|
Several posts over the past days have discussed vibrations in the car at about 40 mph. I've been there lots of times, and thought I'd recap what I know and try to help. I will assume that you've done what you can to see that your tires are mounted and balanced properly, your brakes aren't dragging, and your wheels themselves aren't bent. The drive shaft alignment is where it all starts. The first point where there's a possibility for problems is the transmission mount isolator (tranny mount for short). This little gem of design is two pieces of steel joined by some silicone-impregnated rubber, and is designed to flex slightly under the torque of acceleration and deceleration. That's fine, but when the rubber gets tired, it sags. When that happens, the drive shaft goes out of alignment. The drive shaft has two or three U-joints in it, depending on whether it's a NA (3) or TT (2 plus a CV joint at the differential). These come under stress when the alignment of the transmission and drive shaft changes as the mount sags. The tranny can move around more, which also stresses the first U-joint. Vibrations start to get serious. The drive shaft assembly has a rubber-mounted carrier bearing in the center, which is designed to take some of those vibrations out of the drive shaft, and smooth out the ride. When the shaft goes out of alignment, the increases in stress get multiplied, and the carrier bearing will start to 'fail' - transmitting more vibration to the car, and that's what most people feel inside. The seats start to wobble a little, and there's a deep rumble in some cases, all at about 40 mph. The second U-joint starts to twist as well, and can fail in cases where the vibrations have been going on for a long time. The third U-joint can fail also (NA's only) because of the multiplier effect. The last component in the system is the differential. The weight of the diff itself will set up some sagging over time - especially the front mounts [which are easiest to get at, thank goodness! Nissan used silicone-impregnated rubber in these mounts also]. As the vibrations get multiplied down the drive shaft, the front mounts take the punishment, and will start to degenerate also. There's some on-car checks you can run to see whether to start to tear into the drive line. Get the car up in the air enough to slide underneath the rear end. Look as carefully as you can at the center bearing for cracks and tears. Move the drive shaft around by hand: if it's very loose, then you have a clue that the center bearing is 'soft'. Check the runout of the drive shaft first, before checking the carrier bearing any more - it still could be working properly (next item). Now, check the drive shaft to see if it's rotating properly. Take a measuring device, like a dial indicator, and measure the runout of the back end of the drive shaft about halfway to the carrier bearing (NA coupe and TT) or a third of the way from each end (2+2). Runout should be NO MORE than .024. Don't panic if it is. Try what Nissan does: unbolt the drive shaft from the differential, rotate the holes one position in one direction, bolt it back up again, and measure again (this process is called 'clocking' the drive shaft). Do this through the entire number of bolt positions (4 or 6). Not within spec? NOW you can panic! You should start to fix the problem by starting at the front of the car, with the tranny mount. It's only about $50, and pretty easy to change without taking anything else apart. I have some pictures showing the old and new: sagging was almost half an inch! Re-measure the drive shaft runout. If it's back in tolerance, you may have solved the problem, and you're done for a while. Next up is the drive shaft itself. Get it out of the car and see what shape the front U-joint is in (procedure is in the Tech section). Once it's out, rotate the front U-joint through its range of motion: if you feel hesitation or hear clicking, it's shot. [A good test is to see if it will stand out straight when the drive shaft is laying on the floor. If it does, it's done for.] Check the other U-joints in a similar manner. TT'ers do not need to check the movement of the CV joint, but look for wear on the shaft, and make sure that it doesn't wobble much. If it does, you can re-pack it with fresh grease and get by for a while, but you'll end up replacing the drive shaft later.... Now, check the carrier bearing: do you see any cracks on the rubber? Is it soft and bend easily, or hard and stiff [no wise cracks here - I'm trying to help!]. If it's soft, it WILL crack soon.... This is the point in time to decide whether to repair or replace the drive shaft. A two piece aftermarket unit from Powertrain Industries are about $225, AIR. The one piece shafts are a little more than that, but not much more, in steel, and about $300 in aluminum. This is your call. If you decide to repair the drive shaft, you can get the U-joints replaced with ones that can be greased, the center bearing replaced, and the shaft re-balanced for about $175. When you put the new shaft in, check the runout and go through the same process of clocking the drive shaft until the alignment is within spec. Finally, you should check the condition of the differential mounts: if they appear cracked, or have black goo all over them, then replace them. It's relatively easy to do, but you need to support the front end of the differential with a floor jack while the bolts are out. If you STILL have vibrations, it's probably not in your drive line. It COULD be that the differential has been worn or damaged by a long period of operating with a bad drive shaft. That's another story, though..... Hope this helps! Tom Bell
IZCC 2802
(Dr. Vibes)
|
Drive line vibrations (long) - 
