How to Rebuild a Truck Transmission

Transmission Rebuilding can be done three ways: "Bench building" where you take your unit out of your truck and haul that to a service shop, or overhauled in your own shop if you have mechanical expertise, service literature and tools, or by leaving your "whole baby" at the Transmission Shop. (Sigh.)

Difficulty: Challenging

Instructions

Things You'll Need:

• Pick set for removing seals and springs
• Rebuild hard parts, frictions and steels, seals
• Vehicle Lift
• Transmission Jack
• Oil collection/ Funnel Bucket
• Hardened sockets, extensions and impact wrenches
• Transmission Clamp
• Service Bulletins and Technical data for the unit you are building.
• Shop Rags and Mineral Spirits
• Replacement Fluid, & Assembly Lube

1. Automatic transmissions are operated by compartmentalized fluid, which is sent through first a filter from the pickup tube in the reservoir, then to various shift valves in the valve body, then onward through the gear assemblies and governors to lubricate and cool the moving parts. For the purpose of this article we will use one of the most frequently used truck transmissions, the THM 700R4: Fully automatic and using a three element hydraulic torque converter with its own clutch, the 700r4 uses five multi-disc clutches, one roller clutch, a sprag and an apply band to operate its compound planetary gearset. These provide four forward speeds and one reverse. We will consider the torque converter to be a separate issue. The electrical harness sends speed and torque signals from your engine to the transmission to adjust gear ratios for maximum efficiency under various throttle settings. Assuming you are not scared off by the thousand or so parts in your transmission, and this very technical overview, we can use service literature to explain both the components and the rebuilding process. Getting your unit out of your truck, however isn't covered in the technical manual, and you may need to consult Chilton. Follow this general procedure:
   
   
   
   
   In order to rebuild it, Service and Repair books, showing a schematic and an exploded parts view of the components will absolutely be necessary. ATSG publishes a Techtran Manual and Haynes a "Techbook." In addition to literature you will need specialized tools that probably aren't in your toolbox now, like special pliers used to open scarf cut snap rings and picks to remove old seals. When you order the rebuild kit for your specific year and model, request any critical service bulletins. These will alert you to which seals to use with which model years, upgrades to components, etc. GM made many modifications and changes over the years, including the addition of lube holes in the forward sprag assembly and undersized frictions and steels to drop back into the clutch housing more easily following an apply.

2. Pull the dipstick. Disconnect the Throttle Valve cable or kickdown cable. Set the parking brake and raise the truck on a lift. Drain the unit, by loosening and tipping the pan if necessary. Disconnect the shift linkage. Remove the drive shaft at the differential and slide the yoke out of the rear seal. Remove the torque converter to flexplate bolts. Support the unit with a suitable jack, and using sockets and extensions, remove the transmission to engine bolts. Remove the transmission cross member from the unit or remove the tail mount.
   Once the unit is free, hold the unit as level as possible while lowering and hold the torque converter so it doesn't fall. Drain any remaining fluid from the rear extension housing. Now you are ready to either take your unit to a professional for a "Bench Rebuild" or refer to service literature and do it yourself. I built one on my dining room table once, when my back was out, but it's not for the fearful or the physically feeble. Decide now which way to go. If there are/were only mileage and wear considerations, you can probably just replace the worn friction parts and get it back on the road. Burned fluid with an accumulation of metal fragments on the magnet in the pan may mean you need to take it to a professional with dial indicators and micrometers for a more sophisticated overhaul. Otherwise, clean the exterior of the case thoroughly and begin disassembly:

3. Note the depth from the edge of the housing for reference, then slide the torque converter out, place it over a drain bucket. Tighten a holding clamp onto the case, and set it into the base mounted on your work bench.
   Servo: Use a servo compressor or an extension to release pressure, and remove the servo cover snap ring. Remove the O ring and the servo cover. Discard old seals and clean parts in mineral spirits. Set them on a clean surface, keeping them in groups. Remove the 2-4 servo, the 4th apply piston and the servo return spring. Remove the e-clip, washer and apply pin spring. Remove the 2nd apply piston from the servo pin. Use a compressor or a c clamp to remove the second apply piston snap ring, then the cushion spring retainer and spring.
   Extension Housing and Governor: Remove the housing bolts attaching it to the transmission body and pull it free. Remove the output shaft sleeve, if there is one. Push the tab of the speedometer clip down, and tap the gear off of the output shaft carefully. Tap around the governor cover flange with a small flat blade or punch carefully to remove it. Remove and inspect the governor and the case bore for wear/damage. Flip the case over and remove the oil pan. Remove the filter, and the o ring with a small pick. You must get it out: If the old seal stays in place, it can cause the filter to drop into the oil pan, and no sumping will take place, burning up your transmission.
   Solenoid Harness: Remove the solenoid bolts, solenoid and harness. Remove the case connector and o ring seal. Remove the 1-2 accumulator cover bolts, the cover, the piston, seal and spring. Remove tension plate bolts and tension plate. Remove the manual detent spring, the throttle valve lever and bracket assembly. Remove the remaining valve body bolts and lift the valve body straight up off the manual valve link. Set it aside. Next, following the directions in your service manual, remove the oil pump assembly, the reverse input drum, the input housing , the 2-4 band and the input gear set. Remove the Low-Reverse clutches and reaction gear set, the Low-Reverse clutch apply piston and snap ring. You can blow shop air into the reverse apply passage to remove the piston. Next remove the parking linkage components.

4. Clean the inside of the case and blow it out with compressed air. Set your technical service bulletins, rebuild guide and new parts out, and begin reversing the steps, carefully following all directions. Check the operation of clutch assemblies with shop air. Frictions and Steels should drop back into the housing smoothly after an apply. Replace early 3 hole sprags with the new design. Tighten valve body bolts with hand tools, not air tools, to 20 ft/lbs and no more.

Tips & Warnings

• When reinstalling the torque converter, rotate it until the drive ears in the pump engage the notches, and make sure that it drops all the way into the housing to prevent damage to the pump when reinstalling. Confirm by measuring.
• Replace fluid with new high mileage types like MAX LIFE for longer life.
• Requires physical strength and specialized experience & tools.
• Don't rush, allow plenty of time, be organized.
• Get training on safe use of hydraulic lifts and air tools.
• Obtain the special plastic seal installer tools to avoid damage to square cut seals and O rings in the THM 700 R4

[ScarlettOHairy]

on 3/14/2009 If I had a truck, I'd rebuild my transmission using your excellent tips and tricks. Truly, lots of good info for someone wanting to rebuild the trannie.

[79CourierChick]

on 1/13/2009 I need a full guide to rebuilding a automatic dodge truck transmission. Check my profile and email me if you have anything I can use as far as instructions.