Missing gears on my AOD!

[Alexei]

Dear friends,

Yesterday when cruising, I lost my 3rd and Overdrive gears! Any thoughts of why, could this have happen?

Thanks!

 

[Sheriff41]

Did you check/set your throttle valve (TV) pressure when you installed the AOD?

 

[Alexei]

Yes, but, I will check again. What pressure do you suggest?

 

[Sheriff41]

I don't recall off the top of my head, but I know @mrgmhale posted a helpful video on setting the pressure.

 

[Alexei]

Perfect! Thanks a lot!

 

[Precision Transmission]

Sounds like the typical 3-4 clutch failure. The other possibility is the drive hub in the torque convertor for the inner input shaft has striped out. Used to see both quite frequently. Another possibility, but not common in a broken inner input shaft.

A way to help diagnose is to run down a hill until you are at a speed that allows the valve body to shift into OD. In OD, the fwd clutch disengages, and all input goes through the inner input shaft and the 3-4 clutch. With the fwd clutch disengaged, if there is no input though the inner input shaft and/or the 3-4 clutch, the trans will feel as thought it's in neutral.

 

[Alexei]

Sounds like the typical 3-4 clutch failure. The other possibility is the drive hub in the torque convertor for the inner input shaft has striped out. Used to see both quite frequently. Another possibility, but not common in a broken inner input shaft.

A way to help diagnose is to run down a hill until you are at a speed that allows the valve body to shift into OD. In OD, the fwd clutch disengages, and all input goes through the inner input shaft and the 3-4 clutch. With the fwd clutch disengaged, if there is no input though the inner input shaft and/or the 3-4 clutch, the trans will feel as thought it's in neutral.

Thanks!

If the 3-4 clutch is gone, should I rebuild the all the transmition?

 

[Bobby]

Thanks!

If the 3-4 clutch is gone, should I rebuild the all the transmition?

Yes but don’t buy cheap parts. Get good quality. Minimum pressure should be 35psi but if that is what you set it before, then you should set it at 40 psi.

 

[Fabrice]

I second @Alexei here, my 91 T-bird had same all of a sudden and it was simply the friction plates gone for these clutches.
At the time I had a second trans and swapped them as it was my daily and needed be back on the road right away.

But for my 73, I've rebuild this trans and it's pretty straight forward, Most work went into beautifying the dirty corroded body that I started with. I have ordered by Ken at Badshoe.com, why, well because you need for 2 clutches a compressor and these happen to be quite pricy not too mention the very specific piston gaskets with L shaped sides that you cannot insert unless compressed. Ken offers a very simple PVC made+hardware compressor and most important some moulds that receive these gaskets. The trick being to put them in freezer for a few hours prior install and they fit right in. They return to their soft tight position in matter of minutes) The tools are on Ken's site offered at a very affordable price. Same goes to his quality kits.
May you want to see, https://7173mustangs.com/threads/fabrices-429cj-71-project.30729/page-29
I have detailed the rebuild there...

As about the pressure and complicated tests. I don't know why this legend persists... The valve body is designed in a mechanical way that it needs to be at a certain position (assuming here a functioning trans with oil level as it should be) to obtain the ideal pressure. To do this, you need to simply have the tension cable at rest with nearly no play. You can see on mustangs, t-birds etc, that they have a special "lock" on the cable, exactly made for this. So a non restrained cable must be at throttle linkage with zero tension. That's it, no need to check because by design it is the ideal pressure defined by the volume of oil entering the valve body via this small valve with a properly functioning pump. If you give a tad tension, the trans will shift early, you add lack, it will shift later on. The equilibrium position is highly recommended. You of course can check the pressure, but that's pointless as it's designed by the amount of oil that can go thru that orifice. Reading the pressure and adjusting it will give you the exact position of the cable with no lack, no tension. If you'de find another pressure level, this means there is not the right amount of oil or the trans is ready for a revision, which in your case, you know already... its is not like you need to tune the trans for this. It is by design always at the ideal pressure IF the trans is healthy and the cable is set in this rest position at the throttle. Again, not saying measuring is wrong, but it's not relevant for the proper install of this trans as reading vacuum to tune a carb for instance would be.

Another detail worth mention for this trans (also detailed in my post) is if you go for a lock converter and have a heavy right foot. Is that you kinda must replace the inner shaft as it will be too weak. And for the non lock version like the original, if the trans is pre 91' ( where the clutches bodies are made of cast pot metal vs iron) if the car was pushed, it is likely that the shaft has been hitting the casing and it then can go too far in, which causes it to block one passage of oil just 1/4 inch away. For this and as prevention, know that there are rings available that you can install by just machine a tiny groove on the shaft. The ring will then never allow the shaft to do damage anymore. Another detail is a better snap ring for the reverse clutch(that the last one you see before install the pump back)), that can actually come loose. That is also available and a should be part of the rebuild. I have all the parts refs in my post may you'd want to rebuild it yourself, which is really not complex at all.

 

[Alexei]

I second @Alexei here, my 91 T-bird had same all of a sudden and it was simply the friction plates gone for these clutches.
At the time I had a second trans and swapped them as it was my daily and needed be back on the road right away.

But for my 73, I've rebuild this trans and it's pretty straight forward, Most work went into beautifying the dirty corroded body that I started with. I have ordered by Ken at Badshoe.com, why, well because you need for 2 clutches a compressor and these happen to be quite pricy not too mention the very specific piston gaskets with L shaped sides that you cannot insert unless compressed. Ken offers a very simple PVC made+hardware compressor and most important some moulds that receive these gaskets. The trick being to put them in freezer for a few hours prior install and they fit right in. They return to their soft tight position in matter of minutes) The tools are on Ken's site offered at a very affordable price. Same goes to his quality kits.
May you want to see, https://7173mustangs.com/threads/fabrices-429cj-71-project.30729/page-29
I have detailed the rebuild there...

As about the pressure and complicated tests. I don't know why this legend persists... The valve body is designed in a mechanical way that it needs to be at a certain position (assuming here a functioning trans with oil level as it should be) to obtain the ideal pressure. To do this, you need to simply have the tension cable at rest with nearly no play. You can see on mustangs, t-birds etc, that they have a special "lock" on the cable, exactly made for this. So a non restrained cable must be at throttle linkage with zero tension. That's it, no need to check because by design it is the ideal pressure defined by the volume of oil entering the valve body via this small valve with a properly functioning pump. If you give a tad tension, the trans will shift early, you add lack, it will shift later on. The equilibrium position is highly recommended. You of course can check the pressure, but that's pointless as it's designed by the amount of oil that can go thru that orifice. Reading the pressure and adjusting it will give you the exact position of the cable with no lack, no tension. If you'de find another pressure level, this means there is not the right amount of oil or the trans is ready for a revision, which in your case, you know already... its is not like you need to tune the trans for this. It is by design always at the ideal pressure IF the trans is healthy and the cable is set in this rest position at the throttle. Again, not saying measuring is wrong, but it's not relevant for the proper install of this trans as reading vacuum to tune a carb for instance would be.

Another detail worth mention for this trans (also detailed in my post) is if you go for a lock converter and have a heavy right foot. Is that you kinda must replace the inner shaft as it will be too weak. And for the non lock version like the original, if the trans is pre 91' ( where the clutches bodies are made of cast pot metal vs iron) if the car was pushed, it is likely that the shaft has been hitting the casing and it then can go too far in, which causes it to block one passage of oil just 1/4 inch away. For this and as prevention, know that there are rings available that you can install by just machine a tiny groove on the shaft. The ring will then never allow the shaft to do damage anymore. Another detail is a better snap ring for the reverse clutch(that the last one you see before install the pump back)), that can actually come loose. That is also available and a should be part of the rebuild. I have all the parts refs in my post may you'd want to rebuild it yourself, which is really not complex at all.

 

[Alexei]

I second @Alexei here, my 91 T-bird had same all of a sudden and it was simply the friction plates gone for these clutches.
At the time I had a second trans and swapped them as it was my daily and needed be back on the road right away.

But for my 73, I've rebuild this trans and it's pretty straight forward, Most work went into beautifying the dirty corroded body that I started with. I have ordered by Ken at Badshoe.com, why, well because you need for 2 clutches a compressor and these happen to be quite pricy not too mention the very specific piston gaskets with L shaped sides that you cannot insert unless compressed. Ken offers a very simple PVC made+hardware compressor and most important some moulds that receive these gaskets. The trick being to put them in freezer for a few hours prior install and they fit right in. They return to their soft tight position in matter of minutes) The tools are on Ken's site offered at a very affordable price. Same goes to his quality kits.
May you want to see, https://7173mustangs.com/threads/fabrices-429cj-71-project.30729/page-29
I have detailed the rebuild there...

As about the pressure and complicated tests. I don't know why this legend persists... The valve body is designed in a mechanical way that it needs to be at a certain position (assuming here a functioning trans with oil level as it should be) to obtain the ideal pressure. To do this, you need to simply have the tension cable at rest with nearly no play. You can see on mustangs, t-birds etc, that they have a special "lock" on the cable, exactly made for this. So a non restrained cable must be at throttle linkage with zero tension. That's it, no need to check because by design it is the ideal pressure defined by the volume of oil entering the valve body via this small valve with a properly functioning pump. If you give a tad tension, the trans will shift early, you add lack, it will shift later on. The equilibrium position is highly recommended. You of course can check the pressure, but that's pointless as it's designed by the amount of oil that can go thru that orifice. Reading the pressure and adjusting it will give you the exact position of the cable with no lack, no tension. If you'de find another pressure level, this means there is not the right amount of oil or the trans is ready for a revision, which in your case, you know already... its is not like you need to tune the trans for this. It is by design always at the ideal pressure IF the trans is healthy and the cable is set in this rest position at the throttle. Again, not saying measuring is wrong, but it's not relevant for the proper install of this trans as reading vacuum to tune a carb for instance would be.

Another detail worth mention for this trans (also detailed in my post) is if you go for a lock converter and have a heavy right foot. Is that you kinda must replace the inner shaft as it will be too weak. And for the non lock version like the original, if the trans is pre 91' ( where the clutches bodies are made of cast pot metal vs iron) if the car was pushed, it is likely that the shaft has been hitting the casing and it then can go too far in, which causes it to block one passage of oil just 1/4 inch away. For this and as prevention, know that there are rings available that you can install by just machine a tiny groove on the shaft. The ring will then never allow the shaft to do damage anymore. Another detail is a better snap ring for the reverse clutch(that the last one you see before install the pump back)), that can actually come loose. That is also available and a should be part of the rebuild. I have all the parts refs in my post may you'd want to rebuild it yourself, which is really not complex at all.

I thank you for the thoughtful answer!