07-10-2008, 08:08 PM
Rising Sun Land Use Coordinator
Join Date: Aug 2005
Location: Denver CO
Another one from Mark Whatley: May/June 2005 Toyota Trails
Originally Posted by Mark Whatley
"I live in Richmond, CA. I have a rebuilt 1978 FJ40. I love this vehicle and have owned it over 10 years. Bought it off my friend who bought it brand new off the lot in 1978 in New Mexico. It's my peace of mind at night living on the Hayward fault. Over the last 7 years, I've had it customized by a specialty shop in Concord, CA (Chevy V8, power steering, higher suspension, bigger tires). He's extremely talented but we've run into a problem he can't seem to figure out and thus this email. In 4-wheel drive high and low, when you let off the gas there's a horrible vibration noise, but only when you let off the gas and only when you're in 4 wheel drive. It's coming from the front end and Ron has changed out almost everything in an attempt to track it down without any luck. Unfortunately, I'm not a mechanic so I can't really offer him any advice, so I decided to try doing some online research and that's how I found you. If you have any suggestions, it would be most appreciated." Cathy Stonehill
The most likely source of this vibration is an out of phase driveshaft. Both ends of the driveshaft need to be in line with each other. It is common for people to accidently misalign them by 90 degrees if they don't look carefully during reassembly. It is also possible to misalign them by differing amounts if you get careless. The next most likely reason is misalignment of the flanges at each end of the shaft. The faces are supposed to be parallel. Changing of drivetrains, modification of suspension and other similar undertakings can result in inadvertent modification of the relationship between the flanges. You can get away with a small amount of misalignment, on the order of 2-3 degrees. Any more than that causes the type of vibration you mention.
The reason behind these potential causes of vibration is that a u-joint does not transmit the rotation smoothly when it is deflected. The deflection creates a situation where the constant speed that is input to the assembly is changed into a cyclic speeding and slowing scenario. The output from the assembly speeds and then slows twice for each revolution. The greater the deflection, the greater the difference between the fast and slow speed. When all is working as intended, the u-joint assembly at the other end of the shaft receives this pulsing input and changes it back to a smooth output on the other end. For this to happen correctly, the u-joints have to be aligned so that they are trying to pulse in phase with each other. They must also be deflected by the same amount so that the pulse is of the same amount. Any difference in the timing or the size of the pulse creates vibration.
Less likely to produce a severe vibration but a possible source is a very worn pinion bearing (differential) or output bearing (transfer case). A bent or even dented driveshaft can also cause this. But I would look to an out of phase (driveshaft ends) or an out of alignment (flanges) condition.
While I'm thinking about it, an "out of balance" is seldom really the problem. If a driveshaft is straight, as it is supposed to be, it does not need to have weights added to it to "balance it." The purpose of weights is to compensate for slightly less than straight shafts. A shaft that is not straight will vibrate. The weights will dampen this out and offset it due to the vibration they cause themselves. While this makes sense for a factory that is turning out thousands of driveshafts a day, there is no reason that any shaft that is custom made or modified for your rig to need any weights. It is supposed to be straight. Insist on it when you have a shaft made (or lengthened or shortened).
I'm betting on a bad output (t-case) bearing myself, considering mine is more of a growling anyway...
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