Taking apart any automatic transmission requires considerable care, not only to avoid damaging components but also to avoid losing any of the small bits that tend to fly all over the place if you’re not careful. Many things are under spring pressure and can shoot out suddenly and cause injury. Study the service manual diagrams to see where the springs are and take the appropriate safety precautions. Also many surfaces must be kept smooth and free of scratches to provide a proper seal, so take extra care removing O-rings, gaskets, and seals.
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One of the most common problems with older automatics is that seals and O-rings harden over time and become brittle. This causes morning sickness, where the tranny has problems until it warms up and the seals begin to work better. Sometimes you can minimize this effect with special additives, but a rebuild ultimately is required. After you’ve decided to go that route, it’s best to simply replace certain items preemptively to ensure you don’t have problems later.
Transmission rebuild kits are usually not very expensive and they include all the gaskets, seals, and O-rings, etc., you need, as well as other components like springs. Springs may look good but may have relaxed over time, thus affecting their function. You can also step up to special performance-oriented rebuild kits, which provide faster, firmer shifts, and which may also upgrade the various friction materials to handle higher power levels. These are available for most automatic transmissions used in early Mustangs.
Regardless of the level of service you choose for your transmission, it is critical you keep everything as clean as possible when you put it back together. C4 transmissions, for example, are known to have problems with the governor sticking if dirt gets into it. Clean off the exterior of the tranny before disassembly to help prevent loose dirt from getting inside. After you open it up, you will surely find some areas where particles of worn friction material, etc., have accumulated. This is normal, up to a point. If the filter is virtually clogged with dirt and/or the bottom of the pan has a thick layer of so-called mud, you can reasonably expect to find more problems as you disassemble the rest of the tranny. Be prepared to do more than just replace parts in such a situation. Ideally, you see little accumulation of worn material, and you can simply replace the friction materials, seals, O-rings, etc., without replacing major components.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #1
Our C4 is a 1968 model, so we have the side vent tube setup shown here. Remove the tube carefully if it’s stuck in the transmission housing, to ensure it will still seal properly when it’s put back in. It won’t seal or if it’s bent or scratched. The bolt for the mounting bracket (one of the four for the piston housing) and the similar one on the opposite side of the tranny can be removed now as well. This provides the first real look into two of the many internal pressurized fluid paths of the tranny. The disassembly of these covers is a prime example of where a spring is applying pressure to them, so their bolts must be removed carefully.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #2
The small amount of sediment present in the fluid is not a problem, especially since the color of the fluid is fine and there was no burned odor. The friction materials wear, and the smaller particles will not always be caught by the filter, if they even get that far. The bores in the cover shown at left and in the main housing are of greater significance and showed no signs of abnormal wear or damage. The seals on the piston were also fine. These often harden over time and cause problems, especially when the transmission is not up to temperature. They are replaced anyway as a matter of course during the rebuild. Not shown is the pilot shaft that locates the piston. These rarely have any problems, but if they do show signs of excessive wear or damage, they can be replaced, as is evident from the retaining clip seen on the piston.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #3
Before removing the neutral safety switch, you first must remove the downshift control cable. Make a note of how it was positioned. Use the shift lever to make sure the transmission is in the neutral position and then insert a pin/nail into the alignment hole to keep the switch internals properly lined up for reinstallation. Remove the switch by simply undoing the two mounting bolts. The shift lever assembly is removed later. Note the location of one of the band adjustment bolts to the left of the switch assembly.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #4
Removing the pan confirmed our initial assessment. There was minimal accumulation of sediment in the pan or on the filter, and the inside of the transmission looked fairly clean. There were no signs of debris, broken parts, evidence of abuse, or burned fluid—all good signs. The valve body can come out next, but first it should be noted what type of valve body it is and where the different-size fasteners go. The shape of the “port” just above the filter is one clue to remember, as is the type and color of the spring that’s visible below it. The bolts holding the valve body and filter are different sizes; make a note of how or where the various-size bolts go.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #5
If the pump housing had an O-ring on the sealing face, there would also have been an extra hole in the housing between the two machined surfaces shown. Our car is a 1968, so we have neither the O-ring nor the hole. The minor dirt buildup seen here is typical and of no concern.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #6
Original factory bands were generally cast, like the one shown here. This could cause problems with the lugs that contact the adjusters breaking off under certain conditions. Replacement bands are now often made of steel straps instead, thus eliminating this issue. In either case, the bands should generally be replaced during a rebuild because they all have some wear. If the friction material shows any signs of extreme or uneven wear, then find the cause and resolve the problem before using the tranny again. In many cases, this is a matter of an improper adjustment leading to slippage and wear.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #7
With the front drum assemblies out of the way, inspect the rear components and the forward part of the output shaft. There shouldn’t be any missing rollers or springs missing in the clutch and there shouldn’t be any scratches or grooves on the shaft. The rollers and springs should always be replaced when the tranny is being rebuilt, as should the thrust washer underneath this sprag assembly.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #8
The governor support housing on the rear of the main case should always be removed and inspected for any signs of excess wear or damage on the inner bore. Ours still had the cross-hatch pattern from the factory hone. The “stripes” are from the sealing rings on the output shaft and are normal as long as they are not too deep. You should be able to run your finger nail across them without even feeling them. This housing just gets a little time in the parts washer before it is dried and ready to be put back on.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #9
While it isn’t always mandatory that you replace thrust washers, such as this one, it generally is a good idea because they aren’t expensive. The main reason for not doing so is either that the particular one you need isn’t readily available or that the replacement doesn’t fit as well as the original. If it’s best to reuse an original thrust washer, you must verify it’s within specification and then lightly rub out any minor scratches with fine emery cloth. Any significant scratches, grooves, or embedded dirt means replacement. Make sure any washers used are clean and lubed with a thicker assembly-type lube, rather than just transmission fluid or oil, when you reassemble them into the transmission.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #10
Replace the bushing and the seal in the tailshaft housing because it tends to be one of the more likely sites for trouble. A loose bushing causes the seal to wear unevenly (and thus leak), and can also cause a very noticeable driveline vibration. Both are relatively easy to replace; most rebuild kits include them. Make sure the notches in the bushing face to the rear (toward the driveshaft) and line up with at least one of the passages in the housing. The seal should only be driven in with evenly applied pressure via the correct seal installer, socket, or similar. Apply some RTV or a similar sealer to the seal before it’s driven in further to prevent leaks.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #11
Check several items in the planetary gear housing before using it again. First, check the condition of the thrust bearing. If it is available, it is best to replace it. Otherwise, it can be cleaned and lightly polished to clear up any minor imperfections. The planetary housing itself needs to be checked for any excessive wear at any of the rubbing surfaces, plus each gear needs to be closely examined for imperfections. The play of each gear in the lateral and radial directions must be measured to ensure they are within spec. While these parts generally do not exhibit problems, many are still possible and can have major consequences if they are not resolved before reassembly.
The friction plates are replaced as a matter of course when the transmission is rebuilt. These looked exceptionally good, but they still did show some wear. The replacements had more material, plus it was of a newer, superior specification. Never mix friction plates; only replace and use them as a matched set from the same manufacturer. The spacer plates or “steels” as they are sometimes called generally don’t need replacing unless they are excessively worn or damaged. They can normally be lightly buffed with emery cloth or a ScotchBrite disc to remove any minor flaws. Be sure to fully wipe and lubricate them prior to installation.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #12
The drums and their internal components rarely need replacement, but it is still advisable to replace the springs simply as a precaution. The inner bushings are getting harder to source in some cases, and thus may force the reuse of the original part if it is not worn or damaged. If you can find one, it’s much less expensive that having one custom made. A special fixture is often used to compress the springs while the retaining clip is removed. Carefully avoid sending parts flying when removing the retaining ring; use the tool very slowly. Generally, the drums will simply be cleaned and reused if they are okay.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #13
In some cases, there will be an O-ring or other type of seal inside the drum. These must always be replaced. Use a pick or similar tool to carefully remove the item as to not scratch or otherwise damage the sealing surfaces or the retaining groove. Note that while it appears there is wear on the inner walls of the drum, these are actually just surface rub/polish marks that are not deep enough to be of any concern.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #14
We examined our transmission’s pump housing/stator surfaces and found no signs of excess wear or damage that would prevent its reuse. This is a good indicator that the bushings they contact are also probably in good shape. If these machined surfaces had deeper grooves, scratches, or pitting, we would have repaired or replaced it. The sealing rings are also in good shape and don’t even need to be replaced except as a precaution. The whole assembly can just be put in the parts washer for a rinse and then it can be wiped and dried until it’s ready to go back in. However, that’s assuming we don’t find any other issues. While it is okay to lightly buff these surfaces with emery cloth or a fine abrasive pad, be careful to not damage the sealing rings either directly or due to residue getting down into their grooves. Unless the imperfections are more than just very superficial (similar to what is shown) it’s probably best to not use any abrasive material to try to smooth things.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #15
When taking the pump assembly apart, it is important to note several subtle aspects of how parts fit together. One of the more critical things is that the inner gear has a beveled edge on the bore that must face the housing when the parts are reassembled. Each gear should also be inspected for any imperfections or other signs that there may be a problem with the contact pattern. If excessive dirt and/or debris were in the fluid, we would surely have seen the effects of it here because the pump gears would have pits or other problems. Our tranny had no signs of any surface imperfections on the gears or on the sliding surfaces of the housing/ stator. Even the front shaft seal looked good enough to reuse but this is also one of those items you should just automatically replace.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #16
The outer pump gear is often directional. One side of the gear may be marked with a dot as shown. When reassembling the pump/stator assembly, this side of the gear must face the inside of the housing (same direction as the bevel on the inner gear). All of the same criteria for the inner gear apply to the outer gear. Therefore, the gear and where it makes contact with the housing must be free of damage or marring. Fortunately, these gears and the pump assembly in general rarely exhibit any problems unless they have been exposed to dirt or debris. If the filter is clean, these will usually be fine too.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #17
The pump/stator housing surfaces that contact the gears must also be inspected to ensure they are not excessively worn, pitted, or otherwise damaged. In addition to replacing the seal as indicated previously, the bushing must always be replaced as well. In the case of a C4 transmission like ours, you should use a C6 bushing because it is slightly larger and will provide more support. Whichever bushing you use, however, be sure to install it so the hole on the edge is away from the seal and so it lines up with the left (furthest, counterclockwise) machined hole in the housing. As you can see, that is not the case here. When we put in the new bushing, we will make sure it properly lines up with the hole.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #18
Special tools, such as this fixture, can facilitate the removal of the drum assemblies. It compresses the springloaded plate and holds it down so the retaining clip can be removed. The fixture has a ratchet mechanism and is adjustable for various size components. With the retaining clip removed, the lever must be slowly lifted to relieve pressure so the springs don’t fly all over. Trying to do this without the proper tools can damage the components and some of the smaller parts can be easily lost.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #19
Disassembly of the drums varies with the drum you’re working on, as well as the transmission you’re dealing with. For example, 1969 and earlier transmissions will often use multiple smaller springs for the drum shown, while 1970 and later models are more likely to have a single coil spring. As has been the case this far, the O-ring and friction plates should automatically get replaced, but the steels may be reused provided they are not worn or damaged. While it’s okay to lightly buff these steels with an emery cloth or pad (only if necessary to remove slight flaws) before reinstalling them it is not generally advisable to do so with components made of aluminum due to the potential for damage.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #20
The outer surfaces of the drums must be inspected as well because the friction bands make contact with surfaces in some cases. Make sure there is no excess wear or damage. It’s also advisable to check any splines or gear teeth to ensure the same holds true for these. The splines on this part show some slight wear, but it’s little more than a mild, surface scuffing. Had they been deeper, it could mean replacement.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #21
When disassembling the valve body, it is especially important to be aware of the possibility that small parts (check balls, for example) can come out as the parts are separated. The best approach is to remove the necessary bolts and then flip the assembly over so that the smaller casting (on the right) is on the bottom when the parts are split. This will keep the check balls in place rather than allow them to fall out. Note the location of each check ball because this varies by application. And there may be differences in the check balls themselves, so the balls need to be matched to their specific locations as well. Some sediment in the fluid passages is normal and will usually come off in the parts washer.
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #22
One of the distinctions of the various valve bodies you may come across is the configuration of the linkage for the manual control valve. The 1969-and-earlier versions like the one shown here have the detents on the valve body itself. The 1970-and-later versions tend to have the detents on the case. This affects the assembly procedure to the extent the linkage must be properly lined up when they are reassembled. The factory service manual should have the proper procedure for your particular application.
When disassembling the valve body, it’s most important to recognize that parts are under spring pressure. Virtually all of the small valves, plungers, etc. will fly out of the valve body and possibly get lost or damaged, unless you prevent it. It’s extremely critical that you note each part’s location; improper assembly almost always has very negative consequences. If any of the parts are a bit sticky as they come out, then the plunger and/or the bore should be lightly buffed with an emery cloth or pad to remove any imperfections. Any residual abrasive material must be fully flushed out before reassembly. Most rebuild kits will also include replacement springs. High-performance rebuild kits may provide different color springs with different pressure levels to change the shift characteristics. Pay particular attention to the condition of the aluminum components; they are more prone to wear and/or pitting.
I show the main steps involved in taking an automatic transmission apart, in this case the C4 that came with our 1968 J-code GT, so it can be inspected and evaluated to determine what needs to be done. I only hit some high points and thus recommend you use the factory service manual for the detailed step-by step procedure.
Our tranny was in really good shape, so I also won’t show any major problems being resolved or any modifications. only show the highlights of a straight factory rebuild, but I include some extra information on special things to consider that are not covered by the factory service manual. A very experienced transmission shop (Big 4 Transmissions in Paramount, California) rebuilt our transmission. This is the type of shop you should use if you decide to farm the job out.
The factory manual can only help so much, and diagrams are not always as clear as they need to be. There were numerous production variations, so it’s not always possible to find the correct information for your specific transmission. If you are doing the work yourself, be sure to document how everything came apart—photographing the transmission at each critical step is very important, if not essential. You also need to properly tag and bag parts as you go, so all the components are clearly identified and organized should you find you are unsure of how to put something back together.
Since ours was a basic rebuild, I concentrate mainly on replacing worn friction materials, seals, O-rings, gaskets, and the like. I do, however, point out things to look for regarding the major components, which may indicate the need for more extensive repairs and/or the replacement of those parts. If you get to that point, and it’s more than simply replacing a part, you may be better off having the work done professionally. When it’s just a replacement, there is still a need for a certain amount of expertise. Components may have subtle differences or require specific installation procedures that may not be readily apparent. Something as simple as installing a bushing without properly lining up a slot or putting a gear in upside down can cause big problems.
Written by Frank Bohanan and Republished with Permission of CarTech Inc