TH400

Rebuilding a transmission, such as the TH400, is not relatively as easy as changing your car filter or making an oil change. It is a challenge that will take a good mechanic almost all weekend. But if you don’t possess the skills of a mechanic, you shouldn’t attempt rebuilding your transmission. It is highly recommended that you leave this project in the hands of professionals. Here’s what you need to know about TH400 transmissions and how to rebuild them for faster and better performance.

Overview of TH400 Transmission

The TH400 transmission – or Turbo 400 transmission – was General Motors vehicles’ brainchild in 1964. This transmission was designed to substitute the ST300 2-speed automatic transmission. The TH400 transmission was used in Pontiacs and Cadillacs in the first year but somehow found its way to Chevrolets, Oldsmobiles, and other models by 1965.

The TH400 transmissions from 1965 to 1967 featured a mechanical device known as a variable pitch stator. This feature helped the torque converters characters in the transmissions and can easily be identified by the 2-prong plug on the case. By the time the ’70s arrived, the TH400 transmission was already used in heavy-duty GM trucks in 2WD and 4WD versions. 

In 1990, this transmission changed from TH400 to 3L80, resulting in longitudinally positioned 3-speeds and 8,000lbs. GVW or Gross Vehicle Weight. This name change was necessary as it made readily identifying the transmissions easier. This was also the year an early introduction of the 4L80-E transmission, the successor of the Turbo 400 Transmission that has overdrive, was made.

The TH400 transmission is an electronically-controlled, heavy-duty version of a TH350. The Turbo 400 Transmission required a kick-down switch by the gas pedal to inform the transmission when to kick down a gear. Military and heavy-duty GM vehicles still use this transmission as it is still produced today.

Features of the TH400 Transmission

The Turbo 400 transmission is a highly durable gear mechanism that offers improved performance over its predecessors. One common feature of the TH4oo transmission is its iron and aluminum construction. In addition, it has three tail shaft lengths and bell housing integrated into the transmission.

• First gear – 2.48:1
• Third gear – 1:1
• Reverse gear – 2.08:1

The TH400 transmission weighs 135 pounds minus the fluid.

Identifying the TH400 Transmission

The Turbo 400 transmission has a main cast aluminum case measuring 24-3/8 inches long. This aluminum case is smooth. The rear mounting face of this transmission showcases a bold hex pattern with several ribs that run forward longitudinally. The shape of the fluid pan is somewhat irregular, likened to a distorted Texas pattern. The TH400 transmission is admittedly the largest of the regular General Motors transmissions.

Nevertheless, it is surprisingly compact, considering the immense power it can handle. General Motors created two significant variants of the Turbo 400 transmission. The first variant, i.e., the TH375 transmission, was made public from 1972 to 1976 in smaller displacement vehicles. It was used extensively in several small vehicles and easily identified by the ‘375-THM’ lettering carefully etched on the bottom or underside of the tail housing.

The other variant of the Turbo 400 transmission was the TH475; a heavy-duty transmission used extensively in large trucks from 1975 onwards. You can quickly identify the Turbo 400 transmission by checking out its kick-down assembly. This transmission uses an electric slide switch that controls the throttle linkage mechanism. On the other hand, the TH350 transmission uses a mechanical relay or cable kick-down mechanism attached to the throttle linkage.

Rebuilding the TH400 Transmission to Your Engine

The TH400 transmission is respected and loved due to its versatility and durability. However, there will come a time when the old transmission shows signs of wear and tear. Even the best transmissions are rebuilt at one point or the other. Wholly and correctly rebuilding an automatic transmission like the TH400 is ordinarily not beyond the capabilities of the average DIY automotive enthusiast. But there is always something that keeps away even pro automotive mechanics or technicians when rebuilding an automatic transmission.

Some claim not to have the necessary tools, while others say they don’t have enough time to carry out the task, so they whisk off the tranny to a specialty shop. As mentioned earlier, rebuilding a transmission is not an easy task. But the truth is that most of the tools required are standard hand tools. If you are not 101 percent sure of what you’re doing, best leave this task to professional mechanics. In this light, here are some of the tools required for rebuilding your TH400 transmission:

• Safety equipment (eye protection, nitrile gloves, heavy-duty gloves, hearing protection, etc.)
• Wrenches and sockets
• Screwdrivers
• Spring compressors
• Snap ring pliers
• Measuring devices
• Pans for small components
• Case holding features
• Work surface
• Busing removers and installers
• Cleaning cases, etc.

When racing a Turbo 400 transmission, it is crucial to upgrade the direct and forward drums. The key revolves around more durable and stronger components and boosting efficiency for better performance, safety, and consistency. You can get a radical edge over other users by upgrading your TH400 to maximize drum function with unique configurations of more robust and lighter components. This development significantly boosts the efficiency and performance of your TH400 transmission for more power to the ground and stability/traction on the shifts.

Conclusion

The Turbo 400 – or TH400 – transmission is one of the automotive industry’s most widely used and versatile performance transmissions. This remarkable transmission has proven to be capable of handling massive power and is incredibly adaptable. This is not evident only in General Motor applications but also adapted behind Chrysler, Ford, and other engines.

You can maximize the reliability and performance of the TH400 transmission to make it deliver that exceptional result you trust at any power level. However, unless you are a certified mechanic or technician, it is highly recommended that you allow pros to handle the rebuilding of the TH400 transmission on your behalf.

 The TH400 or Turbo 400 is a good transmission that was used in several cars years ago. This popular transmission was found in GM’s cars, as well as, Rolls-Royces, Jeeps, Jaguars, Ferraris, etc. The TH400’s high-level of adoption could be tied to its remarkable features, and it proved that people can once again rely on GM transmissions.

But what were these features sported by this durable and legendary transmission, and generally, what set it apart from other transmissions from this manufacturer? An overview of the TH400 transmission will give you a better idea. Read on to find out!

 

The GM TH400 Transmission

The GM TH400 transmission is a three-speed automatic transmission that was longitudinally positioned behind the engine. This placement is aimed at providing the best power ratio and durability for rear-wheel drive. The TH400 has been rated at 450 ft. lbs. of input torque, nonetheless, aftermarket designs that have been modified may go beyond that rating.

 

History of the TH400 Transmission

The TH400 was launched by General Motors in 1964, as a replacement to the ST300 2-speed automatic transmission. It was used in a wide range of vehicles, hence, it was not just exclusive to GM cars.

Upon its launch, the TH400 was evident in Buicks and Cadillacs, and a year later, it was used in Chevrolet and Oldsmobile cars. Buick, Olds, and Cadillacs that were later launched between 1965-1967 also featured the transmission but with an innovative variable-pitch stator. This stator helped to vary the torque converter’s characteristics, and you can tell a car has this stator by the two-prong plug on the case.

In the 1970s, the TH400 was used in heavier-duty GM trucks and it was available in 2wd and 4wd configurations. The naming of the TH400 was changed to 3L80 (3-speeds, Longitudinally positioned, 8000 lbs. GVW or Gross Vehicle Weight) in 1990 to make it easier for buyers to identify the transmission.

The change in naming does not come as surprise since some GM transmissions were also renamed (for example the 700R4 to 4L60). Around the same time when the TH400’s name was changed, an overdrive version called the 4L80-E was launched.

This is an electronically controlled transmission that is still being produced and used in a range of heavy-duty GM and military cars/trucks. Generally, here’s a list of cars that used the TH400 transmission:

• • 1965-1967, 327 V8 (Rambler/AMC)
  • 1968-1971, 350 V8 (Buick)
  • 1969-1972, 225 V6 (Buick)
  • 1972-1975, 232 I6 (AMC)
  • 1975-1979, 258 I6 (AMC)
  • 1972-1979, 304 V8 (AMC)
  • 1972-1979, 360 V8 (AMC)
  • 1974-1975, 401 V8 (AMC)
  • GM
  • Jaguar
  • Ferrari
  • Rolls-Royce

 

Features of the TH400 Transmission

The features of the TH400 Transmission enabled it to provide improved performance over previous transmissions. It was also durable, and these characteristics can be tied to the following:

• • Aluminum and iron construction.
  • Weighs 135 pounds without fluid.
  • Bell housing integrated into the transmission.
  • First gear: 2.48:1, Reverse gear: 2.08:1, and third gear is an even 1 to 1.
  • Came with three tail shaft lengths.

 

Turbo 400 Transmission Identification

The Turbo 400 transmission can be identified in the following ways:

1. External Build

The TH400 transmission comes with a case made of cast aluminum alloy to give it strength and durability. The case is very smooth and its length sits at length of 24-3/8″ long. This TH400 is the largest of the common GM automatic transmissions, however, the transmission still maintains a compact design.

There is a hex bolt pattern on the rear mounting face of the transmission and the ribs run forward longitudinally. In line with that, the fluid pan has an irregular shape.

2. Variants of the TH400

There are two variants of the TH400, and these are the TH375 and TH475. The TH375 was used in smaller displacement cars between 1972-1976. In contrast, the TH475 was used in larger trucks from 1971 since it is an extra-heavy-duty version. One can easily different either these variants apart by looking out for the “375-THM” designation cast on the bottom of the tail housing.

TH350 vs. TH400

A simple way to differentiate them is to check the kick-down mechanism. The TH400 transmission takes advantage of an electrical slide switch controlled by the throttle linkage. On the other hand, the TH350 uses a mechanical cable kick-down that is connected to the throttle linkage. Despite the difference, the TH400 is considered the heavy-duty version of the Turbo-Hydramatic 350.

 

Transfer Case of the TH400 Transmission

The TH400 can easily be adapted for use in most Jeeps longer than CJ5s, and as such, it is an excellent conversion transmission. Whether it’s the 2wd and 4wd versions of the Turbo 400, either of these can be used. Nonetheless, the 1976-1979 AMC case is similar to that of the TH400’s, from the collar of the case and back, however,  it is tilted about four degrees.

 

Engine Compatibility and Adaptability

The engine compatibility and adaptability of the TH400 transmission include:

1. AMC/Mopar Jeep

The earliest Turbo 400s in Jeeps featured a factory adapter plate, whereas older AMC versions came with a dedicated AMC style case. It is entirely possible to make the Chevy 400  compatible in the AMC I6 & V8 engines. As an upgrade of this nature can result in a more enhanced Jeep powertrain over 727, 999.

2. Chevrolet

The front face of the TH400 is compatible with the Chevy 90 degree “Small Block” or the “Big Block” patterned engines.

3. Buick / Olsmobile / Pontiac / Cadillac

The TH400 is compatible in Buick engines especially when you use a Buick V8 or V6 version of the transmission.

 

Common TH400 Problems

The TH400 may have offered improved performance over its predecessor, the ST300, but it still came with its own problems. Notable among this is an early shift and less efficiency when the engine revs high. The latter occurs when the kick-down switch, responsible for shifting between gears and maintaining maximum RPMs, stops working. The switch would’ve helped in, efficient power usage.

On the other hand, the cause of this problem can be tied to the wrong placement of the vacuum hard-line, moving from the intake manifold with a pliable rubber line. The heat generated by the engine and transmission leads to the deformity of the rubber, thereby failing to hold the pressure properly. Another problem of the TH400 is the possibility for its transmission fluid to leak gradually into the hose if the seal is not fixed properly.

 

The Bottom Line

An overview of the TH400 transmission shows it is a durable transmission that can give a good performance. The TH400 was used in a wide range of cars from GM as well as other manufacturers. Interestingly, the transmission can be adapted for use in several engines today. In the end, whether it’s a new or aftermarket TH400 you settle for, you are bound to get the full benefits offered by this transmission.