Loud Violent Clunk When Shifting FMX Into Gear. Any Transmission Experts?

TBH, that is a lot of movement in the diff. Here's a video of the 2.75 rear I just removed from a car. I consider this excessive, and yours is much worse.

Hemikiller said:

TBH, that is a lot of movement in the diff. Here's a video of the 2.75 rear I just removed from a car. I consider this excessive, and yours is much worse.

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It doesn't take much wear on the differential bearings to dramatically increase the backlash. Harsh transmission engagement with a small amount of backlash will make a noticeable bang.

Did you ever do that Control Pressure check?

I drive the car often. It shifts just fine.
The whole issue still is, and always has been, shifting into gear and getting a loud clunk that jerks the whole car.
I am just trying to figure out if the culprit is the rear end or the transmission.
It seems as if the overwhelming opinion, when I brought this up almost a year ago, and then started this post again recently, is that its the rear end.
I wanted to know:
A. Can a transmission that has a potential problem, engage reverse and drive with more force than it should, and cause this bad clunk and jerk?
As I have said, I can rev up my other 2 early ford's to 1000rpm, drop them into gear and they don't jerk and clunk like this Mach.
It doesn't seem like the group supports a transmission issue.
B. My 67 has more supposed slop in the rear end using the rotate the pinion while in neutral method. It doesn't clunk and jerk like the Mach.
Can it be something in the rear end that is so damaged or compromised, that IT IS the problem and the "slop" check ideas really mean nothing
in my case?

Was hoping for an easier answer as I don't want to rebuild the rear if not necessary.
The transmission leaks between the sections where it is bolted together. However, tranny rebuild probably twice the price.

Appreciate the input! (still on the fence)

Should have included or been more clear:
It was always my understanding that the vacuum modulator has to do with the shifting of the transmission.
It was led to believe that would not be relevant when shifting from Park into Revers or Drive. Never moving. Sitting stationary.
I didn't really mention that in last post.

andy72 said:

Should have included or been more clear:
It was always my understanding that the vacuum modulator has to do with the shifting of the transmission.
It was led to believe that would not be relevant when shifting from Park into Revers or Drive. Never moving. Sitting stationary.
I didn't really mention that in last post.

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Check out the modulator and the vacuum line to it. It will take ones of minutes since you have a lift.

andy72 said:

Should have included or been more clear:
It was always my understanding that the vacuum modulator has to do with the shifting of the transmission.
It was led to believe that would not be relevant when shifting from Park into Revers or Drive. Never moving. Sitting stationary.
I didn't really mention that in last post.

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Changes in vacuum changes the control pressure, when the vacuum modulator is functioning properly. Lower vacuum results in higher control pressures. No vacuum, as in vacuum modulator with blown diaphragm results in continuously high control pressures.

Sounds like you're searching for something you want it to be instead of what you don't want it to be. I fell into that line of thinking at times and it just dragged out the final fix. Yank the 3rd member and have it examined by a competent mechanic. You'll either find the problem or eliminate that from being the problem. I have had issues in the past where a C4 or a C6 would lurch violently into reverse but never in drive. That's a matter of line pressure and idle speed being the culprits. This sounds like a differential problem from all I've read here.
Good luck.

@andy72

I wouldn’t say that the group doesn’t support the ideas that it may be a transmission issue. Don brought up the idea of it being a modulator issue, and I am also onboard with checking it as it is cheap and easy. A modulator is tens of dollars, and vacuum hose is ones of dollars.

Please take a few minutes to look this over, mainly the “vacuum leakage” section.

andy72 said:

Should have included or been more clear:
It was always my understanding that the vacuum modulator has to do with the shifting of the transmission.
It was led to believe that would not be relevant when shifting from Park into Revers or Drive. Never moving. Sitting stationary.
I didn't really mention that in last post.

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With the exception of the parking pawl, the transmission is shifted by means of hydraulic oil pumped through various circuits, past check valves and shuttle valves, and ultimately to some form of a piston device to apply force to either a band or a clutch. Making a drastic change to the pressure of that hydraulic oil would have an effect on how rapidly the fluid is delivered to, and subsequently, how quickly those piston devices react. The vacuum modulator varies the hydraulic pressure to change how rapidly and when pressure can reach certain piston devices. There is also a governor that varies hydraulic pressure. I don't know the circuits well enough to tell you, for certain, which ones are and are not affected by the modulator, but it's certainly plausible. If it shifts normally, it's probably not the modulator, but disconnected and plug the vacuum to it and take it for a drive....it will shift differently if it's functioning at all.

If this was my car or I was in charge of troubleshooting, I would put someone in the driver's seat, raise the car on that lift, have them replicate the noise while I listened from under the car.

basstrix said:

If this was my car or I was in charge of troubleshooting, I would put someone in the driver's seat, raise the car on that lift, have them replicate the noise while I listened from under the car.

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That should be Step 1.

Any resolution?