I know this topic has been discussed before, but a search did not return any related topic so please bear with me. My 1976 3\4 ton supercab with 390 4bbl and c6 trans has a slight vibration that you feel through the seat and floor board (the steering wheel stays steady) at speeds of 30 to 40 mph. Above and below this speed the truck is very smooth and quite. Also, if you let off the accelerator the vibration goes away. The funny thing is that you hear the vibration as much as feel it as though the engine is in a certain rpm band that produces this shaking. Has any one else expereinced this and been successful in fixing it?

The trans is new, the carrier bearing is new, new flywheel and transmission mount. The rear end is quite and seems tight and the u-joints seem to be okay. Since this only happens at a certain speed I hope I am correct in assuming that these part are in okay shape. I do plan on repalcing the engine mounts when the weather gets cooler.

Also, the engine was rebuilt by a previous owner and as best I can tell was not a high dollar\quality job. Although, it does run smoothy at highway speeds and has proved to be dependable.

Any thoughts and recommendation would be greatly appreciated. Thanks for your time.

 

Mark,
U-joints.
KingFisher

 

Kingfisher,
Thanks
Mark

 

Also if your truck has a slip joint in the driveline,, check that,, it gets loose also

 

mt,
How do you service a slip joint?
KingFisher

 

If there is play in the slip joint the driveline has to be removed and taken to a shop that does driveline repair where they will well a new end on one side and replace the other

 

You didn't mention if you replaced your tourqe converter, I had one go bad and it sounds like the same prob as what you have.

 

Marv: The converter, transmission and carrier bearing have all been replaced within the last six months.

Thanks for your interest in my situation.

 

[QUOTE]Originally posted by Mark76f250
[B]
has a slight vibration that you feel through the seat and floor board (the steering wheel stays steady) at speeds of 30 to 40 mph. Above and below this speed the truck is very smooth and quite. Also, if you let off the accelerator the vibration goes away. The funny thing is that you hear the vibration as much as feel it as though the engine is in a certain rpm band that produces this shaking. Has any one else expereinced this and been successful in fixing it?

Hi Mark - I have a very similar problem at 55-65+mph (4spd OD manual tranny). Let off the accelerator, OR, tramp it down and the vibration goes away.

To eliminate the engine as the source of the vibration, with the tranny in park or neutral, bring the rpms up to the vibration rpm and see how it feels. If its still there, look at the belt driven accersories and their mounts.

Also try driving down the road at the critical speed and shift it into neutral and see how it feels. If the vibration is still there its in the tranny output shaft, u-joints, drive shaft alignment/balance, driveline angles, pinion shaft, flange yoke, differential gears, or axle bearings. (The vibration frequency is too high to be caused by the much slower turning brake drum or wheel balance - at least in my case.)

My next 'tuit' will be to pull out my 3.25 center section and put back in my old 2.75 section to see what that does.

 

I know this might be a long shot, but swap your front and rear tires to see if the vibration changes. What caught my attention was the speed. My truck has a set of tires on the front that that have begun to vibrate. They begin to vibrate at 30mph, shake like crazy at 38-40, then it dissapears at 42. Hey, at least it doesn't cost anything to check.

 

I forgot that i had a vibration once and found out it was the clutch fan bearing about ready to seize.

 

yeah i had to put the spare tire on my truck and i notice the same thing at 45mph and then fast or slower of that its fine thats would i would check like warren said ...i also had a problem similiar to that on a vette i was helping a buddy work on ..they all thouth something expensive and i said well why not check out something cheap first ...got the tires balances and it doesnt have the hum or vibration anymore ..just thought this might help ..take it easy

 

Your driveshaft may need balancing. I have never been able to replace joints in one of those three piece shafts without needing a balance. I never had a problem with a 1-piece shaft...

Have you lifted or lowered the truck?

Are your spring bushings OK?

Is your pinion bearing adjusted properly?

 

Eric:

I just replaced the u-joints last night but have not had a chance to test drive it yet. I will take the shaft to a shop to have it balanced as a precaution. The truck was so worn out that just about everthing else has needed attention so I am fairly cetain the drive shaft is no different.

Thanks for your input and to all others.

 

I just wanted to thank all those that replied to my post last week. I installed 3 new driveshaft u-joints on "Brutus" my 76 f250 supercab and all vibration disappeared. It has never driven so smoothly.

Thanks again.