In slippery road conditions, an open differential sends the torque to the wheel with the least amount of resistance and therefore traction is limited. You have probably experienced the single-wheel burnout.
The easiest way to check if you have a limited-slip differential and if it is still somewhat functioning is to jack the rear wheels off the ground. With the transmission in park, rotate one tire forward and watch the direction of the other tire. If it spins in the same direction, then you have a limited-slip differential. If it spins in the opposite direction, you have an open differential or a worn-out limited-slip unit.
This Tech Tip is From the Full Book, FORD DIFFERENTIALS: HOW TO REBUILD THE 8.8 AND 9 INCH. For a comprehensive guide on this entire subject you can visit this link:
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Step-1: Pre-Lube Clutch Plate Kit (Professional Mechanic Tip)
Before assembling the differential, soak the friction plates in friction modifyer for at least 30 minutes and ideally overnight. Make sure not to stack them so they are properly coated. This soaking process helps prevent the new clutch pack from chattering after assembly. I am installing the carbon fiber clutch plates that were first available on the 2003–2004 Cobra Mustangs and then the 2005–2013 GTs along with 2007-on GT500s. You can purchase a Ford Racing rebuild kit (PN M-4700-C) that includes six carbon friction plates, eight steel reaction plates, two shims, preload spring, and differential pin-retention bolt for about $115. It features a clutch plate pack that is premeasured and has the correct shims, which takes the measuring and guesswork out of the process. I highly recommend purchasing this new clutch plate kit for your performance rebuild.
Step-2: Purchase Friction Modifier (Important!)
Always use the correct friction modifier. The Ford friction modifier can be purchased from a local Ford dealer or performance parts shops, such as Summit Racing or Jegs. The modifier also helps to hold the plates together during the assembly process. One important note here: If you are using a new pre-shimmed clutch pack, do not mix up the plates and shims. They must be kept together to ensure the correct clutch plate stack thickness. Soak the plates in a container.
Step-3: Arrange Clutch Pack
Over the years, clutch plates and steel reaction plates have come in many different orders. The most recent and effective is shown here (left) (from the gear to the shim): side gear, steel reaction plate, friction plate, steel reaction plate, steel reaction plate, friction plate, steel reaction plate, friction plate, and shim. So this clutch pack includes four steel plates, three friction plates, and one .020-inch-thick shim. Notice that the steel reaction plates are splined on the inside diameter to align with the side gear; the friction plates are tabbed to align with the pockets in the differential case. Ideally you want the entire clutch pack, including the shim, to be about .640 to .645 inch for a stock clutch pack and as thick as .655 inch for a performance pack (right). In my opinion the thicker pack performs better and a few pops during turns from the clutch pack are acceptable because you know what is causing the pops and are not worried. Be sure you understand the pops if you are going to use the thicker pack values.
Here are all of the internal parts and gears of the limited-slip differential assembled just for visual reference. Keep in mind, you cannot install them in the differential case like this, but when the assembly has been rebuilt, all of these parts are arranged as shown. I leave the side gears and clutch packs pre-assembled as shown because they are ready to be installed in the differential case. Also the spherical pinion washers resting on the differential pin are important to include during the re-assembly process because they act as the wear surface between the hardened pinions and the differential case. They are curved to match the differential case and back surface of the pinions.
Step-4: Inspect Differential Pocket
The differential pocket for the clutch plate alignment looks like this. The clutch plate tabs align with and run against this semi-circular pocket.
Step-5: Install Side Gears and Clutch Packs
Install the completely assembled side gear with clutch pack into the differential case. Because there is no preload on pinion gears in the differential case, this is easy. The factory-painted orange side gears distinguish themselves from other gears.
Install the opposite-side gear with its clutch pack and shim in place. At this stage, nothing is holding the side gears in place, so they may fall out. Some people install long bolts with washers to help temporarily hold the gears in place. I typically use a 1/2-inch bolt with large washers that span the 6-inch-long holes. Since this is just an assembly aid, the nut just needs to be hand tight. A second set of hands helps, but I have been able to do it by myself; it just takes more time and patience.
Step-6: Install Pinions
Install the differential pinions with their spherical thrust washers in the differential case. Since the gears are just being partially engaged you can assemble them by hand. Do not forget to include the thrust washers that go between the differential case and the pinion gears. Install both pinions at the same time and rotate the side gear to get the pinions to roll in place. (The home-made tool that was shown on page 39 is used to rotate the side gear. It is just a piece of an old axle shaft with a square plate welded to it.) Notice that this pinion gear thrust washer is in place between the pinion and the differential case.
Step-7: Install Differential Pin (Professional Mechanic Tip)
The pinion gears must be aligned correctly, so the differential pin can slide right into place. Use a punch to push it through the pinion gear and into the correct position. Keep in mind, it is possible to assemble the differential gears one tooth off. If installed one tooth off, the differential pin does not line up correctly (left). You want the pinions to be 180 degrees apart and aligned to the correct side gear. So take the time now to make certain that the pin goes easily in place (right). It will be obvious if you are a tooth off. If so, just remove the pinions and index by one tooth with the side gear.
Step-8: Install Preload Spring
Once the differential gears align correctly, remove the differential pin and install one of the two S-shaped preload springs that came with your kit. For reference, the Ford car spring (PN E0AZ-4214-A) has a free height of approximately 1.510 inches and a Ford Racing truck or heavy-duty version (PN F3TZ-4214-A) is approximately 1.765 inches. The car unit was used from 1985 to 2000 while the truck version was originally used in the 1999 Cobra and all 2000-and-up V-8 Mustangs. (I always use a heavy-duty spring for any high-performance build because they are about the same price at about $2.) Keep in mind that the downside to a tightly assembled and heavily preloaded limited-slip differential is additional wind-up and potential chatter and pops as the plates transition from sticking to slipping. Wear safety glasses.The spring needs to evenly descend into the assembly and between the side and pinion gears. If the spring doesn’t start correctly, it may jump out. You may want to place a shop rag over the spring to help keep it contained as you tap the spring into place with a hammer. You can also use channel-lock-style pliers or a C-clamp to pre-compress the spring. I have found that these aids just get in the way and make assembly more difficult. But some mechanics swear by the method of pre-collapsing the spring in a bench vise and holding it in this state with needle-nose Vise-Grips through a portion of the S-shape and then setting it partially in place prior to final placement with a soft-face hammer or brass punch.
Step-9: Double-Check S-Spring is Centered
Once the spring has been installed, visually check that it is centered. Trial fit the differential pin again. Make sure the through hole in the differential pin is aligned with the hole in the differential case for the retention bolt. If it is not, use the retention bolt to rotate the pin to the correct orientation. This ensures that everything is aligned.
Step-10: Push Pin in Place
When correct alignment of the S-spring has been verified, remove the retention bolt and push the pin in the remaining amount. Be careful not to push the pin in too far; it needs to be centered in the differential case so that the through hole aligns with the retention bolt (shown). Temporarily install the retention bolt. Hand tighten the bolt by just a couple of threads to hold the pin in place. The Traction-Lok limited-slip differential assembly is now completely assembled. You can set this aside now and concentrate on the axle housing.
Written by Joe Palazzolo and Republished with Permission of CarTech Inc