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What is a Mustang J Mod?

Written By: Connor MC

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Simply put, the J Mod is a simple modification done to the valve body of the 4R70W that greatly improves its functionality. Performing a J Mod will result in quicker shifting, faster and firmer engagement, even increased service life. Keep reading to find out exactly how it works, and if you can do it yourself.

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Origins of The J-Mod

Debuting in 1996 and running all the way until 2004, automatic transmission Mustangs came equipped with Ford’s 4R70W - a 4-speed, overdrive enabled transmission. Factory automatics are always designed with some comfort in mind, thereby shifts inside the transmission are slipped so they are smoother, but take longer to perform and generate more heat in the process. The 4R70W worked very well and could handle a good amount of power, but as always someone saw room for improvement. This someone, interestingly enough, was actually a member of the initial 4R70W design team. In order to fix the slow and slushy shifts of the 4R70W, Ford engineer Jerry Wrobleski came up what is now widely known as the ‘J mod’ (J for Jerry). Simply put, the J Mod is a simple modification done to the valve body of the 4R70W that greatly improves its functionality.

The J Mod For Mustang 4R70W Transmissions, Explained

As mentioned, the J Mod is a simple modification of the 4R70W’s stock transmission valve body that gives big results. It involves removing the accumulator springs on the 1-2 shift as well as the 2-3 shift. Furthermore, several holes on the separator plate need to be enlarged. The effect of all of this is the transmission fluid gets to where it needs to go faster, and in doing so, speeds up everything else around it. It really is that simple.


Can I do this modification at Home?

The J Mod does not have very many steps in the sense that you need to take apart a million things, nor does it require any special tools that the regular DIY-er wouldn’t already have. All you need to do is drain the transmission, drop the lower transmission pan and remove the valve body. This in and of itself is not too difficult. Once the valve body is out, then you follow the steps and templates to remove the appropriate springs and drill the corresponding holes in the separator plate. Again, not too difficult. However, this is a job that requires precision. Using a drill is easy, but it is imperative that you do so accurately. Drilling too large or in the wrong spot could seriously hamper performance, if not stop operation altogether. Just make sure to proceed with caution, and read the instructions several times!

Speaking of instructions, a great repository (scratch that, the BEST repository) can be found on the Thunderbird and Cougar Club of America (TCCOA) website, under their transmission article section. It even features articles and comments by the great J-Man himself.

If you don’t feel confident doing this yourself, don’t worry. Any garage ought to be able to do it, or as mentioned, Ford dealers used to do it themselves back in the day. It should only take them an hour two, so it shouldn’t be too expensive. 


What about replacing the valve body with an aftermarket one?

Good argument, and one that frequently comes up on forums across the web. In this instance, the J Mod is actually better in all regards over comparable aftermarket valve bodies. You’ll get an increased shift performance AND increased service life. Usually the latter declines as the former increases. Not so with the J Mod. For  a stock 4R70W, you simply can’t go wrong with a J Mod. Still have doubts? Just recall that Ford is supportive of this mod. You’ll also spend less money. 


No Catches, Seriously?

Yup, that’s right. The J Mod is seriously as good as it sounds. In fact, the J mod is comparable to those "get a 6-pack in 3 days, without leaving your TV chair" type commercials, except that it actually works.

Too good to be true? Sounds like it, but this is an instance where it is exactly as advertised. 


Fitment includes: 1994, 1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, GT, V6, Mach1, Bullitt