To Phase or not to Phase

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Mar 8, 2009
Ashland, OR
I finally got around to pulling the front drive shaft to try to address my vibration issue. The vibration is most notable in third gear at 3K rpm and increases in frequency as I accelerate. It's a very dramatic vibration. Yesterday, after reading several posts on phasing of the flanges, I discovered that my front shaft flanges are in phase, like the rear shaft. After inspecting the output flange of the t case, the marking on this flange suggests that it has been this way since the '92 was assembled from the factory, since there were no markings at 90 degrees. The same is observed on the output flange at the differential. The marks look like it has been this way since the factory. Now it is possible that the output flanges were replaced at some point and the shaft assembled incorrectly with new output flanges. I don't have any way of knowing what work was done.

The question is, should I reinstall it out of phase. There was some question here in the forums whether the '91 and '92 shaft flanges were suppose to be in or out of phase. I thought I had read that CDan had found a supplement to the '92 manual that called for out of phase shaft flanges in the front. So, to phase or not to phase, that is the question. :confused:
Wow! :DJust went for a test drive without the front shaft and the vibration has been reduced by about 75 to 80 percent. This is encouraging for someone minimally skilled as a mechanic. I can't really feel any play in the u-joints, but will probably replace them since I have the shaft out. I'll have the shaft balanced but I still have not decided if I should tell the driveline people to balance it in phase or 90 degrees out of phase. Any suggestions?
I have the feeling that I should reassemble it right now with the flanges 90 degrees out of phase just to see what happens. If I do this, should I change the front or rear flange 90 degrees from the existing marks?

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