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
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #13
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #14
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MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #16
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #17
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
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MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #21
MUSTANG RESTORATION: AUTOMATIC TRANSMISSION DISASSEMBLY- STEP #22
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