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vibration in 4th gear

J

johnboy114

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Good morning - I have a new to me 84 jimmy with a 465 transmission and 350 engine. been driving the truck a good bit to get to know it and i notice it has some vibration in 4th gear that feels like it is coming from the middle of the truck. I can press the clutch and put it in neutral and the vibration goes away. It is lifted with 33's. Any ideas what to check for?
 
johnboy114 said:
Good morning - I have a new to me 84 jimmy with a 465 transmission and 350 engine. been driving the truck a good bit to get to know it and i notice it has some vibration in 4th gear that feels like it is coming from the middle of the truck. I can press the clutch and put it in neutral and the vibration goes away. It is lifted with 33's. Any ideas what to check for?
Click to expand...

u joints. Make sure the drive line was extended for the lift as well.
 
K5Devil said:
u joints. Make sure the drive line was extended for the lift as well.
Click to expand...


will check that - but shouldnt the U joints still vibrate regardless if the clutch is pushed in or not? my vibration stops as soon as i put the clutch in.
 
johnboy114 said:
will check that - but shouldnt the U joints still vibrate regardless if the clutch is pushed in or not? my vibration stops as soon as i put the clutch in.
Click to expand...

drive shaft only spins under acceleration. Stop accelerating (push in clutch) drive shaft stops spinning.
 
circling back on this - the vibration does not happen every time i go into 4th. and when it does if i take it out of 4th and then put it back in gear it usually goes away.
 
the pilot bearing in the crank and front input bearing in trans are sloppy. Try just cycling the clutch when it happens, see if vibration goes away. If it does then it's those bearings
 
I've had a tire out of balance a couple times before that caused a shake/bounce/vibration as soon as it shifted into 3rd or 4th gear. Only at the "bottom" of the gear though, after the rpms increased, it would mostly go away. Oddly, they seem to shake and bounce only when there is a load on the driveline as well... let off the gas and it would go away. I was originally thinking mine was a u joint cause I had issues with them before, but then I decided to rotate the tires to see if the vibration would move to a different part of the blazer, and it did. Went and got the tires balanced, and smooth as can be again (relatively speaking of course) lol
 
My mechanic friend, invested in a Road Force balancing machine, made by, Hunter? I think. When I heard about it, I thought it was a gimmick to sell a balance machine for more money than a regular tire balancer.
Turns out, its a real thing. It seems that some tires, radials more than the old bias belted ones, come out of the factory with the side walls not uniformly strong.
Which means that as it rolls down the road, it flexes more in some spots than others. Or less than the rest of the tire. Therefore, the tire and wheel can be perfectly balanced weight wise, but you will still get a vibration going down the road because the tire moves up and down as it flexes.
The road force machine has a big roller that it presses against the tire as it rolls and looks for different amounts of flex. It turns out, that most tire companies have specs for that they have to adhere to.
My friend has sent back several tires for replacement before they even went on the car. And I know of two cars that he found the vibration that way. Other shops had sent them there because they just could not get the tires to roll smooth.

Just when you think you have it all figured out.........
 
Road force balancing has been a big thing for a while. Some forums will tell you to demand road force balancing despite 90% of the major tire centers out there already use them and that if it's not a Hunter machine then it's not worth it.
 
Pretty sure its a Hunter. Kinda embarrassing, I should know, since I had to fix it after the hurricane.
Just after they got it moved into the new temporary shop after having dug it out of the remains of the old shop, it suddenly died.
Hunter could not get to it for a couple of weeks for some reason, and it was out of warranty I think. So, I got volunteered.
Turned out the computer power supply just died for no reason. It was not a normal size one, but all the outputs were standard. I grabbed a spare from the house, and it all came to life.
I didn't want to build a place to put that one, so we ordered a replacement from the same company that made the original. It came in and almost fit perfectly.
The size and shape were perfect, but I guess Hunter specified a longer than standard output bundle. Fortunately, I had an extension at home, so I had it up and running in about an hour.
Been doing fine ever since.
I ordered 4 new tires for my car, and two of them failed the road force. I was amazed, since they were top of the line Michelins.
 
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