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This is a pretty long post but I feel it is informative and of a great benefit to those with early Mustangs that are tired transmission fluid stains on their driveway. Take from this what you will but it worked for me. If you feel unqualified to perform this repair / modification then print this off and hand it to your transmission shop along with the keys to your car. It shouldn’t be any more expensive than the cost to replace the gear selector shaft / throttle valve shaft seals without the modification.

Well, I have finally, after 9 months, reached a point in my restoration where I could install all the vital fluids, fire it up and check for leaks. I am trying to resurrect the existing engine and transmission without a complete overhaul. I replaced all the easy seals and gaskets during the process but ended up with two leaks anyway. One was a core plug in the back of the right cylinder head which I made short work of and the other was at the Gear Selector (GS) shaft / Throttle Valve (TV) shaft area. I did a little home work and found several forums (not all Mustangs) that discussed this leak on C4 transmissions. While I found no real answers to this common problem, it did help in the diagnosis of my car. This is the only C4 I have ever worked on other than fluid changes but I would guess they all have the same problem to one extent or another that mine did. I have included diagrams and photos for your viewing pleasure as well as a discussion of the mechanism.

FROM THE FORUMS:
This is what I took from the discussions and I would like to dispel a few myths. I am not saying the following solutions are wrong as long as they worked to the posters satisfaction. They just didn’t seem like reasonable solutions after my diagnosis of the problem.

“The TV shaft nut is found to be loose.” Mine was, and if yours is, and the shaft binds when you tighten the nut, you will need to perform the modification I have documented below.

“Add a washer under the nut to seal the o-ring.” This will not work because the o-ring bore has a depth of .129” and the o-ring is only .105” thick. The washer will contact the GS shaft before it compresses the o-ring. The o-ring is not designed to be compressed anyway. When properly installed, it is captured but not compressed.

Transmission is over full. That very well may be true. I checked my reproduction dip stick against the original and the tubes were equal length but the stick was about ¾” too short. So mine was no doubt over full but still this is not the true cause of this leak. When the engine is shut off, the fluid drains back out of the converter and the fluid level raises in the pan and likely above the level of the GS/TV shafts. I reconditioned my original dip stick for a more accurate fluid level check.

“It only leaks after I turn off the engine”. Again probably true for the reasons stated above. But I have spent considerable time on the underbody restoration of my car and I will not accept any leaks whatsoever. Trans fluid burned on exhaust pipes is just nasty.

“You have to remove the valve body to replace the GS shaft seal.” Sad but true. However, it is no big deal. It has like 7 bolts and no springs or balls are going to jump out at you. If you can change the filter and fluid you can perform this modification. Getting the two shafts in and out is a little tricky in that one installs from the outside and one installs from the inside simultaneously. I did mine with the full exhaust and everything else still in the car so access is really not an issue.

“I put a pan under mine to catch the fluid when I shut it off”. PLEASE!

THE SOLUTION:

DISCLAIMER:
Perform this modification at your own risk! All information provided below was taken from my observations on my car. No other cars were studied and this problem may not be common across all C4 transmissions. (But I bet it is) Please consult your physician if you begin to feel faint, dizzy nauseous or drowsy while reading this thread.

I will cut directly to the chase on the solution and then explain later. THE GEAR SELECTOR SHAFT IS TOO LONG FOR THE THROTTLE VALVE SHAFT TO BE INSTALLED CORRECTLY. I don’t know if it was poorly engineered or if it was a quality control issue but it appears that a number of these (most) got out of the factory with mismatched or poorly machined parts. The solution is to simply grind away the threads on the GS shaft that extend past the 7/8” nut that holds it in the case. I actually had to grind part of the nut to get the proper clearance for the TV shaft. Study the attached diagram and you will see the unmodified shaft on top and the modified shaft on bottom. On the left side you will see the flaw in the o-ring bore that causes the leak. If the TV shaft does not extend far enough through the GS shaft the o-ring will try to seat on the “double D” of flat portions of the shaft creating a path for fluid to escape. On the right I have shown where the threads are too long on the top diagram and after removal on bottom diagram. This changes the orientation of the two shafts to each other. I assure you the engineers never intended for this mechanism to operate the way I have pictured it on top which is the way mine came apart. Please understand that I did not come to this conclusion lightly. One would think that internal transmission parts are machined properly from the factory but these are not. There just not. I very carefully studied the mechanism, and measured everything to be sure I wasn’t missing something and that this modification would have no ill effects on the operation of the transmission. I studied each component of the mechanism to determine its intended design, position, and operation. I would like to walk you through a discussion of my findings. You have already read my conclusions so if you would like you can stop here and just fix your leak but I would like to prove up my solution with a body of evidence.

PHOTOS 1,2,3
The first photo shows the external end of the TV shaft. The end is machined to a “Double D” shape to secure the external lever. The lever connects to the throttle linkage by an adjustable cable. The red arrow points to a raised portion of the shaft designed to provide interference fit so the lever actually wedges onto the shaft. The lever will index and freely slide down the treaded part of the shaft and half way down the smooth part of the shaft where it begins to wedge. The nut will push the lever the rest of the way down eliminating any free play in the lever regardless of the nut. The 2nd shot shows the orientation of the lever to the shaft the way it came out of my car. The external lever was not fully seated on the shaft and the nut was loose. The lever would still rotate the shaft due to the “double D” shape and it wouldn’t fall off the car because of the nut. At least not until the nut backed off. So away we go with the kick down working, transmission leak and all. The 3rd photo shows the lever fully and properly seated onto the shaft the way it was intended to be installed once in the car. I know, the lever is backwards but it still demonstrates my point. The point is that the engineer intended for this lever to swedge and fully seat onto the shaft and the retaining nut to be torqued down. Not left loose so the shaft could still rotate in the bore. And the loose nut has nothing to do with the leak by the way.

PHOTOS 4,5,6
The 4th photo shows the distance between the properly installed external lever and the fixed internal lever on the TV shaft. The distance is 3.035”. Because the internal lever is welded to the shaft, and the other end is machined to accept the external lever, there is no way to alter this part. It can’t be made longer so any changes must come from the GS shaft through which the TV shaft rotates. The 5th photo shows the length of the GS shaft and it measures 3.156”. This shaft needs to be shortened at least .121” to allow the TV shaft external lever to fully seat without binding on the GS shaft. This is why you may have found the nut loose on your TV lever. If tightened, the shaft will bind and not rotate. The 6th shot shows the unmodified GS shaft and the orientation of the TV shaft at the o-ring bore. You can see that no part of the round TV shaft is protruding into the o-ring bore. Only the “Double D” is showing and that is what causes the leak. The o-ring can still seat against the GS shaft bore like this but will not seat on the flat sides of the TV shaft and this is the root cause of this troublesome leak.

PHOTOS 7,8
These just show the o-ring bore which is .129” deep and the 8th shows the o-ring at .105” thick. This proves that a big flat washer is not the answer. Even if you tried to double stack two o-rings and a flat washer it would still leak through the threads of the shaft.

PHOTOS 9,10,11
After realizing what needed to be done to fix the leak, I wanted to be sure the internal TV lever would still contact the TV valve plunger in the valve body. The 9th photo shows the GS/TV assembly as it is oriented in the valve body. The lower lever is the gear selector lever and modifying the shaft will not change this levers orientation to the valve body. The gear selector lever is captured by (under) the large 7/8” nut and is indexed to the shaft by another “Double D” machining operation on the GS shaft. Only the TV lever will change positions. So the question that remains is what effects will this have. The 10th shot shows the wear pattern on the throttle valve plunger in the valve body and the wear is to the inboard side of the valve (red arrow). The 11th shot is of the TV shaft internal lever (fuzzy I know) and the wear pattern is to the outboard side of the lever. Not that it really matters but this lever has been riding on the valve off center to the inboard side since it was new. This told me I can easily move the lever outboard and still contact the valve properly. I know it sounds scary to go grinding on an internal transmission part. But understand that the .121” the lever needs to move outboard to make the external o-ring seat properly is only just shy of 1/8”. If it makes you feel better you can bend the lever slightly to center up the contact point on the plunger but it probably won’t need it. Mine didn’t. Understand that this lever only moves just slightly and only when you push the gas pedal all the way to the floor to hit passing gear. The rest of the time it sits idle waiting for its moment in the sun.

PHOTOS 12,13,14,15
The 12th photo shows the modified GS shaft after grinding. I used a simple die grinder on it for an overall length of 3.028”. This gave me around .007” clearance for the TV shaft to freely rotate inside of it with the external lever torqued to specs while still capturing the o-ring inside the bore. The 13th shot shows the new orientation of the TV shaft to the GS shaft at the o-ring bore. You can see that the round part of the shaft is now visible. The 14th shot shows the assembly with the o-ring installed and the o-ring is now in full contact with the bore and the shaft keeping all the fluid inside the transmission where it belongs. The 15th shot is of the whole mechanism ready to go back in the car but with the diagram that makes it the 16th shot so I couldn't upload it.

Oh, one more thing. I saw no evidence that the Gear Selector Shaft seal had leaked although I replaced it while I was there. The new seal is a nice rubber coated seal that went in very smoothly from the outside of the case. Polish all of the areas of the shafts where the seals ride with 400 grit sandpaper, clean and lube with white grease before installation. Your leak should now be fixed.

I hope this may help someone.

Steve
 

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CR Info

Excellent info - this would have been a great help to me once upon a time!

Thanks for sharing.
 

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Discussion Starter #3
Thanks Tripleblack,

I was a little surprised I didn't get more replys on this one. I googled this leak and found a bunch of threads on this and other sites about this problem. It worked for me, mabey it will help others.

Thanks again,

Steve
 

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Too Good

Thanks Tripleblack,

I was a little surprised I didn't get more replys on this one. I googled this leak and found a bunch of threads on this and other sites about this problem. It worked for me, mabey it will help others.

Thanks again,

Steve
LOL, your job was TOO good!

It was apparent that the documentation and pics really showed the job - it looked like a piece from a magazine.

LOTS of people read it, and I suspect more than a few used it to help them out of a jam.

My next project is liable to be an early Mustang resto-mod, with an auto tranny (left knee is packing it in, time to sell the 5 speed).
 

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Just Noticed

Just noticed you are the forums moderator.

Pleasure to make your acquaintance. Has to be a thankless job.

LOL, your job was TOO good!
Good enough for a sticky?

Steve
 

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Classic Tech

Just noticed you are the forums moderator.

Pleasure to make your acquaintance. Has to be a thankless job.



Good enough for a sticky?

Steve
OK, thought it over, stuck it. LOTS of folks can use this to help with C4 projects.
 

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Discussion Starter #9
Thanks to all

Thanks to all!

I have never had a sticky before. I was sort of kidding when I suggested it. I did work pretty hard at documenting my findings so it could help someone else down the road and now it will be easy for them to find.

Thanks again,

Steve
 

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Stuck

We don't get many articles this detailed, with documentation and pics. I have mentioned this sort of advice to people many times (jot down you steps and mis-steps, take pics, do the research), but its rare to see it presented in a neat package.

From my point of view, helping each other with our cars is the primary function of AFM. I'm always amazed how many different people with good information are willing to share.
 

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Discussion Starter #11
I'm always amazed how many different people with good information are willing to share.
I worked on cars proffessionally for a long time and then I made a career change and was out of it for a long time. Only in the last 5 years have I devoted my attention to auto restoration which I never did much of as a pro mechanic. After work the last thing I wanted to do was mess with cars ya know. The thing I am most impressed with now is how easy it is to obtain information over the internet. Back in the day you had magazines and stuff but most performace information was sort of black magic if you know what I mean. I have never worked on many Fords and now if I need to know something, I just type it in at AFM before work and by the time I get home I at least some kind of an answer.

How great is that!

Steve
 

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Got your back

It IS great.

Every time I try to help out someone (often some kid just learning) with advice, somewhere in the back of my mind I'm hoping that when the day comes and I'm banging my head against a wall with a problem, someone will take time out of their busy day and help me out.

Bread on the waters - karma - whatever you want to call it.

It really DOES come back, and often 10 fold.
 

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My little darling seems to be moist in this area and provides a little puddle overnight after a small amount of driving.

Before I look at the operation you have so well described, I am thinking my leak could originate elsewhere:

There is a steel (approx 5mm dia') pipe directly above the gear shift that I guess is the overflow. It seems to be in a good place to drip on the GS shaft and various brackets and is moist around the end.

Should this "bleed or seep" or is it supposed to go somewhere else.

Thanks in advance.

Steve
 

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Discussion Starter #15
Diagnostic

Steve, I am far from an expert on these transmissions but that little pipe is bound to be the vent for the transmission case. It may have a baffle missing on the end (not sure if they even have a baffle) or the transmission may be running a little hot and spewing a little fluid there. A few drops or a little moisture there would not be abnormal, but enough to make a puddle every time you drive it would. An over full condition could cause that so check the fluid level hot first.

My leak was a constant drip and I had not (have not) yet driven the car. As a diagnostic, you can clean the entire area with spray carb cleaner and dry it with paper towels. Then stuff a bunch of paper towels around the vent to catch any fluid that may be coming from there (although the vent shouldn’t leak with the engine off anyway). Let it sit over night or a couple of days and see where the drop forms. If it is around the GS/TV lever arms you likely have the same leak I had. First check to see if the TV shaft nut is loose, (1/2” wrench) and if it is, tighten it and see if the TV lever will still rotate. If it won’t rotate, proceed as documented above.

Good luck,

Steve
 

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Awesome post! Thanks for explaining this as I have this exact problem. If everyone posted like this there would not be any questions! :hihi: Thanks!
 

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An overfull condition will cause the fluid to foam and expand. The lubricating quality of the fluid will diminish because of the air that is in suspension in the fluid. Trans. slippage will also cause the fluid to overheat and expand.

The expansion will force fluid out of the vent. A properly functioning trans. shouldn't be forcing fluid from the vent. Some of the newer C4s don't have tubing for a vent, but just a small vent cap.

A dipstick that is for another model or incorrectly marked as well as a poor quality fluid that is lacking the antifoaming additives needed could be culprits.

I am fortunate that I had the experience to eliminate the the leak prone kickdown shaft leak on my own, but I want to say thanks, tripleblack, for your efforts in helping others with this chronic leakage problem. It's really nice to have a 40 plus year old car that doesn't leave its mark on the pavement. Especially when I park on one of my friend's driveways.
 

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Good stuff, This is what's happening to mine. Thanks for the solution.
 

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This definitely deserves a sticky.:bigthumbsup
I TOTALLY DISAGREE.

This thread was referred to in another forum. The solution above APPALLED me. Here is what I wrote in that other forum:

I'm in the same boat... looking for the seal but also learned a lot from the following post:
C4 Trans Gear Selector Shaft/Throttle Valve Shaft Leak SOLUTION! - Ford Mustang Forums

My car has the leak and I find an Oring in there but I have questions....

I went and looked at that "solution". What I found was MISMATCHED manual control and throttle levers. Ford made TWO different lengths of both levers 5/32" difference in length. His problem was he had the SHORT throttle lever matched with the LONG manual control lever.

By grinding off the threads inside, he now has MISMATCHED contact area for the throttle valve, which may or may not be a problem.

My opinion of that "solution" is that it sucks. By obtaining the CORRECTLY matched parts he could have solved his problem without cobbling together a "solution" like that.
Finding the correctly matched parts is no problem whatsoever. There are still lots of C4s still floating around. And I have 2 ten-gallon buckets full of manual control & throttle levers myself. That is a LOT of levers.
 

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Discussion Starter #20
Hi mmerlinn .

My name is Steve. I use my real name in my signature because I have nothing to hide. I felt that I should respond to your post not in an attempt to defend myself but rather clarify a few things for those that you have so graciously enlightened.

Let’s look at information gathering in general. Thank you for the information on the two different length levers. I was a dealership mechanic for 20 years at one of those "C" places. I seldom worked on C4's except for friends. There are all sorts of little nuances like this across all lines and this type of info can be hard to find. For example, I never found any information at all about the two different lengths or what the symptoms of such would be. I believe that in my original post I even documented what others had posted as solutions. I did not however tell them that they “suck” or that their solutions were “cobbled together”. Were you not appalled at their solutions? I guess I just overlooked your solution in my quest to solve this problem.

You must be a transmission man at some shop somewhere if you have “ten gallon buckets” of these lying around. I am sure you are a very skilled technician but you may want to work on your reading comprehension skills. In my post I said:

“Take from this what you will but it worked for me.”

“This is the only C4 I have ever worked on other than fluid changes but I would guess they all have the same problem to one extent or another that mine did.”

“I don’t know if it was poorly engineered or if it was a quality control issue but it appears that a number of these (most) got out of the factory with mismatched or poorly machined parts.”

“I am not saying the following solutions are wrong as long as they worked to the posters satisfaction. They just didn’t seem like reasonable solutions after my diagnosis of the problem.”

“DISCLAIMER:
Perform this modification at your own risk! All information provided below was taken from my observations on my car. No other cars were studied and this problem may not be common across all C4 transmissions. (But I bet it is) Please consult your physician if you begin to feel faint, dizzy nauseous or drowsy while reading this thread.”

“PHOTOS 9,10,11
After realizing what needed to be done to fix the leak, I wanted to be sure the internal TV lever would still contact the TV valve plunger in the valve body. (read the rest and look at the photos and you will see that it does).”


In closing mmerlinn I would just like to say that I posted my information for the sole purpose of helping my fellow enthusiast. Not to thumb my nose at there diligent efforts to merely render their cars leak free. I spend money every year to have a picture site and I have a corner of it tucked neatly away so the photos in my post will be available to anyone that wants to view them. I do this to help people and I am glad that you found this so I could help you feel better about your vast knowledge of this subject and my apparent stupidity. If you wouldn’t mind posting some part numbers for the levers or maybe some casting numbers of the cases that contain them I see no reason anyone should have a problem with this leak again.

And by the way, please link this to those “other sites”. I am sure they would like to see you shoot me down in flames directly.

Hope this helps,

Steve
 
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