cyclic drivetrain vibration

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str12-340

str12-340

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1970 A-body, late 70s 318 (internally balanced), A500 4 speed overdrive automatic (42RH), lock up converter, 8 3/4 rear.
both overdrive and lock up are actuated with a manual buttons, The driveshaft was recently balanced and driveshaft angles corrected.

Out on the open road in third gear (1:1) I get a cyclic vibration at 1800 to 2500 rpm (45-55 mph). When I hit overdrive it goes away completely. When it gets up to 70-75 mph in OD the rpms are in the same range as where the third gear vibration occurs, but no vibration. Every once in awhile I get up to 55 in third and there is no vibration.

I assume that since the vibration disappears when I shift to overdrive 4th, that eliminates further drive shaft angle issues, or something from the tail shaft back, or it wouldn't disappear.

Any suggestions on what to investigate next???
 
Even though the drive shaft has been balanced, have you tried rotating in on the rear yoke 180*? It's just a thought, but it may lead you in the right direction.
 
Has trans been rebuilt? If so, has the rear output bearing been inspected/replaced?
 
Bad lock up converter clutch engagement. Could be electrical but mine ended up being the direct drive clutch in the converter.
 
Too many possibilities, when you say 3rd gear 45-55 is that with TCC on off or both?
When in OD 70-75 TCC on off or both?
Drive angle corrected, what is it set at?
Sounds like RPM or driveline speed related. Can you duplicate at a stop slightly raising RPM"s
in park or neutral and then in gear power braking bringing RPM" up into range, let us know
 
Disengage the lock up and don't let it apply. See if the shutter goes away. Type in lock up converter shutter. See what comes up in a google search
 
Very first thing I would do is put the rear up on stands and run it to those speeds to check out anything odd.
That way you get an idea if it’s drivetrain or rear tire related.
If it doesn’t do it then, it might be a front tire.
Put your stands under the axle so it sits like it would on the ground.
 
I've found a u-joint to cause similar, double check for tightness in flex, feel for brinelling .
 
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moparlvr166 said:
Too many possibilities, when you say 3rd gear 45-55 is that with TCC on off or both?
When in OD 70-75 TCC on off or both?
Click to expand...
What is TCC?

What I don't get is that if it were something in the driveshaft, rear end, rear tires, why would it go completely away in overdrive? All that spins the same speed at the same mph no matter what gear you are in...

I haven't used the lock-up converter hardly at all. Both the overdrive and the lock-up are actuated by a pair of switches which grounds one of those wires from the trans when pressed. No computer or automatic engagement of any kind that I am aware of...

Trans is freshly and completely rebuilt.
 
str12-340 said:
What is TCC?

What I don't get is that if it were something in the driveshaft, rear end, rear tires, why would it go completely away in overdrive? All that spins the same speed at the same mph no matter what gear you are in...

I haven't used the lock-up converter hardly at all. Both the overdrive and the lock-up are actuated by a pair of switches which grounds one of those wires from the trans when pressed. No computer or automatic engagement of any kind that I am aware of...

Trans is freshly and completely rebuilt.
Click to expand...

I don't know about the O/D but whenever a customer came in and said his car did "whir-whir-whir" at hiway speed, went away faster than "x" speed, I knew from my Dart experience it was a binding or brinelled joint.
I would always confirm customer complaint, and u-joint or steady bearing was usually the prob. (Harmonic vibration?)
Good luck
 
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TCC- torque converter clutch.
Sometimes harmonic vibrations happen at certain rpm' and load on drivetrain so when you shift
into another gear it goes away or sometime gets worse.
So that's why I asked the questions. Now drive it and get into 3rd gear 45-55 and hold the vibration then flip the switch for lock up see what it does if it goes away lightly slow down to 45 and lightly accel up to 55 and see what it does?
 
Trying to learn something new - what is a brinelled joint?
 
str12-340 said:
Trying to learn something new - what is a brinelled joint?
Click to expand...

The rollers in the joint cap, makes ruts in the reciprocal surface of the cross.

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If your joints are new or fresh, likely not brinelled, but may be binding.
Prob easiest to check .
Good luck
 
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