How To Repair Gm Transmission Line

General Motors introduced the TH400 manual in 1964. It was start used in Buick and Cadillac applications, and was quickly adapted to other models. It became the workhorse of Full general Motors' automatic transmissions, used in truck applications and anywhere else there was a need for a potent unit to handle high-torque engines, heavy vehicles, and towing applications.

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TH400s make an excellent highperformance automated transmission, and have been used for decades for all sorts of racing applications. The factory conservatively rated their torque input chapters at 450 ftlbs, but they can handle much more than. The performance aftermarket stepped upward with heavy-duty parts to make them even better.

Rebuilding an automatic transmission may be intimidating to some, but don't let its complexity overwhelm you. Stay focused, complete i step at a fourth dimension, and eventually you'll take completed the task.

The gear ratio for low gear is 2.48:1, ane.48:i for second, and one:ane in third gear. A fully assembled unit weighs nearly 135 pounds, so it's no lightweight. Although frequently taunted every bit a scrap heavy and ability robbing, the TH400 makes up for it with its great strength and long-term immovability in heavy-duty service.

The TH400 is too a very simple unit, and is relatively easy to overhaul. It does not require a lot of special tools, or special procedures. Virtually hobbyists have smashing success edifice their own units at home. The aftermarket offers plenty of heavy-duty and high-performance replacement parts for only well-nigh everything inside a TH400. Even and then, the TH400 doesn't demand a lot of help or special aftermarket parts to be a reliable and durable transmission. (I accost potential weak points and recommended upgrade areas in the overhaul process department.)

Here I recommend and outline the replacement of all of the bushings during the rebuilding process. I am also replacing all of the thrust washers. Although non mandatory for all builds, doing so produces a unit of measurement that has firmer shifting, reduced gear noise, and ultimately lasts longer once placed into service.

Remove External Components

Stride i: Remove Modulator

Remove the bolt for the transmission modulator retainer.

Remove the modulator and the modulator valve.

A strong magnet usually pulls the valve from the case. If it doesn't want to come out, you can use a small flattip screwdriver to help it out of the instance afterward the valve body is removed.

Pace 2: Remove Governor

Remove the governor comprehend bolts, governor cover, and the governor. Inspect the plastic gear closely for nicks, wear, and other harm.

Footstep iii: Remove Tail Housing

Remove half-dozen tail housing retaining bolts and slide the tail housing off the transmission.

Footstep 4: Remove Oil Pan

Remove the oil pan bolts, and advisedly lift the oil pan upwards and off the transmission. Information technology makes a expert place in which to put the valve body and other associated parts removed under the pan until they tin can be cleaned.

Remove Valve Body and Governor Filter

Step i: Remove Oil Filter

Remove the special bolt that holds the oil filter in place. TH400s used several types of filters; the ane shown is the about common. Some heavy-duty truck applications use a deeper pan and extension for the filter. Very early units used a side-mounted filter. Lift the filter off the extension tube, taking care not to break or crack the plastic tube.

Step 2: Remove Filter Extension and O-Ring

Remove the plastic filter extension tube from the case. Remove the O-ring from the case. Information technology's a common practice for builders to install two O-rings in the case, so in that location may be more than than one; just brand sure to become them out of the case at this time.

Pace 3: Remove Valve Body

Remove the valve body bolts and remove the valve body from the case. Three of the bolts are ane/four-inch x 20 threads (shown), requiring a seven/16-inch socket. The balance of the bolts are 5/16-inch ten 18 threads and require a i/2-inch socket to remove them. They are similar to the oil pump bolts but non interchangeable, so continue them separate.

Stride four: Remove Governor Filter

Locate and remove the governor filter. It should be establish under the governor tube at the back of the case. It may be missing; many builders do not install 1 during transmission rebuilding.

Remove Downshift Solenoid and Separator Plate

Step 1: Remove Downshift Solenoid Bolts

Remove the two one/4-inch x 20 bolts that attach the electric downshift solenoid to the instance. At that place were several dissimilar styles used. Some have a seal equally function of the solenoid, while others require a separate seal.

Footstep 2: Remove Downshift Solenoid

Locate and remove the downshift solenoid wire from the terminal, and remove the solenoid from the transmission.

Stride 3: Remove Separator Plate

The separator plate and separator plate gaskets can now exist removed.

Remove Front end Band and Low Band Employ Servos

Footstep ane: Remove Front Band Use Servo

Lift the front end band utilize servo from the case. Exist careful not to let the pin slide out of the piston and lose the clip that retains the jump retainer. It is best to leave this assembly intact so none of the parts become lost or rearranged during cleaning.

Stride 2: Remove Check Assurance

Remove the six check balls from the instance, noting their locations.

Step 3: Remove Depression Band Servo Embrace

Remove the six bolts that adhere the depression ring servo comprehend to the case. Lift off the servo cover, then remove the use piston. Note that within the utilize piston is the ane-2 accumulator and bound; remove them as well.

Stride iv: Remove Parking Pawl Guide

Remove the parking-pawl-guide retaining bolts and lift the guide out of the example.

Remove Oil Pump, Manual Shaft and Speedometer Gear Housing

Step 1: Remove Oil Pump, Manual Shaft and Speedometer Gear Housing

Using a 12-point 3/viii-inch socket, remove the center support retaining commodities. This is a special hollow bolt; oil must flow through information technology to the intermediate clutch. Put it in a rubber place until reassembly of the unit.

Prior to removing the oil pump, information technology is a good idea to take an input shaft endplay reading. The factory used specific shims and selective thrust washers under the pump to set endplay. If you purchase a thrust washer kit, it commonly comes with selective shims and thrust washers to ready endplay during the rebuild. Endplay should be no more than .030 inch, and no less than .015 inch.

Stride 2: Remove Oil Pump

Remove the oil-pump-to-case bolts; in that location may be either six or ten bolts, depending on the year of the transmission. Two of the pump bolt holes are threaded for iii/8-inch x sixteen threads. Using a slide hammer, remove the oil pump from the example.

Pace 3: Loosen Nut

Loosen the nut on the end of the manual shaft.

Step four: Remove Manual Shaft

Remove the retaining pin for the transmission shaft, and slide the manual shaft out of the case. Yous can at present remove the transmission linkage and parking pawl shift rod from the case.

Footstep 5: Remove Speedometer Gear Housing

Remove the speedometer gear-housing retaining bolt and retainer. Pry the speedometer gear housing from the case. Annotation that I have installed the end of an old speedometer cablevision to pry on it to avert damaging the threads.

Remove Forward Drum Clutch, Direct Drum and Band

Step i: Remove Frontwards Drum Clutch

Get a house hold on the input shaft and pull the forward clutch drum from the case.

Stride 2: Remove Directly Drum

Remove the directly drum from the example. It tin be difficult to get a practiced grip on information technology; the outer snap ring and clutch and steel plates can exist removed if needed (left). You can utilise 2 hooked screwdrivers to remove information technology (right).

Step 3: Inspect Band

The band can now be removed from the instance. Check the status of the band lining and the surface on the drum where it applies.

Remove Clutch Pack, Output Shaft and Low Band

Step i: Remove Clutch Pack

With a large screwdriver, remove the intermediate clutch retaining ring, backing plate, and clutch pack.

Step ii: Remove Retaining Ring

Remove the center back up retaining ring, center support, and backing spacer ring (not used on all models).

Footstep three: Remove Outer Shaft

Remove the outer shaft that splines into the dominicus gear.

Footstep 4: Remove Output Shaft

Become a house grip on the inner shaft, and elevator the entire lower assembly/output shaft out of the instance.

Step v: Remove Thrust Washer

Remove the thrust washer backing spacer (left) and low ring (right) from the case

Final Disassembly

Stride one: Remove Solenoid Connector

Pull the downshift solenoid electrical connector from the case. Employ intendance not to damage the unit. Use a modest screwdriver, if needed, to depress the retainers.

Step 2: Remove Manual Shaft

Remove transmission shaft from the instance

Stride 3: Remove Manual Shaft Seal (Of import!)

Use a large flat-tip screwdriver to pry the manual shaft seal from the case. Use care not to damage the diameter around the seal.

Footstep iv: Remove Parking Pawl

Remove the parking pawl spring.

Step 5: Strip and Clean Case

The case is now stripped down for cleaning. A power washer removes heavy deposits from the case. Soaking it with engine cleaner or a heavy-duty degreaser helps the process. The case tin also exist taken to a motorcar shop for professional person cleaning. Information technology must be clean and gratis of all dirt and debris prior to associates.

Written by Cliff Ruggles and Posted with Permission of CarTechBooks

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