WIKI
Traction Lock
TRAC-LOC uses clutching
Traction Control:
Ford Expeditions, whether they are rear-wheel drive or all-wheel drive come from the factory with conventional open-type differentials with a choice of axle ratios. Traction control systems, both mechanical and electronic, were offered as options if needed. Only until recently has traction assist become standard on all Expeditions.
Mechanical traction control (1997–2006)
Both first and second generation Ford Expeditions (UN93 and U222) were offered with an optional Traction-Lok rear limited-slip differential (LSD) with a choice of axle ratios. When rear-wheel slip (spin) was detected, a simple clutch pack inside the differential would engage to transfer torque between the rear-wheels to whichever rear-wheel had better traction. The clutch packs inside the LSD tend to be weak, and they will wear over time, gradually losing their ability to transfer torque. Eventually, if they are not replaced, the LSD will function like an open-type differential.[50] Furthermore, the front differential on four-wheel drive models was always an open-type differential, with no way to control wayward torque flow caused by traction loss. Both these problems were remedied with the arrival of AdvanceTrac electronic stability control for the 2003 model year. 2006 was the last model year for the Traction-Lok limited-slip differential.
Electronic traction control (2003–present)
AdvanceTrac electronic stability control was introduced for the 2003 model year as an optional extra on the second generation Ford Expedition (U222). Bundled with the electronic stability control system is four-wheel electronic traction control (functions at all wheels) for Expeditions with four-wheel drive.[15] Two-wheel drive Expeditions only receive two-wheel electronic traction control (functions at rear wheels). AdvanceTrac was made standard equipment on all third generation Expeditions (U324).
The traction control system uses the Expedition's four-wheel, four-sensor, four-channel anti-lock braking system (ABS) to apply and release the disc brakes on any drive wheel which has lost traction. The ABS brakes aggressively clamp down on the slipping drive wheel(s) "brake locking" the differential, which can transfer up to 100 percent of supplied torque to the opposing drive wheel with better traction. By ABS "brake locking" either the front or rear differentials, the traction control system can simulate front and rear differential locks allowing a four-wheel drive Expedition to keep moving, even if only one wheel has traction, no matter which wheel it is. The system will also allow the Expedition to keep moving even if two of its drive wheels are airborne, completely off the ground.[16][15]
Using the ABS brakes rather than mechanical limited-slip or locking devices inside the axle differentials gives quicker response, more seamless performance, and enhanced durability.[16][15]
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