C-6 Wierd vibration

C-6 Wierd vibration

Joined: October 20th, 2011, 9:57 pm

October 20th, 2011, 10:07 pm #1

I have a 70 mach 1 with a 351 and a c-6. Ok here is the problem, when in 3rd gear starting at 55 mph, It has a cyclic vibration. If I shift into neutral then back to drive the vibration goes away (sometimes it takes a few attempts) and does not come back until it has gone back to 2nd and has to shift into third again. When I do get the vibration to disappear if I shift into neutral or 2nd and then back into drive, the vibration returns.
I have isolated this to the trans by disconnecting the drive shaft and running it on jack stands. Any help is apprecited!!!
Last edited by quebedo on October 20th, 2011, 10:12 pm, edited 1 time in total.
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Joined: October 20th, 2011, 9:57 pm

October 20th, 2011, 10:22 pm #2

I should add, The trans was rebuilt about 6000 miles ago and there is a shift kit in it. I don't know what kit was used, I bought the car this way.
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Joined: July 17th, 2011, 7:07 pm

October 21st, 2011, 12:55 am #3


If you had not mentioned that you disconnected the drive shaft, I would have said it sounds like a bad u-joint.

It is rare for internal trans parts to cause a vibration.

From your description of the problem I would guess that something is slipping... or there are some badly worn out bushings in the trans.
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Joined: October 20th, 2011, 9:57 pm

October 21st, 2011, 1:08 am #4

I have a 70 mach 1 with a 351 and a c-6. Ok here is the problem, when in 3rd gear starting at 55 mph, It has a cyclic vibration. If I shift into neutral then back to drive the vibration goes away (sometimes it takes a few attempts) and does not come back until it has gone back to 2nd and has to shift into third again. When I do get the vibration to disappear if I shift into neutral or 2nd and then back into drive, the vibration returns.
I have isolated this to the trans by disconnecting the drive shaft and running it on jack stands. Any help is apprecited!!!
Could it be the band is too tight and haging up when it goes into 3rd? I am not real familiar with what clutchs lock and when the band is tight. I also should mention, The fluid is not burnt.
Last edited by quebedo on October 21st, 2011, 1:12 am, edited 1 time in total.
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Joined: July 17th, 2011, 7:07 pm

October 21st, 2011, 1:22 am #5


The band is only applied in 2nd gear.

On the 2 - 3 shift the band "lets go" and the Direct clutch applies.

If the band was too tight, the 2-3 shift would have a "bind" feeling, or feel harsh on the shift. Also the band and Direct clutches would burn up sooner or later... probably sooner.

How's the oil? does it smell burnt?

I suppose it's possible that someone did some "trick" inside the trans... and that is causing the problem. Drilling the direct drum comes to mind... Racers sometimes do that. If the drum was drilled "not quite" perfectly it could be out of balance.

The other possibility is .... it's not a trans problem at all.... maybe engine related?

Vibrations can be tricky to track down... Vibrations "travel" and isolating exactly what is vibrating can be tough sometimes.
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Joined: October 20th, 2011, 9:57 pm

October 21st, 2011, 2:00 am #6

The fluid is good. If I bring the rpms up in neutral or park, there is no vibration. I know it's in the trans. It is just so strange that I can make the vibration go away by puting in neutral then back in drive while cruising. Or make it come back by doing the same. I really do appreciate all the help and ideas I can get!! This trans will be a pain to get out.
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Joined: July 17th, 2011, 7:07 pm

October 21st, 2011, 2:05 am #7


How severe is the vibration?

Very noticable, or do you really have to pay attention to notice it?
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Joined: October 20th, 2011, 9:57 pm

October 21st, 2011, 2:12 am #8

Sometimes it is more noticeable then others. Smoother roads makes it more noticeable and it seems like when it's cold it is worse. It my be my imagination but, When it takes a few bumps up to neutral then back to drive, each time the intensity will be different til it finds the right place or something and it goes away.
Also, when it is cruising {under light load} and I give it just a bit of throttle, the vibration instantly gets more intense and closer faster cycles. Instead of a longer duration.
Last edited by quebedo on October 21st, 2011, 2:21 am, edited 1 time in total.
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Joined: July 17th, 2011, 7:07 pm

October 21st, 2011, 2:32 am #9


Ok, here's a long shot... but maybe will it give a clue...

Try disconnecting the vacuum line to the trans modulator. Plug the end that goes to the engine.

This will cause the pressure in the trans to go to maximum and cause the automatic shifts to be extremely/very late, but you will still have manual control of the 1-2 shift.

Now, with the vac line disconnected, go for a drive and get it up to cruise speed where it normally vibrates.

It may be a challenge to keep it in 3rd gear with the vac line plugged/disconnected, but see if the vibration changes or goes away while at cruise in 3rd gear.

Report please.
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Joined: October 20th, 2011, 9:57 pm

October 21st, 2011, 2:43 am #10

I will try that tomarrow. Believe me, I will post all results. I know there are others that have gone through this and got the usual replies. I have replaced and balanced everything behind the trans. Including new tires, driveshaft,axle bearings, third member,brake drums and reset pinion angle. The car is very smooth except the vibe.
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