What light's don't come on ? 4X4 switch ? The GEM - generic electronic module controls the 4X4 system . Checking for DTC'S -diagnostic trouble codes is the first step in the diagnostic process . You miss thing's if you don't . Your best bet , take it to a qualified repair shop. Replacing parts without knowing what's going on never works .
P1804
Transmission 4-Wheel Drive High Indicator Circuit Failure
GEM
REFER to Section 413-01A .
P1806
Transmission 4-Wheel Drive High Indicator Short Circuit to Battery
GEM
REFER to Section 413-01A .
P1812
Transmission 4-Wheel Drive Mode Select Circuit Failure
GEM
REFER to the Symptom Chart.
P1815
Transmission 4-Wheel Drive Mode Select Short Circuit to Ground
GEM
REFER to the Symptom Chart.
P1820
Transmission Transfer Case Clockwise Shift Relay Coil Circuit Failure
GEM
GO to Pinpoint Test A .
P1822
Transmission Transfer Case Clockwise Shift Relay Coil Short Circuit to Battery
GEM
GO to Pinpoint Test A .
P1824
Transmission 4-Wheel Drive Clutch Relay Circuit Failure
GEM
GO to Pinpoint Test E .
P1826
Transmission 4-Wheel Drive Low Clutch Relay Short Circuit to Battery
GEM
GO to Pinpoint Test E .
P1828
Transmission Transfer Case Counter Clockwise Shift Relay Coil Circuit Failure
GEM
GO to Pinpoint Test A .
P1830
Transmission Transfer Case Counter Clockwise Shift Relay Coil Short Circuit to Battery
GEM
GO to Pinpoint Test A .
P1832
Transmission Transfer Case Differential Lock-Up Solenoid Circuit Failure
GEM
GO to Pinpoint Test C .
P1834
Transmission Transfer Case Differential Lock-Up Solenoid Short Circuit to Battery
GEM
GO to Pinpoint Test C .
P1838
Transmission Transfer Case Shift Motor Circuit Failure
GEM
REFER to the Symptom Chart.
P1846
Transmission Transfer Case Contact Plate 'A' Circuit Failure
GEM
REFER to the Symptom Chart.
P1850
Transmission Transfer Case Contact Plate 'B' Circuit Failure
GEM
REFER to the Symptom Chart.
P1854
Transmission Transfer Case Contact Plate 'C' Circuit Failure
GEM
REFER to the Symptom Chart.
P1858
Transmission Transfer Case Contact Plate 'D' Circuit Failure
GEM
REFER to the Symptom Chart.
P1866
Transmission Transfer Case System Concern - Servicing Required
GEM
REFER to the Symptom Chart.
P1867
Transmission Transfer Case Contact Plate General Circuit Failure
GEM
REFER to the Symptom Chart.
P1876
Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Failure
GEM
GO to Pinpoint Test C .
P1877
Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Short to Battery
GEM
GO to Pinpoint Test C .
P1891
Transmission Transfer Case Contact Plate Ground Return Open Circuit
GEM
REFER to the Symptom Chart.
SECTION 308-07A: Transfer Case - General Information
2002 F-150 Workshop Manual
DESCRIPTION AND OPERATION
Procedure revision date: 06/20/2001
Transfer Case - Electronic Shift on the Fly (ESOF) Operation
The electronic shift on the fly (ESOF) system is an electronic shift 4x4 system that allows the operator to choose between two different 4x4 modes as well as 2-wheel drive. The operator can switch between 2WD and 4H mode at speeds up to 88 km/h (55 mph). To engage or disengage 4L range, the vehicle speed must be less than 5 km/h (3 mph), the brake pedal depressed, and the transmission must be in NEUTRAL.
The transfer case is equipped with an electromagnetic clutch which is located inside the case. This clutch is used to spin up the front driveline when shifting from 2WD to 4H mode at speed. When the control switch on the instrument panel is turned, the generic electronic module (GEM) recognizes that a shift has been requested and activates the electromagnetic clutch and the relays which power the transfer case shift motor. When the shift motor reaches the desired position, as determined by the contact plate position inputs to the GEM, power to the shift relays and motors will be removed. When the transfer case front and rear output shafts are synchronized, the spring loaded lockup collar mechanically engages the mainshaft hub to the drive sprocket. Finally, the front axle collar is engaged and the electromagnetic clutch is deactivated.
Shifts between 2WD and 4H can be made at any speed up to 88 km/h (55 mph). Listed below are the inputs and outputs needed by the GEM to execute a change between any of these modes.
Feature Inputs:
- 4WD mode switch (various resistances; go to Table 1).
- Contact plate position inputs A, B, C, D (grounded when closed, open circuit when open; go to Table 4).
Feature Outputs:
- 4x4 shift motor relay outputs (go to Table 2).
- Transfer case clutch relay (grounded when relay is activated, open circuit when relay is deactivated).
- 4x2/4x4 vacuum solenoid (grounded when activated, open circuit when deactivated).
- Cluster indicators (ground when activated, open circuit when deactivated).
When shifting into or out of 4L, the GEM requires that the vehicle speed be less than 5 km/h (3 mph), the brake is applied, and the transmission is in NEUTRAL.
Feature Inputs:
- 4WD Mode Switch (various resistances; go to Table 1).
- Contact Plate Position Inputs A, B, C, D (grounded when closed, open circuit when open; go to Table 4).
- Output Speed Shaft (OSS) Sensor (Sinusoid Signal: 0.7V-20V, 2.2 Hz/mph).
- Brake Input (battery voltage when brake is depressed, open circuit when not activated).
- Transmission Range Sensor (grounded when transmission is in NEUTRAL, open circuit otherwise).
- Start/Clutch Depressed Input (go to Table 3).
Feature Outputs:
- 4x4 Shift Motor Relay Outputs (go to Table 2).
- 4x2/4x4 Vacuum Solenoid (grounded when activated, open circuit when deactivated).
- Cluster Indicators (grounded when activated, open circuit when deactivated).
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