How To Rebuild your Automatic Transmission Part I

Contributed By: Mark Wallace

Preliminary thoughts:

1. Automatic Transmissions have a lot of parts, and are not exactly simple, If rebuilding a carburator or an engine represents a substantial challenge for you you may be better of going to a non-AAMCO transmission shop. On the other hand rebuilding an automatic transmission is definitely within the realm of what a good ametuer mechanic can expect to do correctly.

2. Cleanliness is of utmost importance. Any place where there is wind and dust and dirt is probably unacceptable. I used the kitchen table after coving it with first some plastic garbage bags and then some masking paper. I had some newspaper down too.

3. More likely you will have to buy some tools, and it is not entirely unlikely that those will be expensive tools. Of course keep in mind that my taste in tools tends toward quality name brands, and that you'll probably end up with at least a few new tools that you'll never know how you did without. I tried to include tools used in the pictures whenever I could.

4. You will need a good manual. I used a factory Jeep shop manual for the 81 model year. This particular manual actually has the same cover graphic as the owners manual so it's a pretty cool relic too. I'm writing this how to mainly so that anyone considering this undertaking can get a good idea of what they are getting into from an ametuer's perspective.

5. I am not an expert on this subject. I rebuilt one transmission (a Torqueflyte 727, but I'm sure the TH-400 is similar) and it made it across the country without incident. Had it failed along the way I don't think that I could have witten a workable how to.

With that in mind it's time to break out the tools and get dirty.

1. Transmission Removal: There are a couple of ways to go about doing this, and the various methods probably can be best chosen according to what other work has to be done on the Jeep. Basically what I did was drop the transfer case and remove it out the bottom, unbolt the engine mounts, unbolt the transmission from the crossmember and then unbolt the crossmember from the frame. I then pulled the engine and transmission as a single unit following the transmission with a bucket to catch the fluid. (With the stock tranny pan no matter what you are going to make a mess with the tranny fluid) I then set the transmission on an overturned five gallon bucket and divorced the engine and transmission. If you plan to save your torque converter (more later) you want to try and hold it on the transmission for the time being so that you don't beat up anything important. After the transmission is out the entire transmission should be set in a bucket with the tailshaft assembly pointed down to drain the remainder of the transmission fluid out.

2. Torque Converter Removal: With the transmission divorced from the engine pull the torque converter straight out. Set it someplace safe even if you don't plan on re-using it as there will probably be a core charge for the replacement torque converter.

3. Transmission Oil Pan Removal: With the transmission upside down, unbolt the pan. The pan may need to be pried off. Note whether RTV has been used to glue on the pan. (AAMCO did use RTV to glue on my pan, which is 100% incorrect. Automatic transmission fluid will disolve RTV, so consequently I found a substantial amount of RTV stuck to the transmission filter, and floating around in the bottom of the pan.) Look for an excessive amount of particulate matter. If there is a lot plan on replacing the torque converter.

4. Valve Body: As soon as the pan is off you can see the valve body. This is the hydraulic brain for the automatic transmission. It is full of sliding valves, springs, and check balls. (If AAMCO rebuilt your transmission last it is probably also full of particulate matter, which is not correct. Judging from the dirt that I cleaned out of the valve body I don't think AAMCO had ever even had it apart). Rebuilding the valve body is fairly straight forward, but first it has to be removed from the transmission. It is held on by a few bolts on each end and has the park lock rod attached to the manual shift detent by an e-clip. you can remove the whole valve body without removing the park lock rod from the valve body and the park lock rod just slides out.

5. Rebuilding The Valve Body: This is no more complex that taking the valve body apart, cleaning it carefully, and putting it back together, except for one detail: The valve body for the 727 has 106 different parts and they all look more or less the same. This is where the manual really starts to help. What I did is I laid each part on the paper on my table, drew a box around it, labeled what it was, and then proceeded to do the next part. When I was cleaning the parts I made a new piece of paper so that I did not have to set the clean parts back into the dirt that I had just washed off. I cleaned the parts in a plastic dishwashing tub in paint thinner, which is not the best solvent because it leaves a residue, but it worked okay for me. Safety Clean, or brake cleaner would be better. It is also important not to used either shop towels or paper products to clean the valve budy (or any part of the transmission for that matter because any lint left in mechanisms can cause problems later). When everything is clean and dry reassemble the valve body in reverse order torquing everything to proper spec. (You'll need a torque wrench that reads in inch-lbs.)

6. Rotating Assembly Endplay Measurement: Both the input shaft and output shaft endplay need to be measured before the rotating mass is disassembled. This is because a selective thrust washer is used to determine endplay and it needs to be determined whether the selective thrust washer needs to be replaced. (In my case The endplay was way out of spec and by replacing the selective thrust washer with the thickest thrust washer available I was just barely able to get it back into spec. Once again this is something that AAMCO should not have screwed up) The manual says to use a dial indicator, but I was able to use a straightedge and a dial caliper. Once again the endplay spec is in the manual.

7. Tailshaft Assembly Removal and Contents: On the rear of the transmission there is a short (as in maybe about eight inches in length) aluminum housing that adapts the transmission to the transfer case. It also houses the park sprag, the governor, a ball bearing, a seal, and some of the output shaft. Remove the six bolts that hold the tailshaft assembly to the main case and slide the tailshaft assembly off. The ball bearing should fall out, and the seal can be removed with a punch and a ball peen hammer (or a seal pusher if you have one). The governor is a small valve assembly that is held on the output shaft with a pin that goes through the output shaft. The governor is bolted to the park sprag which rides on a spline on the output shaft. Once this is all apart clean it and reassemble it.

8. Accumulator Piston and Spring: Remove spring from piston. Remove piston from case.

9. Front Pump Removal: This is where you start getting big transmission parts everywhere. First comes the front pump. There should be seven bolts holding the front pump to the transmission case. Remove them. The front pump is now still in the case. Before you go any further you need to tighten the front band until the front band is tight around the front drum. This keeps tranny guts from bouncing out and bending or breaking all over your shop floor. I diverted from what the shop manual siad here. Two of the bolt holes in the front pump have threads in them. The manual said to use two slide hammers, but the amount of force that it takes to get the front pump out really doesn't warrant it. (Also I only have one slide hammer) So What I did is I bought a threaded rod and cut it in half so that I had two shorter threaded rods. I screwed the two rods into the threaded holes in the transmission and measured (approximate is fine for this) the distance between them. I them got a piece of 1x4 and drilled two holes in it at the same distance that the two rods were spread apart. I sild the 1x4 (which needs to be long enough to span across the front of the transmission) onto the threaded rods. I then put some nuts and washers on the rods to complete my primitive puller. Worked like a charm.

10. Why I Did Not Rebuild My Front Pump: Had I been the first person to tear into this transmission I would have rebuilt my front pump, But due to the incompetence of the people at AAMCO I didn't have that option. The front pump is bolted to the reaction shaft support. First off my reaction shaft support was original. This would have been all fine and dandy except that the front pump and the reaction shaft support really need to work together as one unit. If you repace one you need to replace the other. The other reason is that even without my original front pump my reaction shaft support was badly gouged on a seal surface (AAMCO had put it back in that way). So I purchased a warranteed good condition used front pump and reaction shaft support.

11. Front Band and Front Clutch Removal: Loosen up the front band (the one you tightened up two steps back) and slide the band strut and band out the front of the transmission housing. Remove front clutch assembly by grabbing (as best you can) the outside of the clutch housing. (Don't pull on the input shaft, that comes later) What you will pull out is a complete assembly that you can (I suggest that you do this) leave together until it's time to rebuild it.

11. Input Shaft and Rear Clutch Removal: Pull on the input shaft and the rear clutch assembly will slide right out of the case. There is a also a thrust washer that goes between the input shaft and the output shaft it may stick to either shaft or it may fall off into the transmission. Either way you'll need it later.

12. Driving Shell and Output Shaft Assembly: Pull driving shell and output shaft straight out of case. The driving shell is a big bell shaped thing (rings pretty nice too). Care should be taken to protect machined surfaces during removal.

13. Rear Band and Drum Removal: Pull the drum straight out of the case. Loosen the rear band. I diverted from the manual again on this step. The manual says to thread a 1/4 inch bolt into the actuating lever pivot pin and pull it out with pliers, but I was able to get a small punch on the other side of it and lightly tap it out with a ball peen hammer. Your transmission case should be getting really close to being empty.

14. Overrunning Clutch Removal: The overrunning clutch is basically a hub and a bunch of little springs and rollers that will not allow the rear drum to turn the wrong direction. Remove the hub, springs and rollers and store them. The spring support stays in the transmission housing unless it needs to be replaced.

15. Front Servo Removal: The front servo takes a fair amount of force to get out. There is a plug on the transmission housing that leads into the front servo bore. Remove it so that when you compress the front servo piston you can stick a screwdriver (#2 Phillips works well) in to hold the servo in place while you remove the snap ring that holds it in. (This may take a buddy) For reference the front servo actuates the front band.

16. Rear Servo Removal: Fortunately the rear servo (which actuates the rear band) is much easier to get out. With this one you just compress it and remove the snap ring.

You should now have an empty transmission case with a whole bunch of hopefully labeled transmission parts laying around. Now it's just a matter of rebuilding subassemblies. Actually for the most part you just clean, inspect, and put back together. You replace a fair amount of seals (a lot of which are made out of the same kind of steel as piston rings) and a handfull of clutches. Now would be an appropriate time to set up what ever you are using to clean parts. The next several steps do not necessarily need to take place in any particular order.

17. Accumulator Piston and Spring: Clean the accumulator piston and spring. Inspect for nicks, scratches, etc. Inspect spring for cracks, etc. Replace piston ring with the correct (you'll just have to match it up) piston ring from the rebuild kit. (The rebuild kit comes with absolutely no instructions) The piston ring should rotate freely in it's groove.

18. Rebuilding the Front Clutch: The front clutch contains a piston retainer (big metal shell), a piston, a couple of springs, a spring retainer, some clutch plates and driving discs, a pressure plate, a couple of seals and two snap rings. the first snap ring is a big waved snap ring that holds in the pressure plate, clutch plates, and driving discs. It can be removed with a screwdriver. Then the clutch plates and driving discs should just fall out. After that you have to remove the second snap ring, a spring retainer, all the clutch springs, and the piston, but first you have to compress the clutch springs in order to get the spring retainer off. The manual shows a tool (they even give a Jeep part number) that if you were so inclined you could probably buy. But I decided that there was a better (cheaper) way. So I compressed the spring retainer with three c-clamps. I was then able to remove the snap ring and get the remainder of the front clutch apart. I cleaned everything in the front clutch assembly, replaced the clutch plates and driving discs (they are in the rebuild kit) and reassembled the front clutch. The rubber seals need to be prelubricated with petroleum jelly, and the clutch discs need to be prelubricated with automatic transmission fluid.

18. Rebuilding the Rear Clutch: The rear clutch is much like the front clutch except that insetad of a whole bunch of coil springs behind a spring retainer it has a single piston spring made out of a stapmed piece of spring steel. This makes it a little bit easier to deal with. The whole clutch can be gotten apart without the use of c-clamps. Once again the seals all need to be prelubed with petroleum jelly and the plates and driving discs need to be prelubed with transmission fluid. Assembly is very similar to the front clutch. I never found it necessary to remove the rear clutch from the input shaft.

19. Planetary Gear Assemblies: What you should find here is two planetary gear assemblies. Because they are both fairly similar (they're not so similar that you will get them mixed up) I'll only write one section on them. They contain an aluminum planetary gear carrier that has either three or four steel planetary gears in it. You don't need to remove the planetary gears from the aluminum carrier to clean up the assemblies. The planetary gear assembly goes inside a steel annulus gear. The annulus hear is just a gear with the teeth on the inside. As with just about everything in your transmission clean, inspect, set aside until later. If the the gears don't turn freely you'll probably have to replace the planetary gear assembly as a unit.

20. Play With All The Parts: The rotating mass should be finished now. This is just a suggested step, but it really does take a lot of the black magic out of the way these things really work. Basically set the parts together as they would be inside the transmission and turn the parts against each other. Grab where the bands grab with your hands (the rear band is reverse, the front band is second...pretty neat huh?)

21. Clean The Case: If you haven't already now is an appropriate time to clean the tranny case. I took it to the pressure wash and fed it quarters until I was certain that it was clean. The case is really light without anything in it. That won't last for too long. Reassembly starts very soon.

22. Overrunning Clutch: Clean and inspect the springs and rollers. If the Cam (carrier) is loose in the case tighten up the set screw.

23. Front Servo and Band: The front servo is a little more complicated than one might expect. But again it's dissassemble, clean, inspect, re assemble. It should have two springs, a washer, a piston, a piston rod, a few seals and a snap ring. Apply petroleum jelly to the piston and piston bore of the transmission case. This is the servo with the extremely heavy spring, and the hole for the screwdriver in the side of the case. If you have a buddy (strongly recommended for this step) have him insert the screwdriver into the hole while you compress the servo. A word to the wise: tell your buddy exactly where that screwdriver goes (into your fingers) when he shoves it in as hard as he can. Otherwise you'll definitely need to cuss a blue streak. With the screwdriver holding the servo in it's bore place the retaining snap ring back inside the bore. The screwdriver can now be removed and the plug can be replaced. Inspect the band for excessive wear (the spec is in the manual) and set it aside for later. (I was able to re-use both my bands) Actually you could just use a c-clamp, but it seems that I didn't think of that.

24. Rear Servo and Band: Fortunately this one doesn't have the excessively stiff spring. It also has a few less pieces. Other than the screwdriver trick repeat step 23.

25. Overrunning Clutch Installation: Place the overrunning clutch hub inside the clutch cam. Place all the rollers and springs back around the clutch hub as they were before. It works better to put the rollers in first and then the springs. It will also help to have the case upright.

26. Rear Band and Low-Reverse Drum: Install rear band in case. Thread rear band adjustment screw in enough to hold bad strut in place. Install low-reverse drum in overrunning clutch hub. The whole pivot mechanism and the lever arm all goes in together. Try turning the low reverse hub in both directions. It only turns in one direction...neat huh?

27. Planetary Gear Assembly: Place rear annulus gear over rear planetary gear assembly. Place rear planetary into low reverse drum with rear planetary carrier lugs going into their slots in the low-reverse drum. There are thrust washers in this general vicinity each of which needs to be prelubricated with petroleum jelly before assembly. Install output shaft through the whole rear drum with front planetary assembly.

28. Front and Rear Clutch Assemblies: Once again I didn't like what the manual said so I found another way. I quote from the 1981 Jeep Technical Service Manual(p 2C-65) "Cut a 3 1/2 inch (9 cm) hole in a workbench, in the end of a small oil drum, or a large wooden box strong enough to support the transmission" Well I decided that that was too much work and that I didn't really want to cut a hole in the kitchen table anyway. Had I had the gumption I think a 5 gallon plastic bucket would have worked, but I decided to set the edge of the transmission on the end of the table and just lean on it to support it. I placed all the parts within an arms reach and that worked just fine for me. Prelubricate all thrust washers with petroleum jelly, align front clutch inner splines and position assembly in rear clutch on input shaft. Align rear clutch inner splines, and lower clutch assemblies into the transmission into the case with the input shaft. it may take some coaxing and cajoling to get the front clutch lugs to fully engage in the driving shell.

29. Front Band: Slide the front band over the front clutch assembly. Install the band strut and the servo actuating lever. Tighten band adjusting screw enough to hold the the band, the band strut, and the actuating lever in place.

30. Front Pump: Prelubricate the front pump seal with petroleum jelly. Put seal around front pump, and place the front pump gasket in the transmission case. Slide front pump and reaction shaft assembly over input shaft. Rotate input and output shafts to see if any binding exists. If not bolt the front pupl to the transmission case. Torque to spec.

31. Governor and Park Gear: Install park gear and governor body on output shaft (this may take a few tries because a hole through the shaft has to line up with a hole in the governor body) Install governor valves in the governor and governor valve shaft through the hole that goes through the governor body and the output shaft. There is also a snap ring that holds the governor body on the output shaft.

32. Adapter Housing: Install big roller bearing and seal in adapter housing. Bolt adapter housing to transmission.

33. Pressure Test: With compressed air you can test all the servos and pistons. There is a good diagram in the factory Jeep manual.

34. Valve Body and Accumulator Piston: Install accumulator piston assembly in transmission case bore. Force park lock rod into hole to engage park sprag into park gear. You may have to rotate the output shaft to get park lock rod in. If the neutral safety switch is in the transmission remove it. Place valve body manual lever in 'drive' position and place the valve body into the transmission case. Connect park lock rod to manual shift valve mechanism with e-clip. Torque valve body to spec and check shift shaft for binding. (The shift shaft has a seal that is really hard to put in without bending. I'm still dealing with a small leak in this area) Install lever mechanisms to shift shaft and kickdown shaft.

35. Band Adjustment: Adjust front and rear bands to spec.

36. Oil Pan and Gasket: I could not get my stock stamped steel pan to not leak. (it was badly bent, and AAMCO had used RTV to seal it up which is an automatic transmission don't because ATF eats up RTV) So I purchased a cast aluminum pan that had extra depth, cooling fins, and a drain plug. (It was a TCI unit, but other companies make similar pans) This just barely fit and because the sealing surface is flat on the aluminum pan rather than recessed as it is on the stock pan you have to use the rubber gasket that TCI provides. A cork gasket just leaked. Also you can't get the TCI pan off without pulling the crossmember which you can do with the stock pan. It doens't take away any ground clearance though. I didn't like the TCI filter so I used a NAPA filter which seemed a little bit better built.

37. First Time Running: Once the transmission is installed in the Jeep again it'll take a little bit of running to get it pumped up. (If I had it to do again I'd put the Jeep on jack stands) but you have to start up the engine and at first drive feels just like neutral. With the motor running you just have to cycle it through all the gears and eventually it'll pump up.

38. Enjoy Your Transmission: Wipe off your tools, put them back in your toolbox. Marvel at all the transmission fluid on the driveway. Try and clean the driveway. Drive across the country. Do some 'wheeling. Decide Jeep is getting way too ugly and start making plans for next project.

Mark Wallace
81 Wagoneer
Boston MA