Differentials transfer engine power from the transmission to the axles so a truck will move forward. A posiloc differential has clutches to distribute the torque to both wheels which increases the traction by allowing the opposite wheel to pull if one is simply spinning. These can be damaged or worn, requiring a rebuild.
- Diagnosing the differential
1) Check the differential to determine if it needs maintenance. You will want to decide if it is time to tear down the differential, and there are a few ways to decide if a tear down is needed.
2) Set the truck on a lift, or jack up both rear wheels so they can be rotated by hand. Make sure the vehicle is supported if you use a jack, use solid blocks or jack stands designed for this purpose.
3) Check the differential lubricant. Remove the filler plug, and use a clean screwdriver or other slender object to dip inside the differential to check the fluid level. If the fluid is very low, continue with other diagnostic steps before filling it, since a tear down will require draining and refilling the differential, and any gear oil you add will soon be discarded.
4) Go to one rear tire with the truck in gear or park, and rock it forward and backward, listening for noises that indicate loose metal parts or grinding. The tire should move freely a few inches before the opposite wheel begins to turn. If one of the clutch packs has failed, metal parts will cause a grinding noise when the wheel is turned. Bearings and shims can also produce noises if they are substantially worn or damaged.
5) Shake the wheel from side to side, then up and down to see if there is any loose motion. If there is, an axle bearing has failed, and the differential will need to be disassembled for a repair to be made.
6) Inspect the pinion flange and driveshaft coupling. Vibrations and noise while driving can be caused by a universal joint or pinion gear bearing failure. From underneath the truck, with the vehicle out of gear, rotate the driveshaft forward and backward while feeling for and listening for sounds or loose play in the universal joints. From near the tail end of the drive shaft, any noises produced by manually rotating the pinion assemble should be easy to locate.
7) Road test the truck. Lower the truck to the ground, and prepare for a road test. You will want to locate a wide, flat, smooth area to road test the vehicle: a large, empty parking lot would be ideal. Drive the truck slowly across the pavement, making sharp turns both to the left and the right while listening through an open window for grinding noises. Since the wheels turn at different speeds from each other on a tight radius, this extreme turning may produce sounds indicating a problem.
8) Proceed to the next set of steps if you have decided a condition will require maintenance of your differential. Otherwise, top off the gear oil with a mixture of 75w90 gear oil and friction modifier and continue driving the truck as you normally would, paying attention to any sounds coming from the rear end while driving.
- Preparing to disassemble the rear end
1) Jack the truck up and support it with jack stands. You will need to get the back wheels as high as you can so you will have room to work underneath it. Make sure the emergency brakes are in good working order and, if possible, do this work on a concrete driveway or on a garage or shop floor. If you have to work on dirt or grass, it is a good idea to lay plastic sheeting down and then place two sheets of plywood beneath the truck so parts won’t be lost while you work.
2) Get an oil drain pan or other container to catch differential lubricant when the differential is disassembled. You will also want lots of clean rags or paper towels on hand.
3) Borrow or buy a differential tube adjusting wrench so the preload adjusters can be reversed. If you cannot find one, you won’t be able to remove the carrier and disassemble the differential. One can be made by welding a 1½ inch nut on the end of a 30 inch (76.2 cm) long piece of 1 inch (2.5 cm) steel tubing, and grinding the flats on a taper down to 1¼ inch.
4) Gather wrenches, including 12mm, 13,mm, 15mm, 17mm, and a 5⁄16 inch (0.8 cm) box end wrench, along with a pry bar, and a pair of locking pliers like Vice Grips. It helps to have a workbench with a solid wooden surface, a bench vise, and bearing pullers, as well. For removing bearing races from the axles and differential, you will also need a heavy (3 or 4 pound) hammer and a length of 1½ inch steel pipe.
5) Locate and price out a full parts list. You can wait until the tear down is complete if you like, but since some parts will require special ordering, it is better, if practical, to locate them before beginning the actual rebuild. For a complete rebuild, you will need:
- Axle bearings and seals
- Clutches and clutch retaining clips
- Differential bearings and races
- Pinion bearings, crush sleeve, and seal
- 3 quarts 75w90 gear oil and 4 ounces of posiloc friction modifier
Remove the drive shaft from the pinion flange. There should be four bolts, requiring a 15mm socket or wrench, holding it in place. You will need to pry the drive shaft toward the front of the truck to allow it to drop down, then the rear end can be swung around out of your way. Make sure you keep track of the bolts you have removed.
Remove the pinion nut. You may have to lock the emergency brakes to prevent the pinion gear from spinning while you break it loose. To remove the nut, you will need a 1¼ inch socket and a breaker bar.
Pull the pinion flange off the pinion gear splines and set it on a clean surface, then remove the pinion oil seal and the outer pinion bearing. A screwdriver or small pry bar may be used to pry out the oil seal.
Remove the differential cover. The bolts fastening the cover in place are 13mm (½ inch), and two should have tags on them, identifying the gear box ratio and assembly information. When all of the bolts are removed, position a large oil drain pan underneath differential cover, then pry the cover off slowly and carefully. When the cover breaks free from the gasket adhesive the oil will dump into the catch pan.
Inspect the differential carrier. You may want to spray an engine degreaser or use mineral spirits to clean the differential carrier after the oil has been drained. You will now be able to see the spider gear set, wrist pin, ring gear, and the back of the pinion gear. If you look closely, you can also see the clutches, behind the axle end gears. Look into the oil remaining in the bottom of the differential for metal parts or residue, and on the magnet located in the lowest part of the bottom for accumulated parts/metal residue.
Rotate the carrier until you see the wrist pin lock bolt. It will have a 5/16 head and be located on the right side of one end of the pin. Remove the lock pin by turning it counterclockwise until it is free of its threads and then pulling it out. Only the shoulder of the pin is threaded, the rest slides through the wrist pin lock bolt.
Push the wrist pin out of its hole until it is beyond the ends of the axles. Make sure it is rotated so that there is enough room to slide it far enough to clear the axle bolts without it binding on the pinion gear or the differential housing. If the wrist pin is pushed too far out, the spider gears may drop out of the carriers. This isn’t really desirable at this point, but if they do, simply collect them and their shims and place them in a safe, clean location.
Remove the lock clips (horseshoe shaped clips on the inside ends of the axles) by sliding the axles in, toward the center of the differential, and then rotating them. Make sure you have released your emergency brakes. You may want to remove the wheels and brake drums to make the axle assemblies lighter, although this isn’t absolutely needed.
Pull the axles out of the axle tubes. They will be coated with gear oil, so set them down on a clean, level surface to be prepared for reinstallation later. You can inspect the axle bearings and seals once you have removed the axles, but this is optional, and not a part of the differential rebuild. Still, if you suspect wear and tear on these parts, now is the time to deal with them.
Reposition the wrist pin in the center of the carrier, and thread the locking pin back into the whole far enough to prevent the wrist pin from sliding out again. This will keep the axle end gears and spider assembly together when the carrier is dropped out of the differential housing.
Remove the locking clips from each of the carrier bearing support caps. These are located in the center of each cap, and will require a 12mm wrench. They do not have to be completely removed, but only loosened enough so that the internal preloading nuts can be rotated. Taking them all of the way off will, however, make cleaning these parts a bit easier.
Slide the preloading nut tool through each axle tube and engage the nut on the end of the axle tube adjacent to the differential housing. You will turn these counterclockwise until the bearings are free, about 3 to 4 turns each. If you are not going to take time to set up the rear end later, you need to count exactly how far you back these nuts off so they can be returned to the same position during reassembly.
Remove the carrier bearing cap bolts. These hold the bearings which in turn support the carrier, which weighs nearly 50 pounds, so you may want someone to help support the carrier when these bolts are removed. Place each carrier cap with its bolts in separate places on their respective sides so they will be reinstalled the same way they came out.
These are cast iron clamps, with one side and one surface machined to fit, and should not be reversed.
Remove the carrier from the differential housing. Lay it on some clean rags and place it on a workbench or in a convenient place to do the bearing/clutch replacement later. Look at the ring and pinion gears for signs of significant damage now, to determine if their condition will allow favorable results in a rebuild. If there are severely damage teeth on either, or if the spider gears are damaged, it might be time to consider a complete replacement, rather than a rebuild.
Remove the pinion gear by tapping the threaded end with a soft faced hammer. The assembly should slide out without too much trouble, but if a heavier force is required, take care to use a wooden block or other protective method to prevent damage to the threaded end and splines.
Clean all of the parts you have removed thoroughly. You will want to be able to inspect the rollers in the bearings, the splines in the axle end gears, and the mating machined surfaces for obvious damage or signs of wear before continuing. Any serious damage to machined surfaces or gear teeth will require replacement parts not covered in detail in this article.
Remove the wrist pin as you did in Step 10, above. This will allow you to roll the spider gears out of the carrier, and to remove the axle-end gears. When removing the axle-end gears, keep the clutch plates and clips together. The flanged clutch plates fall in an alternating order between the grooved discs, and the clips keep everything in position. If there is no obvious damage to these parts, you may choose to reinstall them, but replacement kits are available from several different sources.
Remove the carrier bearings from the carrier. A bearing removal tool makes this easier: You simply slip half the tool over each side of the bearing, then install the bolts, and tighten them with their tapered edges forcing the bearing away from the mating surfaces. You can also tap the bearings off with a large cold chisel, rotating it and hitting it in different places to prevent it from binding as you go.
- Remove the pinion bearing and races
Remove the crush sleeve from the pinion gear. It should slide off easily, but may require light tapping with a hammer and punch. Set the pinion gear on a flat, solid surface and tap or press the bearing off the assembly.
Drive the pinion bearing races out of the differential assembly. You can use a section of steel pipe that fits through the housing from behind the race, or a solid bar, but make sure your tool fits squarely against the edge of the bearing race, and not on part of the housing. Tapping with a hammer as you move around the race should bring it out of the housing without too much trouble.
Flush the differential housing to remove oil and metal particles from it. You can use engine degreaser, or mineral spirits to accomplish this, but be careful since these are flammable. Use a container to catch the washout and dispose of it properly.
Inspect the gears and bearings for wear and damaged surfaces. Since gears are forged, then machined, they may have manufacturing imperfections like nicks in non-contacting surfaces, but the smooth faces of the gears should not have cracks, chips, or discolored areas that show evidence of overheating or binding.
Decide on whether the critical surfaces of the pinion gear and shaft are suitable for reinstallation into the differential. If not, you can order a replacement (usually paired with a matching ring gear of the same drive ratio as the original).
- Reassembling the pinion gear
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Install the new races in the pinion housing of your differential. If you do not have access to an installation tool, you can use the old races as a driving tool when you tap the races into place, or use a very large socket that is slightly smaller than the outside of the race. Tap the race around its circumference by working on opposite sides and keeping it aligned with the machined surface of the housing. If the race becomes off-centered, it may bind or even break, so proceed carefully.
Install the bearing on the pinion gear shaft, tapping it using the old bearing’s inner race or a section of smooth pipe the same outside diameter as the bearing’s inner race. Again, use care, since hitting the roller bearing cage or the bearings themselves will damage the assembly. After each bearing is installed on its respective shaft, lubricate it lightly with 75w90 gear oil.
Install the pinion gear into the differential housing. You may find it helpful to have a helper hold the pinion in place inside the differential housing while you install the crush sleeve and outer bearing. Once the pinion is installed, place the crush sleeve on from the outside, then set the outer bearing into position. It should slide onto the shaft with little resistance.
Install the outer pinion shaft flange onto the pinion shaft, then the washer, and finally the nut. Tighten the nut enough to hold the pinion assembly in place. If you have access to a wrench that will hold the flange to prevent it from turning, you can tighten the assembly enough to compress the new crush sleeve (it requires between 380 and 410 foot-pounds of torque, so it is very difficult to set the crush sleeve with ordinary tools). If not, you may have to wait until the carrier assembly is installed to lock the pinion in place while tightening it. Do not install the oil seal on the pinion shaft until final assembly (later on).
Tighten the pinion nut tightly enough to crush the sleeve to specification. If you are using a torque wrench and torque multiplier, use the above mentioned torque specs, otherwise, tighten until there are two or three threads showing outside the pinion nut with the flange pulled all of the way into the front of the pinion gear housing.
Back the pinion nut off enough that the pinion shaft can be turned by hand. You will need to set the bearing preload, this should be between 12 and 20 inch-pounds of torque. This is the force required to rotate the pinion gear, and the tighter the bearings, (hence, the pinion nut), the higher the preload torque. Setting the preload too high will cause the bearings to overheat, and subsequently fail, whereas, too loose will cause whining when the vehicle is driven.
Use an inch-pound torque wrench to adjust the pinion bearing preload. 15 inch (38.1 cm)-pounds of torque is generally sought, but a torque in the 12 to 20 inch-pound range is acceptable.
- Replacing the bearings and carrier assembly
Install the new bearings on your carrier assembly. You can press these into place if you have a hydraulic press and suitable dies, otherwise, you can simply tap them in, using the same technique as with the pinion bearing. Make sure the taper is set with the larger diameter toward the carrier housing.
Reinstall the old clutches (or new ones), keeping the C-clips in position as you slide the axle-end gears and clutches into position. Once they are in place, you will need to hold the carrier in a level position while installing the spider gears and shims. Using a sturdy table with a cloth to hold the assembly may help.
Install the spider gears. Make sure the shims (concave washers that fit between the smooth face of the spider gear and the housing) are in position as you rotate both gears in opposition to each other so they mesh with the axle end gears. When the spider gears rotate so they are aligned with the holes in the carrier, you can reinstall the wrist pin.
Install the wrist pin, making sure the retaining pin hole is on the side of the carrier with its corresponding hole, and turned to align with it. Set the retaining pin into the hole and thread it a short distance to hold the spider gears in place.
Paint gear-marking paint on the ring gear’s mating surface if you are going to reset the gear position. Not setting up the gears may cause the ring and pinion interface to be out of tolerance, since new bearings and wear can change the dynamics of the ring and pinion gear’s contact. Since you have a preload reference for the gear assemblies, you may decide to take a chance replacing the carrier in its original position.—that choice is up to you.
Install the carrier into the differential housing. You will want to get it into position facing the bearing sockets resting on the housing bottom before installing the outer races. Otherwise, they will fall off while it is being handled. Slide the races on the bearings, and then shove the carrier into position.
Install the carrier bearing caps onto the carrier bearings, and install the bolts that fasten them into place. You won’t need to fully tighten the bolts yet, if you are going to set up the ring and pinion gears using the gear marking paint mentioned earlier.
Using the internal axle tube wrench, turn these nuts back the same number of turns you backed them off earlier. If the bearings fit too tightly to allow the full number of turns on each side, decrease each side the same amount so that they are near the same center as before. This would be unusual, but is possible if the new bearings are slightly wider than the original equipment.
Check the carrier for side-to-side play. If you can pry the carrier from left to right and right to left, there isn’t enough preload on the assembly, so you will need to tighten the preload adjusting nuts more, going the same amount from each side. A small amount of end play may be tolerated when the carrier is set.
Use a dial indicator to determine if the ring gear is turning in a true fashion if you want. This will let you know if the ring gear or carrier has been warped, and, after the true-ness of the ring gear is established, you can use the dial indicator to measure the gear run out or distance from side to side it can travel.
Turn the ring gear one full turn forward, then one full turn reverse, to mark the gear contact pattern in the gear marking paint you applied earlier, if you are going to do this. This will ensure the pinion gear is set in properly and that its teeth engage the ring gear correctly. You can use an online chart or a reference guide to compare the wear pattern to determine if your gears are contacting correctly.
Look behind the carrier to see if the pinion gear’s teeth are set flush with the end of the ring gear teeth if you are not using gear marking paint to establish their position. If the teeth of the pinion gear are sticking out past the end of the ring gear teeth, the crush sleeve hasn’t been tightened sufficiently. If the pinion gear teeth do not reach the toe end of the ring gear teeth, the sleeve has been crushed too far and will have to be replaced.
Retest the torque on the pinion shaft with the ring gear engaged with the pinion gear. The preload on the combined ring gear bearings and pinion gear bearings should be about 45 inch-pounds. If it is higher, you can back off the preload nuts on the carrier slightly, if it is lower, tighten these a bit.
Reinstall the axles when you are satisfied the ring and pinion gears are set up correctly and the bearing pre loads are adjusted to specification. If you haven’t torqued the pinion nut to crush the sleeve, you will have to install the axles to lock the pinion gear shaft in place to do so, then fall back to earlier steps in order to preload the pinion bearings and set up the ring and pinion gear.
Remove the lock pin from the carrier wrist pin. Slide the wrist pin out far enough to allow the axles to be installed far enough for the axle retainer clips to fit the groove at the end of each axle.
Slide the axles back into the carrier, fitting the splines in the axle end gears, and pushing through so the clip grooves are beyond the inside edge of the end gears. If the clutch spines are not perfectly aligned, you may have to twist the axle back and forth as you apply a steady pressure to get everything engaged properly. Do not tap or bump the axle, as this can warp or damage the thin clutch discs.
Reinstall the wrist pin and the wrist pin retaining pin and tighten it snugly. Some people use thread locking compound on the retaining pin but if you tighten it sufficiently, it shouldn’t be needed.
Tighten the bearing caps for the carrier bearings, then the preload nut retaining clips. Check the torque specifications in the service manual for specific torque pounds for each bolt.
Clean the differential cover when you are satisfied with the set up and preload on the carrier bearings, ring gear, and pinion gear and bearings. Use a silicone gasket adhesive, and reinstall the cover. Tighten the cover bolts uniformly and allow the adhesive to set.
- Installing the pinion oil seal and final assembly
Remove the pinion flange and nut. Install the pinion shaft oil seal by tapping it into place. Put a little grease on the rubber sealing surface to protect it during installation of the pinion flange.
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Install the pinion washer and nut. Use red thread locking compound on the pinion nut to make sure it doesn’t back off during driving. Tighten it to specified torque (80-90 foot-pounds), then turn the pinion to make sure the assembly rotates at the previous preload setting.
Reinstall the drive shaft, using thread locking compound on the fastening bolt threads. Tighten them securely.
Fill the differential with 75w90 gear oil, adding the friction modifier between the first and second quart of gear oil. The capacity of the differential is about 2⅔ quarts, total.
Remove the jack stands and lower the vehicle back to the ground. You can now test drive it to see if your set up is correct. Grinding, roaring, or other noises from the rear end of the vehicle indicate an incorrect gear set up or bearing preload. If you hear these, cut the test drive short and disassemble the differential to look for problems.