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Turbo 400 Rebuilding Tips and Tricks - Four Wheeler Magazine

When it comes to full-size 4x4 trucks and SUVs, the General Motors Turbo-Hydramatic 400 (TH400) automatic transmission is perhaps one of the most coveted automatic transmissions in existence. Thanks to a healthy production run from 1966 to the early 1990s, core units are both plentiful in wrecking yards and easy to upgrade, thanks to a steady supply of rebuild kits and replacement parts offered through the aftermarket. Truck builders favor the TH400 over the more abundant TH350 because the internals are larger and feature more robust support hardware. The TH400 is also desirable over the TH350 (both have cast aluminum cases) due to the former's input/output shaft strength, and the fact that a TH400 has a center shaft support. For those reasons, we decided it was time to highlight a few builder tips for making the Turbo 400 the best it can be. Our plan was to swap the one shown here in place of the NV4500 manual gearbox in our newly revived project SuperBurb. As such, we wanted this transmission to handle 450 lb-ft of torque and a 7,500-pound truck without issue. We enlisted the help of Quality Transmission in Seaside, California-a shop with a knack for ensuring repeat customers. Follow along as we show off a handful of improvements everyone should consider when rebuilding a TH400 automatic.

general Motors Th400 Engine turbo 400 Photo 27667603

PhotosView Slideshow The first tip to consider when rebuilding the proven TH400 is the inclusion of an aftermarket shift kit such as this one from Transgo. The contents of this blue box will resolve nearly 75 percent of all common problems encountered with the Turbo 400. Items such as improved gaskets, seals, and springs are just a few examples of what one would find inside this kit.

The first tip to consider when rebuilding the proven TH400 is the inclusion of an aftermarket shift kit such as this one from Transgo. The contents of this blue box will resolve nearly 75 percent of all common problems encountered with the Turbo 400. Items such as improved gaskets, seals, and springs are just a few examples of what one would find inside this kit.

Upon disassembly, Quality Transmission's technician found the likely cause of our donor's demise: this plastic filter tube had a small chunk missing, which would have caused air to leak into the vital lifeblood (ATF) of the valve body. It's funny how a $2 part can destroy a perfectly good transmission.

This is how the filter tube is supposed to look.

After our donor unit was completely disassembled, the case was placed in a parts washer to remove any metallic debris from the internal cracks and crevices. While our donor case was being cleaned, Quality's technician showed us the end result of improper operation and, essentially, neglect. Shown here are the Third gear apply steel discs-the metallic discs between the friction clutches. Notice the discoloration and warpage of these steel plates. This is caused by low pressure, which causes slippage between Second and Third gear shifts. The frictions found between these plates were in similar shape-worn out and definitely in need of replacement.

Here you can see the new clutch frictions and Kolene steel discs that make up the Third gear apply cluster. These products come from Alto Products and are designed for high-torque applications. They use a special high-friction material to provide improved holding force while also increasing longevity and life span.

Another place we found obvious signs of wear was in the Second gear drum apply band-essentially, it's what makes engine braking possible. Note the burnt color of the center area of this band. This discoloration is usually caused by excessive heat, and is typical for vehicles that see heavy downshifting.

PhotosView Slideshow Here you can see the new heavy-duty Second gear apply band from Transtar. Loaded with proprietary friction elements to improve performance, this heavy-duty band will help grab the drum and hold it with minimal slippage. This band should enable our SuperBurb to get the torque to the ground and deal with the added heat of off-highway use.

Here you can see the new heavy-duty Second gear apply band from Transtar. Loaded with proprietary friction elements to improve performance, this heavy-duty band will help grab the drum and hold it with minimal slippage. This band should enable our SuperBurb to get the torque to the ground and deal with the added heat of off-highway use.

Another attribute of the Transtar Second gear apply band is the way the steel backing terminates at the eyelet for the anchor pin. Note how the unit on the bottom is welded on the end in addition to the two small spot-welds; this improves strength and ensures that the forward band does not fail under load.

One essential procedure that every TH400 rebuilder should consider is block sanding the valvebody mating surface. This helps ensure a leak-proof seal between the valvebody and its mounting surface. If overlooked, this vital intersection may have slight warpage and would likely develop a pressure drop and/or fluid leak within the transmission-which contributes to direct clutch failure.

PhotosView Slideshow The other internal element that greatly benefits from a light block sanding is the cover for the Reverse apply piston. These stamped steel covers tend to warp, causing low pressure for the Reverse band servo.

The other internal element that greatly benefits from a light block sanding is the cover for the Reverse apply piston. These stamped steel covers tend to warp, causing low pressure for the Reverse band servo.

Most TH400 transmissions came from the factory with the forward sprag shown here. This design is prone to failure because of its intricate internal spring arrangement with few rollers to distribute torque loads over the drum.

Quality Transmission replaced the factory sprag with the superior 32-element design shown here. This part comes from Transtar and requires a special drum that virtually eliminates any issues associated with breaking the sprag-a common failure point in the TH400.

PhotosView Slideshow This photo illustrates the difference between the stock springs and the heavy-duty springs that come with the Transgo shift kit. The shorter spring on the left is constructed from thicker, higher-quality wire and prevents the clutches from applying centrifugally at high rpm.

This photo illustrates the difference between the stock springs and the heavy-duty springs that come with the Transgo shift kit. The shorter spring on the left is constructed from thicker, higher-quality wire and prevents the clutches from applying centrifugally at high rpm.

Cast aluminum versus stamped steel: when it comes to the TH400's clutch apply pistons, aluminum is the clear winner. Here's why: the stamped steel unit consists of two individual pieces that press together. Sometimes this press fit loosens over time, and fluid pressure loss is the unavoidable result. The cast piston on the left is a single part, and leaves no room for a seal to leak or gap to develop in the first place. Another argument in favor of the aluminum-type piston is that of rigidity. The aluminum piston will flex much less than a steel unit, thus preserving the outer diameter seal and sealing surface from wear. When changing out pistons, builders must take note that a check ball exists in either the piston or the forward/direct drum; without one, you are asking for trouble.

Transgo provides improved Teflon seals for the center support unit. These seals replace steel-type seals and are far superior at sealing the direct clutch circuit.

PhotosView Slideshow

Ideally, you want to fit six frictions inside the direct clutch apply drum on the TH400. To achieve this, a transmission builder must select the thinnest combination of steel discs and friction elements. The more you fit, the longer the life span of the clutch material will be. However, if you install too many clutch packs, you will not have enough endplay and this can cause a harsh reverse engagement. Ideally, you want 0.004 to 0.006 inch of endplay in the direct clutch pack.

This shot shows the three available output shafts for the TH400 automatic, the shortest of which is the one required for mating the TH400 to the popular Atlas II or Off-Road Design 203/305 Doubler kit. Our plan is to use the Doubler, which you will be able to read all about in next month's issue.

Also included in the Transgo shift kit is this shift valve along with a cup plug, which enables compression braking in all forward gears. This option is imperative for anyone who uses low-range when on the trail, or for those who tow heavy loads over mountain passes. Essentially, this part lets the engine slow the vehicle when you let off the gas in gear or downshift to slow the vehicle. The one on the left is the new Transgo unit.

The TH400 features two filters inside that help remove debris from the ATF. The small screen filter shown here eliminates contaminants from entering the governor unit. This is super important to replace when rebuilding because metallic debris can jam the governor unit and cause the transmission to shift erratically.

Every transmission rebuild should include a new filter. The one shown here is for the deep pan commonly found in heavy-duty applications such as 3/4- and 1-ton pickups.

The torque converter we used for this build came from PDQ Transmission Parts and features a stock stall speed and anti-balloon plate. This converter also uses a heavy-duty enclosed Allison bearing and features furnace-brazed turbine impeller and vane plates for long-lasting durability. FW

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Turbo 400 Rebuilding Tips and Tricks - Four Wheeler Magazine

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