2008 f150, 4x4 is stuck in high. fuses are good. so are relays. what do i check next

check to see if your 2 wheel high button is not sticking. Next I would check fuses and relays.

It is your 4 x 4 electronic shifter at fault. Located on your transfer case it is mostly black/brown plastic electronic switch that engages and disengages your transfer case from low, high 4 x 4 or 2 wheel drive.
Check connections for electric continuity and replace it if there is no power...

P1825 Transmission 4-Wheel Drive Clutch Relay Open Circuit
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tacho pro 2008

I am going to take a SWAG that this is an issue with the IWE solenoid. There is a TSB for water getting into the vacuum system, and the IWE gears cannot be held out. Still want to check this SWAG.

Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vacuum is removed the spring engages the IWE. The IWE solenoid is what controls vacuum to the system, and is activated either by the switch or the 4x4 lever on the MSoF version.
Block diagram of the Vacuum system


Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
1.Shut of the truck ( still jacked up, and supported ) remove the vacuum lines from the IWE. The spring should push the gears in, without the vacuum line on.
2.Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
3.If the vent line is clear, and with the vacuum lines off the IWE should be engaged ( both front wheels locked ).
4.If they lock, the problem is with the IWE solenoid not shutting off vacuum to the IWEs. There is a TSB on the 2004-2006 IWE solenoids, a newer revision has been made, and used from 2007 and newer MYs. TSB # 06-08-15
Right Click Save as :
http://my.voyager.net/~sscully/file_tsb06_08_15.pdf

- If the front drive shaft turns:
1.Check the fuses for the 4WD system ( If ESOF system ) . These are #11, #29, #30 and #111. Use a meter to check the fuses while plugged in, there are pins on the back of the fuse for this. Use a known good ground and check each side of the fuse with the other meter probe, with the meter set to V-DC ( check the scale if a setting that it will read at least 13 V DC ).
2.With the key on, if the ESOF system have someone turn the mode selector from 2WD to 4WD while hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ). Some times the shift motor will get stuck ( lack of use ? ) and the dead blown hammer will free it up.
3.There are 2 relays ( in the small engine compartment fuse / relay panel - ESOF only ) for the 4WD system, one is the CW ( **** Wise ) one is the CCW ( Counter Clock Wise ). These are full size relays, that can be bench tested, or a quick remove and replace test with the high beam relay in the cab fuse panel ( Relays R03 ). 1 at a time, remove the CW & CCW relays and install them in the position for the high beam relay, and test the high beams by pulling back on the MFS ( MultiFunction Switch ). If the high beams work with both relays, the CW & CCW relays are not the issue ).
4.Check the connector on the transfer case for the shift motor, to make sure it is not corroded.

It might not be the TSB issues, it could be a cracked vacuum line causing this anywhere along the vacuum line routing. quick check with a vacuum gauge should show if this is the case.

Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vacuum is removed the spring engages the IWE.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
1.Shut of the truck ( still jacked up, and supported ) remove the vacuum lines from the IWE. The spring should push the gears in, without the vacuum line on.
2.Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
3.If the vent line is clear, and with the vacuum lines off the IWE should be engaged ( both front wheels locked ).
4.If they lock, the problem is with the IWE solenoid not shutting off vacuum to the IWEs. There is a TSB on the 2004-2006 IWE solenoids, a newer revision has been made, and used from 2007 and newer MYs.

- If the front drive shaft turns:
1.Check the fuses for the 4WD system ( If ESOF system ) . These are #11, #29, #30 and #111. Use a meter to check the fuses while plugged in, there are pins on the back of the fuse for this. Use a known good ground and check each side of the fuse with the other meter probe, with the meter set to V-DC ( check the scale if a setting that it will read at least 13 V DC ).
2.With the key on, if the ESOF system have someone turn the mode selector from 2WD to 4WD while hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ). Some times the shift motor will get stuck ( lack of use ? ) and the dead blown hammer will free it up.
3.There are 2 relays ( in the small engine compartment fuse / relay panel - ESOF only ) for the 4WD system, one is the CW ( **** Wise ) one is the CCW ( Counter Clock Wise ). These are full size relays, that can be bench tested, or a quick remove and replace test with the high beam relay in the cab fuse panel ( Relays R03 ). 1 at a time, remove the CW & CCW relays and install them in the position for the high beam relay, and test the high beams by pulling back on the MFS ( MultiFunction Switch ). If the high beams work with both relays, the CW & CCW relays are not the issue ).
4.Check the connector on the transfer case for the shift motor, to make sure it is not corroded.

Not too sure if you have a New Body Style 2004 or a 2004 heritage ( same as the 99-03 body style ) :
Here is both sets of checks for either option

**** 2004-2008 ****

Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vacuum is removed the spring engages the IWE.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
1.Shut of the truck ( still jacked up, and supported ) remove the vacuum lines from the IWE. The spring should push the gears in, without the vacuum line on.
2.Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
3.If the vent line is clear, and with the vacuum lines off the IWE should be engaged ( both front wheels locked ).
4.If they lock, the problem is with the IWE solenoid not shutting off vacuum to the IWEs. There is a TSB on the 2004-2006 IWE solenoids, a newer revision has been made, and used from 2007 and newer MYs.

- If the front drive shaft turns:
1.Check the fuses for the 4WD system ( If ESOF system ) . These are #11, #29, #30 and #111. Use a meter to check the fuses while plugged in, there are pins on the back of the fuse for this. Use a known good ground and check each side of the fuse with the other meter probe, with the meter set to V-DC ( check the scale if a setting that it will read at least 13 V DC ).
2.With the key on, if the ESOF system have someone turn the mode selector from 2WD to 4WD while hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ). Some times the shift motor will get stuck ( lack of use ? ) and the dead blown hammer will free it up.
3.There are 2 relays ( in the small engine compartment fuse / relay panel - ESOF only ) for the 4WD system, one is the CW ( **** Wise ) one is the CCW ( Counter Clock Wise ). These are full size relays, that can be bench tested, or a quick remove and replace test with the high beam relay in the cab fuse panel ( Relays R03 ). 1 at a time, remove the CW & CCW relays and install them in the position for the high beam relay, and test the high beams by pulling back on the MFS ( MultiFunction Switch ). If the high beams work with both relays, the CW & CCW relays are not the issue ).
4.Check the connector on the transfer case for the shift motor, to make sure it is not corroded.


**** 1997 - 2003 ****

Operationally vacuum shifts the front axle between 2WD and 4WD mode. The GEM ( Generic Electronics Module ) applies ground to the solenoid needed for the mode selected on the switch ( for ESOF ) or when the transfer case shift lever is moved ( MSOF ).The contact plates in the transfer case signal the GEM that the transfer case is engaged, and the Vacuum solenoids need to be activated ( either 2WD to 4WD or 4WD to 2WD ).


Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
•With the truck running ( with front wheel back on the ground ), switch from 2WD to 4WD and back again, the vacuum shift motor should move the lever on the front axle.
{ picture of shift motor }
-- If the vacuum shift motor does not move:
1.Check for vacuum on the lines while shifting between modes. Pink vacuum line is 2WD mode, Light Blue vacuum line is 4WD.
2.Check the fuses for the 4WD system and the solenoids.
3.Check the vacuum lines from the vacuum shift motor to the solenoids on the firewall.
4.Check the vacuum lines from the solenoids to the vacuum reservoir box behind the battery.
5.Check the vacuum reservoir box for cracks.
6.Typical solution is the vacuum lines ( to the solenoids or to the vacuum shift motor ) are cracked or the vacuum shift motor has a cracked diaphragm and has water in it.
7.Could be the shift plate contacts in the transfer case are corroded / not making good contact, and not activating the solenoids.
{ Picture of Vacuum diagram )

- If the front drive shaft turns:
For ESOF system
-- Check the fuses for the transfer case, engine compartment fuse panel, 17 & Cab fuse panel fuses 13 and 23 ( verify with owners manual, 97-97 & 99-03 MY can be slightly different on some items ).
-- Try hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ) to see if the transfer case shift motor is hung up .
-- Check the transfer case connector, to make sure the connector pins are clean and making good contact.
For MSOF systems
-- Check that the shift linkage on the transfer case is moving when the lever is moved from 2W to 4H and 4L.

Operationally vacuum holds the IWE ( Integrated Wheel Ends ) out, and once vacuum is removed the spring engages the IWE.

Lift and support the front of the truck so the front wheels are off the ground ( take safety precautions, chock wheels, jack stands, etc )
Key on, engage 4x4 and try to turn the front drive shaft.
- If it turns it is a problem with the transfer case.
- If it doesn't it is a front axle engagement problem.

- If the front drive shaft does not turn :
•Shut of the truck ( still jacked up, and supported ) remove the vacuum lines from the IWE. The spring should push the gears in, without the vacuum line on.
•Check the vent line ( tube that is routed into the engine compartment with nothing attached to it ), for blockages.
•If the vent line is clear, and with the vacuum lines off the IWE should be engaged ( both front wheels locked ).
•If they lock, the problem is with the IWE solenoid not shutting off vacuum to the IWEs. There is a TSB on the 2004-2006 IWE solenoids, a newer revision has been made, and used from 2007 and newer MYs.

- If the front drive shaft turns:
•Check the fuses for the 4WD system ( If ESOF system ) . These are #11, #29, #30 and #111. Use a meter to check the fuses while plugged in, there are pins on the back of the fuse for this. Use a known good ground and check each side of the fuse with the other meter probe, with the meter set to V-DC ( check the scale if a setting that it will read at least 13 V DC ).
•With the key on, if the ESOF system have someone turn the mode selector from 2WD to 4WD while hitting the transfer case with a dead blow hammer ( do not use a metal hammer, it can damage the transfer case ). Some times the shift motor will get stuck ( lack of use ? ) and the dead blown hammer will free it up.
•There are 2 relays ( in the small engine compartment fuse / relay panel - ESOF only ) for the 4WD system, one is the CW ( **** Wise ) one is the CCW ( Counter Clock Wise ). These are full size relays, that can be bench tested, or a quick remove and replace test with the high beam relay in the cab fuse panel ( Relays R03 ). 1 at a time, remove the CW & CCW relays and install them in the position for the high beam relay, and test the high beams by pulling back on the MFS ( MultiFunction Switch ). If the high beams work with both relays, the CW & CCW relays are not the issue ).
•Check the connector on the transfer case for the shift motor, to make sure it is not corroded.

Here is a block diagram of the vacuum system for the IWEs.

check your wheel sensors make sure they are secured and the casing on the wires is not compromised

Well I replaced the Transfer Case Control Module and the 4 button selector swith on the dash, and my 20001 Chevrolet Blazer Lt is still stuck in 4whl. drive high. Does anyone have any advice on this, Ive also already checked all fuses under the hood and in the truck, all are ok.

here is a link to your problem and a solution:
http://en.allexperts.com/q/4-Wheel-Drive-2700/2008/5/C-Clutch-engaging.htm