Date: Wed, 3 Sep 1997 13:26:50 -0400 (EDT)
From: Mark Wallace 
To: Big Jeep List 

Subject: How To Rebuild your Automatic Transmission Part I.

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. Mark Wallace 81 Wagoneer Boston MA
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