My 2005 excursion won't shift into 4x4 high but it shifts into 4x4 low

Have it check for DTC'S - diagnostic trouble codes ?
GEM Diagnostic Trouble Code (DTC) Index
GEM Diagnostic Trouble Code (DTC) Index DTC Description Source Action C1728 Transfer Case Unable to Transition Between 2H and 4H GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. C1729 Transfer Case Unable to Transition Between 4H and 4L GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. P0500 Vehicle Speed Sensor (VSS) Malfunction GEM GO to Pinpoint Test F . P1804 Transmission 4-Wheel Drive High Indicator Circuit Failure GEM GO to Pinpoint Test H . P1808 Transmission 4-Wheel Drive Low Indicator Circuit Failure GEM GO to Pinpoint Test I . P1812 Transmission 4-Wheel Drive Mode Select Circuit Failure GEM GO to Pinpoint Test E . P1815 Transmission 4-Wheel Drive Mode Select Short Circuit to Ground GEM GO to Pinpoint Test E . P1819 Transmission Park/Neutral Position Switch Short Circuit to Ground GEM GO to Pinpoint Test F . P1820 Transmission Transfer Case Clockwise Shift Relay Coil Circuit Failure GEM GO to Pinpoint Test E . P1822 Transmission Transfer Case Clockwise Shift Relay Coil Short to Battery GEM GO to Pinpoint Test E . P1828 Transfer Case Counterclockwise Shift Relay Coil Circuit Failure GEM GO to Pinpoint Test E . P1830 Transmission Transfer Case Counterclockwise Shift Relay Coil Short Circuit to Battery GEM GO to Pinpoint Test E . P1832 Transmission Transfer Case Differential Lockup Solenoid Circuit Failure GEM GO to Pinpoint Test G . P1834 Transmission Transfer Case Differential Lockup Solenoid Short Circuit to Battery GEM GO to Pinpoint Test G . P1865 Transmission Transfer Case Contact Plate Power Short to Ground GEM GO to Pinpoint Test E . P1866 Transmission Transfer Case System Concern - Servicing Required GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. P1867 Transmission Transfer Case Contact Plate General Circuit Failure GEM GO to Pinpoint Test E . P1876 Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Failure GEM GO to Pinpoint Test G . P1877 Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Short to Battery GEM GO to Pinpoint Test G . - For All Other DTCs GEM GO to Section 419-10 .
Your best bet , take it to a qualified repair shop that has the diagnostic tool to trouble shoot .
Principles of Operation
Transfer Case - Electronic Shift
The four-wheel drive, electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4X4 HIGH, and 4X4 LOW. The system mode is selected by the operator through the mode select switch (MSS) on the instrument panel. The operator is informed which mode the system is in by two instrument cluster indicators, one for 4X4 HIGH which is 4X4, and one for 4X4 LOW which is LOW RANGE (in 4X4 LOW both the lamps are on). Shifts into 4X4 HIGH can be made at any speed. When shifting into 4X4 HIGH with the vehicle stationary, tooth blockage may occur, preventing shift completion. When the vehicle is driven above 8 km/h (5mph) the shift will complete. When shifting in or out of 4X4 LOW, the generic electronic module (GEM) requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal be applied, and the transmission in NEUTRAL (automatic transmission) or the clutch pedal be depressed (manual transmission). (The digital transmission range [TR] sensor informs the GEM when the automatic transmission is in the NEUTRAL range position.)
The gearmotor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and the range fork within the transfer case between the 4X4 HIGH, 4X4 LOW, and 2WD range positions.
The GEM uses two relays to control the gearmotor encoder assembly shift between 4X4 HIGH, 4X4 LOW, and 2WD modes. The GEM controls the pulse vacuum hublock (PVH) solenoid which supplies engagement and disengagement vacuum to control the hublocks.
The GEM accomplishes shifts by interpreting inputs from:

• mode select switch (MSS)
• vehicle speed signal (transmitted from the ABS system)
• gearmotor encoder plate position
• brake pedal switch
• digital transmission range (TR) sensor (automatic transmission)
• clutch pedal position (CPP) switch (manual transmission)
• ignition switch

Based on these inputs, the GEM controls the shifts into 2WD, 4X4 HIGH, or 4X4 LOW with the following outputs:

• low to high relay (clockwise)
• high to low relay (counterclockwise)
• pulse vacuum hublock (PVH) solenoid
• gearmotor encoder assembly

The electronic shift-on-the-fly (ESOF) system has a feature which allows the driver to override the vacuum operated hublocks. When the front hublocks are manually turned to the LOCK position, the hublocks are locked at all times, overriding the vacuum operated system. If the front hublocks are manually turned to the AUTO position, the hublocks can only be locked by turning the MSS to 4X4 HIGH or 4X4 LOW position.

Manual shift or Four-Wheel Drive Systems - Electronic Shift I'm assuming you have electronic shift because you refer to relay clicking behind the instrument panel . Principles of Operation Transfer Case - Electronic Shift
The four-wheel drive, electronic shift-on-the-fly feature electrically shifts the vehicle transfer case between 2WD, 4X4 HIGH, and 4X4 LOW. The system mode is selected by the operator through the mode select switch (MSS) on the instrument panel. The operator is informed which mode the system is in by two instrument cluster indicators, one for 4X4 HIGH which is 4X4, and one for 4X4 LOW which is LOW RANGE (in 4X4 LOW both the lamps are on). Shifts into 4X4 HIGH can be made at any speed. When shifting into 4X4 HIGH with the vehicle stationary, tooth blockage may occur, preventing shift completion. When the vehicle is driven above 8 km/h (5mph) the shift will complete. When shifting in or out of 4X4 LOW, the generic electronic module (GEM) requires that the vehicle speed be less than 5 km/h (3 mph), the brake pedal be applied, and the transmission in NEUTRAL (automatic transmission) or the clutch pedal be depressed (manual transmission). (The digital transmission range [TR] sensor informs the GEM when the automatic transmission is in the NEUTRAL range position.)
The gearmotor encoder assembly is mounted externally on the transfer case. It drives a rotary cam which moves the mode fork and the range fork within the transfer case between the 4X4 HIGH, 4X4 LOW, and 2WD range positions.
The GEM uses two relays to control the gearmotor encoder assembly shift between 4X4 HIGH, 4X4 LOW, and 2WD modes. The GEM controls the pulse vacuum hublock (PVH) solenoid which supplies engagement and disengagement vacuum to control the hublocks.
The GEM accomplishes shifts by interpreting inputs from:

• mode select switch (MSS)
• vehicle speed signal (transmitted from the ABS system)
• gearmotor encoder plate position
• brake pedal switch
• digital transmission range (TR) sensor (automatic transmission)
• clutch pedal position (CPP) switch (manual transmission)
• ignition switch

Based on these inputs, the GEM controls the shifts into 2WD, 4X4 HIGH, or 4X4 LOW with the following outputs:

• low to high relay (clockwise)
• high to low relay (counterclockwise)
• pulse vacuum hublock (PVH) solenoid
• gearmotor encoder assembly

The electronic shift-on-the-fly (ESOF) system has a feature which allows the driver to override the vacuum operated hublocks. When the front hublocks are manually turned to the LOCK position, the hublocks are locked at all times, overriding the vacuum operated system. If the front hublocks are manually turned to the AUTO position, the hublocks can only be locked by turning the MSS to 4X4 HIGH or 4X4 LOW position.
Do you know what DTC'S - diagnostic trouble code's are ?
GEM Diagnostic Trouble Code (DTC) Index
GEM Diagnostic Trouble Code (DTC) Index DTC Description Source Action C1728 Transfer Case Unable to Transition Between 2H and 4H GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. C1729 Transfer Case Unable to Transition Between 4H and 4L GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. P0500 Vehicle Speed Sensor (VSS) Malfunction GEM GO to Pinpoint Test F . P1804 Transmission 4-Wheel Drive High Indicator Circuit Failure GEM GO to Pinpoint Test H . P1808 Transmission 4-Wheel Drive Low Indicator Circuit Failure GEM GO to Pinpoint Test I . P1812 Transmission 4-Wheel Drive Mode Select Circuit Failure GEM GO to Pinpoint Test E . P1815 Transmission 4-Wheel Drive Mode Select Short Circuit to Ground GEM GO to Pinpoint Test E . P1819 Transmission Park/Neutral Position Switch Short Circuit to Ground GEM GO to Pinpoint Test F . P1820 Transmission Transfer Case Clockwise Shift Relay Coil Circuit Failure GEM GO to Pinpoint Test E . P1822 Transmission Transfer Case Clockwise Shift Relay Coil Short to Battery GEM GO to Pinpoint Test E . P1828 Transfer Case Counterclockwise Shift Relay Coil Circuit Failure GEM GO to Pinpoint Test E . P1830 Transmission Transfer Case Counterclockwise Shift Relay Coil Short Circuit to Battery GEM GO to Pinpoint Test E . P1832 Transmission Transfer Case Differential Lockup Solenoid Circuit Failure GEM GO to Pinpoint Test G . P1834 Transmission Transfer Case Differential Lockup Solenoid Short Circuit to Battery GEM GO to Pinpoint Test G . P1865 Transmission Transfer Case Contact Plate Power Short to Ground GEM GO to Pinpoint Test E . P1866 Transmission Transfer Case System Concern - Servicing Required GEM CARRY OUT the electronic shift function test. REFER to Function Test - Electronic Shift in this section. P1867 Transmission Transfer Case Contact Plate General Circuit Failure GEM GO to Pinpoint Test E . P1876 Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Failure GEM GO to Pinpoint Test G . P1877 Transmission Transfer Case 2-Wheel Drive Solenoid Circuit Short to Battery GEM GO to Pinpoint Test G .
Your best bet , take it to a qualified repair shop .

ck for power to bulbs ck fuses ck switch

the transfer case shifter linkage may be loose or binding. Check the shift fork for signs of cracks or wear.
Damaged internal gears or viscous coupler prevent the transfer case from shifting into "4-low" or "4-high" drives.
and in evry 15000 miles need to change oil.

you cannot run your truck normally in 4x4 low!!!! the low is designed to go slow slow.try shifting into neutral gear on column tap the high 4x4 then shift to reverse,back up 15feet then shift to drive.it should go into high 4x4 then if you want to go back to 2wheel high shift to neutral tap the 4x4 high until the light goes off,shift to reverse and back up 15feet then shift to drive and the 4wheel drive lights should all be off. then you'll be in 2x2 rear wheel drive. thank you for choosing fixya.com

Without the ability to diagnose the problem further, obvious things need to be eliminated. First step, with engine running and foot on brake, shift transmission to neutral. Then slowly turn Four Wheel Drive (4WD) selector knob out of 4WD Low to 4WD High, and then finally to 2 Wheel Drive. "4x4" and "LOW RANGE" indicator lights on the instrument panel should no longer be illuminated. If this does not work, you should visit FixYa for further help or take it to the shop for proper diagnosis. Hopefully, this will help you to FixYa problem!

4wd shift motor, It is mounted to the transfer case. If it has not been used for a while the windings in the motor usually get stuck. Take a hammer and tap on the shift motor and then try to put you vehicle in 4wd. The shift motor kind of looks like a window motor just a little bigger. Sometimes you have to have someone move the selector as you tap on the motor, BUT BE VERY CAREFUL< DON"T GET YOURSELF RAN OVER> Let me know if this fixes your 4wd. Once you get it going it is always good to put your vehicle in 4wd a few times a year just to keep this 4wd motor going. Good Luck, Roger

When shifting to 4wd hi you can shift at anytime.The only time you have to stop or slow down is when your putting it in 4wd low.Does this mean I have to get my snow blower ready?Brrrrrr.

Encoder motor located on the transfer case is usually the problem. It is the motor that shifts the transfer case.

try putting your truck in neutral and then shifting into 2 wheel drive shift on the fly does not apply to 4 low only 4 high and on electric shift models it wont shift unless your ****** is in neutral.