first let me say thanks to all that use this sight, the info is great and what not. now on the to problem. i have an 84 BII with a 7.5 rear with 373 grears. it originally had the open diff, but i swapped that out with a tru-trac posi unit from a 84 mustang gt (also 7.5) i put my 373 ring gear on it and reinstalled everything. it pulls posi and all but when idleing around the yard in gear or taking really slow corners the rear end pops and snaps, and bucks sparatically. any ideas or solutions would be greatly appreciated.

 

You say posi? Meaning both wheels roll the same around corners. You will hear noise like that. If you used a limited slip unit the wheels would roll at different rates around corners thus no noise. Check your clutch pack for proper clearance. The lash adjustment on the pinion to ring gear should also be checked.

 

What kind of "posi" is the tru-trac you got? clutch type limited slip? geared limited slip? locker of some kind? most clutch type limited slips need a "friction modifier" added to the differential lube to allow for smoother differentiation. automatic lockers pop and snap by nature.
And, since it sounds like you replaced the carrier in the process, if you didn't get the gears set up right (backlash, wear pattern, etc) then they can make some horrible noises and won't last.

 

There's an oil additive required in many limited slip differentials to reduce or eliminate that noise. It's available at many auto parts supply stores, and usually comes in a tube (8 oz.). It's usually put in before refilling the diff with the oil.

I'm curious about that swap, though. Was the backlash readjusted as is usually required anytime differential gears are replaced with a housing? Although, it's POSSIBLE to "get lucky" and have everything mesh just right after replacing the ring and pinion gears using the pinion and carrier shims as they came out of their original locations, more than likely the procedure will have to be redone to get everything just right for reliability.

That's all I can add right now with the information so far given.

 

the posi unit had 2 plates inbetween the spider gears with a series of springs pushing outward "locking" the gears together. this is the first time i ever been into a rear end and the manuel i used to do it was extremly vague on info so most of it was guess work. in detail this is what i did. i took the unit out of the mustang and took the 308 ring gear off. then i took the open diff out of my bronco and took my 373 gear off that and put it on the posi unit from the mustang, i put the unit in the rear end, in stalled my bearing caps, then the c - clips that lock in the axles, then the pin that goes in the diff and bolt for it, finally put the cover back on. { i torqued the bearing caps @ 60ftlbs, if that might have something to do with it}

 

also i use 80w-90 gear oil.

 

Once the differential has been reinstalled in the carrier with its new gears, and no binding was felt DURING and after that phase, it would be necessary to determine the amount of backlask or "play" between the ring and pinion gears. It's usually measured with a dial indicator setup pressing against one of the ring gear teeth sides. The pinion shaft is solidly locked using a special tool attached to the pinion yoke preventing it from rotating.

There is a minimum and most certainly a maximum allowable amount of rotational movement of the ring gear with this setup. That maximum is VERY small, about the thickness of a typical business card. The minimum is enough to prevent binding and to allow a layer of cooling lubricant to flow between the teeth, while allowing for variances due to temperature changes.

There's also a way to check for where the teeth make contact with each other. For longetivity, the teeth must make contact in areas as centered as possible on the teeth surfaces. With the ring gear teeth cleaned of oil, paint a portion with a THIN layer of light colored dye, such as white or gray primer. with a speedy wrench or motorized socket driver, rotate the pinion shaft until you begin to see wear in the paint. If the wear patch is well centered on the teeth surface areas, that part of the alignment is OK.

Assuming everything is cross-compatible between the donor and receiving axles, such as axle sline count and diameters, differential case bearing (outer) race and overall height, AND the donor axle was not making the noises you're noticing now on your truck now, other that what I and others have already commented, I'd be lost for a reasonable explanation as to your problems.

Maybe someone out there has actually done the swap you did and would catch and respond to your complaint.

 

frankenbroncoii, if that's all you did, then you need to get back into the differential and make sure those gears are set up correctly. If you can't find a good set of instructions with specifications (you'll have to use a professional grade manual. DIY manuals will simply say that, "this is beyond the scope of the home mechanic"), you might consider having it done professionally. As mentioned, the tolerances are small, the process can be tedious (trial and error type process that you only get better at with experience), and the cost of getting it wrong is your ring and pinion.