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Posted by Chriscase [75.36.32.213] on Wednesday, April 11, 2012 at 09:50:11 :
In Reply to: Re: Body lift 1/2" ? posted by Wayneh [68.55.55.71] on Wednesday, April 11, 2012 at 07:24:25 :
We can diagnose most tranny noises by which gear it's in when it make the noise.
First and reverse are waaaay to the rear. If they are noisy, it's usually the rear bearing.
Second is closer to the middle. It puts less load on the rear brg, and some more on the intermediate brg between input and out put shafts.
Third puts the most load on the intermediate brg because it is the very closest to it.
Fourth puts no load on the intermediate, since in 4th the input and output shafts are locked together, and the intermediate brg doesn't even turn. And the power path being straight through in-to-out, there is no load on the other bearings either.
In synchro trannies, the gears are always meshed, so chipped teeth are rare. Engagement is by sliding a synchro to lock the gear to the out put shaft.
In sliding gear (crash box) transmissions, the gears actually slide into and out of engagement, wearing and chipping teeth. That would make a noise as the broken tooth makes contact once per revolution.
Your noise in 'under load in 3rd and 4th' sounds to me like some other drive train noise. Probable a speed related noise. Maybe a worn u-joint. Possible a drive shaft out of alignment. Check all three shafts to be sure the u-joint crosses are in proper phase. You want the two legs of the crosses on the 'shaft side of the yokes' to be parallel. NOT 90°, but parallel. I keep the mental picture of 'like a double cardan constant velocity joint'. Every tuck I've ever bought had at least one wrong. Also, some guys actually put the transfer case into a false neutral between low and high- slip under neath, and rotate the in and out of the transfer case to align those yokes the same way. It has to be re-done each time you take it out of high.
And makes sure the lip around the big hole in the floor of the cab is not touching the frame. Most have sat directly on the frame for so long that if the cab is removed there is a groove worn in the frame. This makes the cab act as a megaphone, amplifying innocuous drive line noises.
I would eliminate all those before putting $$$ and effort into another tranny overhaul. They are all common problems in these old trucks.