Thunderbird IRS Traction-Lok Limited Slip

The independent rear suspension (IRS) found on 1989-1997 Thunderbird's and Lincoln Mark VIII's are different from the well known solid axle's found on Mustangs (FOX or SN95), and other rear wheel drive Fords. I will refer to the MN12 Thunderbirds differential here only. The differentials for the MN12 Thunderbirds come in two sizes 7.5 and 8.8 (diameter of the ring gear). All Thunderbird SC's came with 8.8 differentials and depending on the transmission (automatic or manual) either 2.73, 3.08, or 3.27 ring and pinion gear set. If you decide on replacing or upgrading your Traction-Lok Limited Slip there are some important things to keep in mind. When upgrading the 8.8 Traction-Lok unit you will find a lot of 8.8 units that are made to fit the solid axle cars but, will not fit the IRS cars. The MN12 8.8 and 7.5 differentials use a 28-spline shaft. The difference between the standard solid axle and the IRS differential is a "Step" (grove) in both differential side gears in order to retain the Circlip (C clip) that holds the Halfshafts in place (see diagram). A "Step" is a grove that will allow the Circlip (C-Clip) to slip over the Halfshaft and secure the Halfshaft in the differential. The differential has precise tolerances and should not be disassembled or assembled without the proper tools and experience. When replacing or adding a Traction-Lok Limited Slip to your SC that may or may not have one installed the easiest ways is to get a Traction-Lok Limited Slip unit from Auburn Gear Inc. 219-925-3200. High Performance unit for standard OE replacement part# 542079 or Pro Series unit part# 542080 for better traction and faster engagement.  The average street price for the High Performance unit is $340.00 and $410.00 for the Pro Series. Torsen also sells a limited slip differential through Ford Racing part number M-4204-T28 unit is $450.00 through Ford Racing   By, Rich Thomson

Description:

  • The Traction-Lok differential employs two sets of multiple disc differential clutch packs to control differential action.
  • The differential side gear mounting distance is controlled by seven plates (four steel, three friction) and one steel shim by select size to control differential side gear position.
  • The plates are stacked on the differential side gear hub and are housed in the differential case.
  • Also located in the differential case, between the differential side gear, is a one-piece differential clutch spring which applies an initial force to the differential clutch packs.
  • Additional clutch capacity is delivered from the side gears' thrust loads.
  • The four steel plates are stacked in pairs between the three tanged friction plates and are splined to the differential side gear hub. The hub, in turn, is splined to the halfshaft.
  • The friction plates are tanged to the differential case so the differential clutch packs are always engaged.

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    Differential action is accomplished as follows:

  • The pressure between differential clutch packs opposes differential action at all times.
  • When the vehicle turns a corner, the differential clutch pack slips, allowing normal differential action to take place.
  • Under adverse weather conditions, where one or both wheels may be on a low-traction surface such as snow, ice or mud, the friction between the clutch plates will transfer a portion of the usable torque to the wheel with the most traction. The wheel that is on ice or snow will have a tendency to operate with the opposite wheel in a combined driving effort.

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    8.8 Differential Rebuild Kits: