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

[andy72]

Hello Everyone,
I posted this same problem about a year ago but dads health caused me to step away from the car.
I'm going to start over here...

When I shift into gear, especially Reverse, the car has a loud clunk and a violent jerk. It feels like it wants to break something.
What I've done/tested...

351C FMX transmission
Timing 6 BTDC
Idle in Park about 700
new U Joints
Transmission had a fluid replacement and is full.
The car had sat stationary outside for years before I got it in 2015.
It has shifted like this as long as I've owned it.

I disconnected the drive shaft from the rear end and supported it up. No clunk when shifting into gear.
With shaft reconnected, I got under the car and clearly the noise comes from the rear end.
I find it hard to believe its the rear though because I have a 460 with a C6 and a 302 with a C4.
In either of these cars, if I give a little gas to increase RPM to about 800, neither car clunks at all when shifting into gear or jerks so bad like my 72.
Both of those cars just shift into gear like butter virtually noise and sensation free.
The 72 shifts into gear and jerks the car and has a loud clunk.
My 67 with 9 inch Equa-lock seems to have more slop in rear end if I jack tire off ground and rotate axle.
Not sure if this is actually a valid test of rear end slop, but I did it.

Is it possible the FMX is shifting into gear improperly and rotating the drive shaft harder or with more force than it should?
It does this shifting into Reverse or Drive, but it is worse shifting into reverse.
I feel like if I replace the third member the car is going to do the same thing.
Please watch vide below to see and hear the issue.

Any thoughts?
Thank you

 

[Bentworker]

Hello Everyone,
I posted this same problem about a year ago but dads health caused me to step away from the car.
I'm going to start over here...

When I shift into gear, especially Reverse, the car has a loud clunk and a violent jerk. It feels like it wants to break something.
What I've done/tested...

351C FMX transmission
Timing 6 BTDC
Idle in Park about 700
new U Joints
Transmission had a fluid replacement and is full.
The car had sat stationary outside for years before I got it in 2015.
It has shifted like this as long as I've owned it.

I disconnected the drive shaft from the rear end and supported it up. No clunk when shifting into gear.
With shaft reconnected, I got under the car and clearly the noise comes from the rear end.
I find it hard to believe its the rear though because I have a 460 with a C6 and a 302 with a C4.
In either of these cars, if I give a little gas to increase RPM to about 800, neither car clunks at all when shifting into gear or jerks so bad like my 72.
Both of those cars just shift into gear like butter virtually noise and sensation free.
The 72 shifts into gear and jerks the car and has a loud clunk.
My 67 with 9 inch Equa-lock seems to have more slop in rear end if I jack tire off ground and rotate axle.
Not sure if this is actually a valid test of rear end slop, but I did it.

Is it possible the FMX is shifting into gear improperly and rotating the drive shaft harder or with more force than it should?
It does this shifting into Reverse or Drive, but it is worse shifting into reverse.
I feel like if I replace the third member the car is going to do the same thing.
Please watch vide below to see and hear the issue.

Any thoughts?
Thank you

That is wild. Is your idle RPM really 700? Do you have a Detroit locker in it?

Reason I ask about the Detroit is that they have a clunk by design. It is normal with a Detroit to have a 60 degree ish amount of rotational slop between forward and reverse at the driveshaft.

On the engine RPM, the video didn’t look like 700 rpm to my eye. If you have a timing light with a tach second check your idle RPM.

 

[Sheriff41]

Have you checked the play in the differential? If it doesn't do it with the driveshaft removed, then you've eliminated everything forward of the driveshaft. You say the u-joints are good so you've eliminated them. Time to check the play in the diff, I'd say. Mine had significant play in it after 40-some years; it's not unusual for cars as old as these to develop some slop in the ring and pinion gears.

 

[Cleveland Crush]

First off, rotating the wheels back and forth while jacked up is not typically a valid test for excessive rear-end free play (unless something is radically wrong). I'd suggest getting your car up on a shop's lift and isolate this noise with a stethoscope and dig in from there. Another alternative is using an electronic noise sensor to zero in on that noise. Good luck with it!

 

[72conv]

I had a very similar situation with a Plymouth Duster back in 1980. Long story short: the differential was shot.

 

[andy72]

so "72 Conv"...
You say you had a shot differential that would produce such a pronounced loud clunk that can make a car jolt?
That's what I want to know...

No Detroit locker. Just an open 9 inch with 3.25's.
I have a lift. I've been under the car with my hands on the rear end while my son shifts. It is definitely the rear end making the clunk.

As I said, I can rev my other 2 cars to higher RPM than this, about 1000 rpm, shift into gear and they don't clunk like this!

I've had people mention the torque converter as a potential problem. ?
It just engages reverse and drive so hard that it makes me wonder if anything can go wrong in the tranny to do this.
I've had numerous Mustangs in the family. Been around cars my whole life. Friends with cars. Everyone says "Wow, something not right"

 

[Bentworker]

I’d put it up in the air, disconnect the driveshaft at the rear axle and with the parking brake on see how many degrees (approx) play you have turning your pinion back and forth. Won’t cost anything

 

[72mokk1]

You might have a stall converter, when my C6 was rebuilt the guy put a stall converter in and if I put it in gear without my foot on the brake it will chirp the tires in the garage. In other words it bangs pretty hard. The 9 inch rear end is hard to break but it is something to check also.

 

[basstrix]

When you disconnected the driveshaft, did you restrain it from rotating or did it just spin freely when trans was put in gear? If it wasn't restrained, the test would seem to provide only minimal info and doesn't really eliminate the trans from the root cause.

That being said, the rear diff is more likely the problem. WIth the rear wheels restrained (either on the ground or e-brake firmly holding) and driveline removed, rotate the pinion yoke back and forth. The spec on ring gear backlash is .008-.012"...don't know what that translates to in in degrees of yoke rotation, but it's not a lot. you should not be able to move the pinion yoke, other than the small amount of rotation due to backlash. Excessive backlash would likely come from the small pinion gears that run on the splined side gears (the ones that make it a differential). I've seen the support shaft worn badly from getting the vehicle stuck and spinning one wheel for a relatively long time....even worse if the axle was tilted and all the oil ran to one side. Doing long one-wheel burnouts will smoke these gears/shaft too. That's my best guess.

 

[cjstaci]

Sounds like play in the rear end. You should be able to hear it more pronounced at the rear of the car

 

[andy72]

When I disconnected the driveshaft, I supported it up with mule tape so it would spin freely. I understand that this is not a great test of anything, but it did show me that the noise wasn't coming strictly from the transmission itself being put into gear.

As I said it my original post, my 67 seems to have more rear end slop simply by rotating a wheel that is off the ground. I can disconnect the driveshaft from both my 72 and my 67, leave the wheels down on my 4 post lift, and see which one has more slop in the pinion. This will at least compare the two although I'm not sure this provides a definitive answer.

As I said before, I was really hoping that a transmission person could say that "this or that" (transmission problem) could produce that problem.
I doubt that it has a stall converter installed. Can the converter itself be a contributing problem? Everything I read online says a bad torque converter will slip. My car has an opposite problem. A loud bang when it engages gear.
I just find it hard to believe that the huge "jerk" or "jolt" I get is just rear end slop. All old Mustangs have old sloppy rears from 50 plus years on the road and they don't bang like this.

Frustrated

 

[cjstaci]

Can you feel and or hear where it is? With the wheels on the ground and trans in neutral, how much rotation is there in the driveshaft? Are your motor mounts and transmission mount in good condition?
When driving the car, does it shift normally into 2nd and 3rd gears?

 

[Bentworker]

When I disconnected the driveshaft, I supported it up with mule tape so it would spin freely. I understand that this is not a great test of anything, but it did show me that the noise wasn't coming strictly from the transmission itself being put into gear.

As I said it my original post, my 67 seems to have more rear end slop simply by rotating a wheel that is off the ground. I can disconnect the driveshaft from both my 72 and my 67, leave the wheels down on my 4 post lift, and see which one has more slop in the pinion. This will at least compare the two although I'm not sure this provides a definitive answer.

As I said before, I was really hoping that a transmission person could say that "this or that" (transmission problem) could produce that problem.
I doubt that it has a stall converter installed. Can the converter itself be a contributing problem? Everything I read online says a bad torque converter will slip. My car has an opposite problem. A loud bang when it engages gear.
I just find it hard to believe that the huge "jerk" or "jolt" I get is just rear end slop. All old Mustangs have old sloppy rears from 50 plus years on the road and they don't bang like this.

Frustrated

Post up what you find when you put it up on the 4 post…