I have a 1999 Ford Expedition with a 5.4L Engine and don't know what you are talking about. I searched under mileage sensor for the 99 expedition but could only find On-Board Warning Systems.
Are you talking about the Vehicle Speed Sensor?
If so, this is located near or on the transmission and has wires that go into the harness, and basically is a pickup coil that detects movement of the transmission which then controls mileage on the odometer and the speedometer.
On-Board Warning Systems
Turn on the ignition switch in almost any vehicle and watch the instrument panel. The modern automobile has an abundance of warning lights that provide valuable information. The list could include:
Brake system warning
Windshield washer fluid level
Brake fluid level
Headlamp door position
EGR or Check Engine
High beam indicator
Cold engine warning
ABS or Anti-lock Brake System
SRS or Supplemental Restraint System
Brake lining wear indicator light.
The high price of fuel created a demand for yet another light--the fuel economy warning system. When the light comes on, it tells the driver he or she is pushing too hard on the gas pedal. High manifold vacuum equals good gas mileage and vice versa. The system simply reads manifold vacuum from a sensor, and when it drops to a predetermined level, a circuit is completed and the light is lit. This has become in many models what is known as the up-shift light. An arrow on the dash indicates the time to shift into the next gear to obtain optimum fuel economy.
If a warning light comes on, you must find out why. It means there is a problem either in the system being monitored or in the warning lamp circuit. Finding the actual fault is important and not very difficult. However, a wiring diagram may be needed to prevent confusion.
Looking at the typical warning light circuit, you'll see that the bulb is most often supplied with current through the ignition switch. Further examination reveals the most common way of completing the circuit and getting the bulb to light is by means of a sensor, which completes the ground connection. In this case, sensor is a fancy word for a switch that turns on or off according to specific conditions.
Consider a typical oil pressure warning light system. Current from the ignition switch flows through the warning lamp and from there to ground through the oil pressure switch. This particular switch is normally closed and the circuit is complete until the switch opens in response to oil pressure in the engine.
Just the opposite is true with the coolant temperature sensor. It is normally open and only completes the circuit when an internal element expands (in response to heat) to close the contacts. If the vehicle has a cold engine warning lamp, the sensor includes two sets of contacts. One set is normally closed and opens as the internal element expands. This action breaks the ground circuit to the warning lamp. The other set of contacts functions if the temperature rises far enough to close them, turning on the high temperature warning.
If either of the temperature lights is lit while the engine seems normal, just unplugging the wires from the sensor will provide valuable diagnostic information. If the lights remain lit, there's a short to ground in the wiring from the lamp to the sensor. The service needed isn't to the cooling system, but to the warning system.
If the lights go out with the wires unplugged and the engine seems normal, it's entirely possible that the sensor has failed and needs to be replaced. Nevertheless, don't just unplug the wires and forget about them. This could be disastrous for the vehicle owner should a cooling problem develop without warning.
Fig. 1: Typical coolant temperature sending unit location
Fig. 2: Oil pressure sending units may be located in a variety of places on the engine