I got this idea from a suggestion by SlantSixDan in one of his old posts.

Many people have heard how troublesome the lockup torque converter is in the 904 transmission. Personally, I have owned many late 70s and early 80s cars and vans equipped with a lockup torque converter and never had a problem. Several years ago, the transmission in my brother's 83 slant powered van failed. Due to a number of different factors, I ended up installing a 1983 lockup 904T into the van but I had to disable the lockup feature. Here is how I did it.

Step one:

Drain the transmission and remove the pan.

Step two:

Remove the shift and kickdown levers from the post on the driver's side of the transmission.

Step three:

Remove the transmission filter

Step four:

Remove the valve body. You could probably do this with the valve body dropped and dangling from the transmission, but I find it much easier to do work not laying on my back on the cold cement in the pool of transmission fluid that invariably occurs no matter how careful you are draining the transmission.

Step five:

Locate the lockup module on the top of the valve body:



Step six:

Remove the two screws holding the cover plate over the fail safe valve and the lockup valve. BE CAREFUL! Both valves are under a moderate amount of pressure from the springs inside the module and will shoot out of their bores unless you keep a finger on the cover plate as you remove it. The failsafe valve will have the actual piston sticking out of the module and the spring for the lockup valve will also be sticking out of the module.



In the picture above, all I have done is remove the cover plate. The spring on the left is the lockup valve spring, the piston on the right is the failsafe valve.

With the transmission off, the lockup piston is supposed to bottom out in its bore. As engine RPMs and line pressure increases, the piston travels up the bore. At a certain point, the piston uncovers a hydraulic passage (presumably, I am no transmission guru) that engages the lockup feature of the torque converter. This is why installing a stiffer spring behind the lockup piston will raise the lockup point of the transmission.

The idea behind this modification is to fashion a spacer or plug that will install between the piston and the cover plate blocking the piston to its position at the bottom of the bore. In this way the transmission will never lock up because the piston will never move and will never uncover the lockup fluid supply passage (for lack of a better term).

Step six:

Remove the spring for the lockup valve. Using a micrometer, measure the inner diameter of the spring. Next, using a plunge micrometer, measure the distance form the face of the lockup module to the crown of the lockup piston as it sits bottomed in the bore.

When I did these measurements, I ended up finding that a length of metal transmission cooling line cut to the right length would slip inside the spring perfectly.



On the right is the lockup spring, on the left is the length of transmission cooling line I cut down to fit. The section of tubing is VERY short.

When cutting the tubing, you want to cut it slightly longer than the distance between the crown of the piston and the face of the module. This is because the edge of the piston is bevelled. The transmission cooling line will slide down the sides of the piston a bit due to the bevel. The best way to do this is to cut the tubing overlength and then trial fit it by placing it in the bore on top of the piston and seeing if the cover plate will seat against the module body. If the plate sticks up at all, you need to shorten the piece of tubing.

I trial fit mine several times and had to file it down using a file before I got it to fit perfectly.

Step seven:

Install your plug. Reassembly is the reverse of disassembly (the parking lock rod can be a pisser). While you are in there, you might consider changing your transmission filter, adjusting the bands, and refilling the tranny with ATF+4.

Your transmission will now never lock up until you remove the piece of tubing from the lockup module. I did this modification on the transmission in my brother's van and it cured a nasty shudder the transmission had between 30-35. We had previously tried installing a stiffer spring (as is recommended by Chrysler to fix lockup shudder) but it didn't cure the shudder. Disabling the lockup feature solved the shuddering problem and the transmission now works great.

The most complex tool you need for this modification is a micrometer. I made my spacer using a hacksaw and a hand held file. You do want to be sure that whatever you use to make the spacer is clean and smooth. You don't want any nicks in the bore or any metal shavings making their way into the transmission.

You still need to use a lockup torque converter with the transmission, but it will effectively become a non-lockup torque converter and the transmission will work like a non-lockup transmission.

Realistically, lockup is a good thing to have. However, I was at my budgets end with this transmission project and needed to get a working transmission installed in the van pronto. This modification solved the problem and got a working rebuilt transmission into the van. If I had more time and money I would have tried to keep the lockup feature. But I didn't have the money or time. C'est la vie.

Good luck!

From whom? The only "trouble" I'd talk about is that some of them are calibrated to lock up at too low a road speed, which can be fixed with a stronger and/or shimmed spring.

Interesting disable info, though.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

I have read many posts on other websites where people would complain long and loud about how the lockup 904/727 transmissions are always failing and should just be replaced with a non-lockup unit. I never said I agreed with those statements, just that I have heard many people complain. Maybe I just have seen more than usual.

I will say that the lockup problem in this transmission was NOT cured by a spring much stiffer than the stock spring, or even the replacement spring Chrysler recommended to fix the lockup problem (my brother ordered three of those and they were all weaker than the spring in the transmission). Like I said above, I don't think the lockup shudder problem is that difficult to fix, but for some reason this particular transmission wasn't cured by a stiffer spring. Disabling the lockup worked great, however.

In general, I advise keeping the lockup functional since it helps get a bit more fuel economy. I certainly wouldn't do this modification to a transmission that worked fine. I only did it to the transmission in my brother's van because a very stiff spring didn't cure the lockup shuddering problem.

Sounds like you might have an ENGINE problem. I usually see weak ignition system, egr or carb/efi stumbles when the torque converter locks up. That's when you have low rpm & hi load- which really stresses things out.

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63 Valiant Wagon
225 - 4 bbl

No EGR, ignition system seems fine. The transmission didn't shudder with four other transmissions (long story, don't ask), so the lockup shudder problem was definitely something internal to this particular transmission.

Thanks Reed, it is duly noted in my 'Big Book Full Of Good Tips That Might Come In Handy One Day''.

Olaf.

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Aspenized

I just rediscovered this post. For purposes of posterity and to avoid misleading anyone who comes behind me and finds this post, I ultimately determined that the shudder in the transmission was caused by worn clutch discs, not by anything to do with the lockup feature of the transmission. So, this post serves more as an answer to the trivia question of "how do you disable the lockup feature in a 904" rather than as an example of any really cure to a problem.

Just as a comment, the most common method of disabling done by transmission shops is to use a ball bearing rather than cutting a piece of tubing. I think it's a 1/4" ball, as I recall. It is placed inside the spring and prevent the valve from moving.