1998 Expedition 4AWD engaugeing at low speed

How about the water pump ? Who replaced the parts ? A qualified repair shop ? Were codes checked ?
The PCM controls the electro-viscous fan clutch engagement. The PCM regulates a 12-volt pulse width modulated signal (PWM) to the cooling fan relay. The PWM signal determines the ON time of the relay. As the commanded state of the fan clutch increases, so does the ON time of the relay. This ON time directly effects the amount of time the solenoid, which is internal to the fan clutch, is energized. When the solenoid in the fan clutch is energized, it opens the spring loaded valve and allows fluid to flow from the storage chamber to the fluid coupling of the cooling fan clutch, increasing the fan speed. When the solenoid is de-energized, the spring loaded valve closes, and blocks the path of the fluid to the fluid coupling of the fan clutch, reducing fan speed.
The fan has the ability to create a feedback signal, so the PCM has an actual fan speed input. This is done with a hall effect sensor internal to the fan clutch. The PCM supplies a 5-volt reference and a low reference to the hall effect sensor. The hall effect sensor returns a signal pulse through the cooling fan speed signal circuit in response to the relucktor track passing by the magnetic field of the hall effect sensor.
Was electrical tested before fan clutch was replaced !

Check the
Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 43) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling. That same VSS signal is use for the tachometer as well !

If lights are on, trouble codes are stored. Get he codes read. That's always the starting point.

the vss is a variable reluctance or hall effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. if the vehicle is moving at a relatively low velocity , the sensor produces a signal with a low frequency. as the vehicle velocity increases,the sensor generate a signal with a higher frequency. the powertrain control module uses the frequency signal generated by the vss (and other inputs) to control such parameters as fuel injection, ignition control, transmission shifting and torgue converter clutch scheduling

I'm assuming you have Transfer Case -Automatic Four Wheel Drive (A4WD) An not Transfer Case -All-Wheel Drive (AWD) Two different system's . The automatic system is electroniclly controlled . When it has electrical problems it set DTC'S - diagnostic trouble code's in the 4X4 control module . Before replacing part's or trying this an trying that , having the vehicle checked for DTC'S is always the first step in any diagnostic's with these vehicles . Then if not familiar with this system read factroy service repair information . Trying to fix not knowing what your doing never works .
System Function
Hall effect sensor monitors input and output shaft speed.
In the A4WD (C-Trac) system, the GEM varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the GEM activates the transfer case clutch a minimum duty cycle (percentage of time the clutch is turned on), which allows for the slight difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. Under any of the following conditions, the GEM will increase the duty cycle in order to prevent or control slip:

• Slip is detected (by using the Hall Effect Sensors)
• Heavy acceleration (throttle position).

Feature inputs:

• Brake ON/OFF switch.
• 4WD mode switch.
• Digital transmission range sensor.
• Clutch switch (manual transmission vehicles only).
• Vehicle speed signal (transmitted from the ABS module).
• Front/rear driveshaft speed sensors.
• Throttle position information from the PCM.
• Contact plate position inputs A, B, C, D.

Feature outputs:

• Solid state clutch relay (pulse width modulated signal: grounded when relay is on, battery potential when relay is off).
• A4WD indicator (ground when activated, open circuit when deactivated).
• 4x4 shift motor relay outputs.

Shifts between A4WD and 4x4 HIGH can be made at any speed. Listed below are the inputs and outputs needed by the GEM to execute a change between any of these modes.
Feature inputs:

• Front/rear driveshaft speed sensor.
• Vehicle speed signal.
• Throttle position information.
• 4WD mode switch.

Feature outputs:

• Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).
• 4x4 HIGH cluster indicator (ground when activated, open circuit when deactivated).

When shifting into or out of LOW range, the GEM requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature inputs:

• Throttle position information.
• 4WD mode switch.
• Contact plate position inputs A, B, C, D.
• Vehicle speed signal (transmitted from ABS module).
• Brake ON/OFF switch input (battery voltage when brake is depressed, open circuit when not activated).
• Digital transmission range sensor (ground when transmission is in NEUTRAL, open circuit otherwise).
• Start/clutch depressed input (manual transmission only).

Feature outputs:

• 4x4 shift motor relay outputs.
• 4LOW cluster indicators (ground when activated, open circuit when deactivated).
• Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).
• The GEM they talk about is a generic electronic control module .
• Do you know what DTC'S are ?
• P1763 Transmission Neutral In Tow Indicator Circuit Short to Battery GEM REFER to Section 413-09 . P1764 Transmission Neutral In Tow Indicator Circuit Fault GEM REFER to Section 413-09 . P1804 4WD High Indicator Circuit Failure GEM GO to Pinpoint Test C . P1806 4WD High Indicator Short to Battery GEM GO to Pinpoint Test C . P1808 4WD Low Indicator Circuit Failure GEM GO to Pinpoint Test C . P1810 4WD Low Indicator Short Circuit to Battery GEM GO to Pinpoint Test C . P1812 4WD Mode Select Circuit Failure GEM GO to Symptom Chart. P1815 4WD Mode Select Short Circuit to Ground GEM GO to Symptom Chart. P1820 Transfer Case CW Shift Relay Coil Circuit Open or Short to Ground GEM GO to Pinpoint Test B . P1822 Transfer Case CW Shift Relay Coil Short to Battery GEM GO to Pinpoint Test B . P1824 4WD Electric Clutch Relay Circuit Failure GEM GO to Pinpoint Test A . P1826 4WD Electric Clutch Relay Short to Battery GEM GO to Pinpoint Test A . P1828 Transfer Case CCW Shift Relay Circuit Failure GEM GO to Pinpoint Test B . P1830 Transfer Case CCW Shift Relay Coil Short to Battery GEM GO to Pinpoint Test B . P1836 Transfer Case Front Shaft Speed Sensor Circuit Failure GEM GO to Symptom Chart. P1837 Transfer Case Rear Shaft Speed Sensor Circuit Failure GEM GO to Symptom Chart. P1838 Transfer Case Shift Motor Circuit Failure GEM GO to Pinpoint Test B . P1846 Transfer Case CONTACT PLATE "A" Circuit Failure GEM GO to Pinpoint Test B . P1850 Transfer Case CONTACT PLATE "B" Circuit Failure GEM GO to Pinpoint Test B . P1854 Transfer Case CONTACT PLATE "C" Circuit Failure (GEM Only) GEM GO to Pinpoint Test B . P1858 Transfer Case CONTACT PLATE "D" Circuit Failure (GEM Only) GEM GO to Pinpoint Test B . P1866 Transfer Case Maintenance Required (GEM Only) (4-wheel drive operation can be restored by clearing the DTC and cycling the ignition.) GEM GO to Pinpoint Test B . P1867 Transfer Case Contact Plate General Circuit Failure (GEM Only) GEM GO to Pinpoint Test B . P1874 Automatic Hall Effect Sensor Power Circuit Failure GEM GO to Symptom Chart. P1875 Automatic Hall Effect Sensor Power Circuit Short to Battery (GEM Only) GEM GO to Symptom Chart. P1891 Transfer Case Contact Plate Ground Return Open Circuit GEM GO to Pinpoint Test B .
• Your best bet , have it towed to a qualified repair shop . You have not the knoweldge or diagnostic equipment to fix it.

E150?

Vehicle Speed Sensor The Vehicle Speed Sensor (VSS) (Figure 36) is a variable reluctance or Hall effect type sensor that generates waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition timing, transmission/transaxle shift scheduling and torque converter clutch scheduling.


Typical VSS

Yours is mounted on the rear axle at the differential (pig).

wires are:

Red/Pink

LG/BK

No ,not tunex . Qualified repair shop . Code's stored in 4X4 control module. Need a factory or some other professional scan tool to read code's .
SECTION 308-07A: Transfer Case - General Information 2000 Explorer/Mountaineer Workshop Manual DESCRIPTION AND OPERATION Procedure revision date: 06/17/1999 Transfer Case -Automatic Four Wheel Drive (A4WD) The A4WD system is an electronic shift 4x4 system that allows the operator to choose between three different 4x4 modes. The operator can switch between A4WD and 4WD HIGH modes at any speed. To engage or disengage LOW range, the vehicle speed must be less than 5 km/h, the brake depressed, and the transmission must be in NEUTRAL.
The front/rear driveshaft speed sensors (Hall effect) are mounted to the transfer case and detect the speed of the front and rear driveshafts. This input is used by the generic electronic module (GEM) primarily to control A4WD operation.
The throttle position output signal is provided to the GEM from the powertrain control module (PCM). This signal is used by the GEM in controlling the A4WD clutch.
The shift motor sense plate, an integral part of the electric shift motor, informs the GEM to the position of the transfer case.
The digital TR sensors are located on the LH side of the transmission; these sensors inform the GEM when the transmission is in the neutral position.
The electric shift motor is mounted externally at the rear of the transfer case. It drives a rotary cam which moves the mode fork and range fork within the transfer case between the HIGH range (A4WD, 4HIGH) and LOW range positions.
The 4x4 shift motor uses a module containing two relays which, under the control of the GEM, shifts the transfer case shift motor between HIGH range (A4WD, 4HIGH), and 4LOW modes.
The solid state clutch relay is used to activate the A4WD clutch within the transfer case.
System Function
Hall effect sensor monitors input and output shaft speed.
In the A4WD (C-Trac) system, the GEM varies the torque sent to the front driveline by controlling the transfer case clutch. At rest and under cruising conditions, the GEM activates the transfer case clutch a minimum duty cycle (percentage of time the clutch is turned on), which allows for the slight difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. Under any of the following conditions, the GEM will increase the duty cycle in order to prevent or control slip:

• Slip is detected (by using the Hall Effect Sensors)
• Heavy acceleration (throttle position).

Feature inputs:

• Brake ON/OFF switch.
• 4WD mode switch.
• Digital transmission range sensor.
• Clutch switch (manual transmission vehicles only).
• Vehicle speed signal (transmitted from the ABS module).
• Front/rear driveshaft speed sensors.
• Throttle position information from the PCM.
• Contact plate position inputs A, B, C, D.

Feature outputs:

• Solid state clutch relay (pulse width modulated signal: grounded when relay is on, battery potential when relay is off).
• A4WD indicator (ground when activated, open circuit when deactivated).
• 4x4 shift motor relay outputs.

Shifts between A4WD and 4x4 HIGH can be made at any speed. Listed below are the inputs and outputs needed by the GEM to execute a change between any of these modes.
Feature inputs:

• Front/rear driveshaft speed sensor.
• Vehicle speed signal.
• Throttle position information.
• 4WD mode switch.

Feature outputs:

• Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).
• 4x4 HIGH cluster indicator (ground when activated, open circuit when deactivated).

When shifting into or out of LOW range, the GEM requires that the vehicle speed is less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature inputs:

• Throttle position information.
• 4WD mode switch.
• Contact plate position inputs A, B, C, D.
• Vehicle speed signal (transmitted from ABS module).
• Brake ON/OFF switch input (battery voltage when brake is depressed, open circuit when not activated).
• Digital transmission range sensor (ground when transmission is in NEUTRAL, open circuit otherwise).
• Start/clutch depressed input (manual transmission only).

Feature outputs:

• 4x4 shift motor relay outputs.
• 4LOW cluster indicators (ground when activated, open circuit when deactivated).
• Solid state clutch relay (pulse width modulated signal; grounded when relay is on, battery potential when relay is off).

does your cruise control work? If not, check the wiring from the VSS to the speedo cluster. Check VSS connector closely for pin pudhout or corrosion

Sometimes, the VSS siganl is converted by the instrument cluster and sent to the PCM and speedo.

Check engine light on?


Speedometer
The electronic speedometer (17255) receives a speed signal from the vehicle speed sensor (VSS) (9E731). This sensor is also used to sense vehicle speed for the engine management system and to regulate the cruise control when equipped.
----------------------------------------------------------------------------------
Vehicle Speed Sensor The Vehicle Speed Sensor (VSS) (Figure 53) is a variable reluctance or Hall-effect type sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.

the flashing O/D light usually means you have a bad speed sensor that sends speed info to the engine & trans computer.

Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.
-------------------------------------------------------------------
Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 42) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.

-------------------------------------------
Some trans use an OSS, Output Shaft Sensor instead of a VSS. Depends on which trans you have, 4R100 or 4R70W

Transmission Control Switch (TCS) and Transmission Control Indicator Lamp (TCIL)
The transmission control switch (TCS) is a momentary contact switch. When the switch is pressed, a signal is sent to the powertrain control module (PCM) to allow automatic shifts from first through fourth gears or first through third gears only. The PCM energizes the transmission control indicator lamp (TCIL) when the switch is off.

The TCIL indicates overdrive cancel mode activated (lamp on) and electronic pressure control (EPC) circuit shorted (lamp flashing) or monitored sensor failure.

Output Shaft Speed (OSS) Sensor
The output shaft speed (OSS) sensor is a magnetic pickup, located at the output shaft ring gear, that sends a signal to the powertrain control module (PCM) to indicate transmission output shaft speed. The OSS is used for torque converter clutch control, shift scheduling and to determine electronic pressure control.

if this helps, please rate as fixya. thanks.

Vehicle Speed Sensor The vehicle speed sensor (VSS) (Figure 43) is a variable reluctance or Hall-effect sensor that generates a waveform with a frequency that is proportional to the speed of the vehicle. If the vehicle is moving at a relatively low velocity, the sensor produces a signal with a low frequency. As the vehicle velocity increases, the sensor generates a signal with a higher frequency. The PCM uses the frequency signal generated by the VSS (and other inputs) to control such parameters as fuel injection, ignition control, transmission/transaxle shift scheduling and torque converter clutch scheduling.



For the 4R70W trans vehicles

Vehicle Speed Sensor (VSS)

The vehicle speed sensor (VSS) is model dependent. Possible sources of vehicle speed input are, anti-lock brake sensor (ABS), a gear-driven vehicle speed sensor (VSS), or the transmission output shaft speed (OSS) sensor. On 4x4 applications with a manual shift transfer case, the transfer case speed sensor is the source of vehicle speed. The VSS signal is either an AC signal whose frequency changes with vehicle speed, or an SCP data message depending on the source. Some vehicles have both. The vehicle speed signal is an input to various vehicle sub-systems such as the powertrain control module (PCM), instrument cluster (speedometer and odometer), speed control systems, etc. The vehicle speed source must be operational to enter output state control mode for diagnostics.

If you have a 4R100 trans, it is different.