

The oil pan's stamped nature does not provide any rigidity benefits to protect the case against physical failure under stressful applications. Cast aluminum deep pans further enhance the rigidity of the case.

The High Pressure Oil Pump

As with the valve body, the 6L80-E and 6L90-E high pressure oil pump contains multiple valves that are responsible for the proper oil flow about the transmission, and as a result, suffers from similar issues. These valves can stick, wear at the bore, springs can break and more.

However, the added complication exists that the oil pressurizing gears at the front of the transmissions pump can wear excessively much like an oil pump in a motor. This causes the rate with which pressure is produced to slow down, and also minimizes peak pressure figures.

Uniquely, the boost valve inside the pump suffers from high probability of bore wear due to how many times it cycles throughout operation of the transmission. The frequent mobility of this valve made it critically important for a sleeve and valve system to be created, facilitating tremendous increases in the longevity of this valve system. Further, this restores lost line-pressure and is a must for TCM tuning to produce its full effect.

Inside the pump is also a pressure regulator valve system, similar to elder years of General Motors transmission. It tends to suffer from bore wear, rendering it unable to capacitate the high line pressure figures necessary to produce reliable motoring experiences in our clients. Common symptoms of this are transmissions that can function fine at low power input, but begin to misbehave at higher engine loads. Our use of a unique steel valve is a dramatic enhancement to this problematic piece.

There beyond, the mating surfaces of the pump are frequently flexed beyond what can be safely machined. As a result, the front half of the pump is often bad during the rebuild process. A bell housing must be purchased from the OEM to resolve this problem, incentivizing the remanufacturing of this transmission before it fails.

The Transmission Electro-Hydraulic Control Module (TEHCM)

Inside the transmission, and adjoined to the valve body, is the transmission electro-hydraulic control module. This system is an hub for all oil being directed throughout the transmission, and is controlled by electrical impulses controlled by the on-board computer of the vehicle. It is responsible for the activation and deactivation of solenoids that control where oil goes and when.

Further, it is married to the vehicle by VIN #, and is used in conjunction with the onboard anti-theft system to prevent putting the vehicle in drive or reverse without the key being detected. These are intelligent small systems that often fail at the pressure-diaphragms due to erratic line pressure caused by the aforementioned failure points in the valve body itself.

It is common practice to remanufacture the TEHCM, but unfortunately, this is incredibly unreliable and often does not provide long-term results. The only reliable way to resolve the TEHCM debacle is to simply purchase a new OEM TEHCM. Further, all TEHCM's used in our products or sold individually at Next Gen Drivetrain are programmed to the vehicle by VIN # ahead of time, allowing for direct OEM compatibility and maximum longevity.

The 4-5-6 (Overdrive) Clutch Pack

As with most transmissions, and foreshadowed in the intermediate shaft section prior, is the problem of overdrive. Overdriven gears have the highest torque multiplication factor of any gears, contributing to added heat, stress and detracted longevity. All applications, to one extent or another, should fear this expensive failure.

For modest applications, a simple upgrade in friction and steel quality will ensure long-standing reliability. However, for considerable added power or stress, adding clutches becomes necessary. In conjunction with a billet intermediate shaft, we can add multiple friction elements and steels to this clutch pack, further fortifying it for added power.

Regardless of application, all 6L80-E and 6L90-E transmissions can justify making enhancements to this portion of the transmission. Further, these upgrades are valid insurance for those who spend a lot of time in upper gears, such as heavy towing applications, high power applications and commuting applications.