Why won't it engage 4WD?

http://www.carforum.net/transmissions-drivetrain/15664-isuzu-rodeo-4wd-wont-engage-clicking-noise.html
see post 9

post 3 and still the part you state is named wrong.
my best guess, is the
front alxe lock, up, device. with 2 lines
is that it.
?????
there are only 2 ways, 2 lines
is it so hard to try both ways, on 2 solenoids?

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 ( this is what you seem to have, the transfer case is engaged and will drive the front shaft mine was a key on engine off test ).

- 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.

-- 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.


Usually the case is cracked vacuum lines from the solenoids, the next common is a cracked vacuum reservoir box, and 1 time I have seen the mount for the vacuum disconnect motor broken ( motor floating free so it could not push against the axle )

For the 2002 MY ( this is in the 2006 Section, just want to call it out to others that might find the solution, this does not apply to the 2006 MY )

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.


- If the front drive shaft turns:
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.

I am going to take a SWAG that it is 1 of 2 places, both cases the front drive shaft is locking into place.

1. The front vacuum shift motor is not moving ( vacuum or vacuum line issue ).
2. The contact plates for the shift motor are not making contact and are not signalling the GEM that the transfer case state has been changed.

I'd start with the front axle off the ground test, and see which way to go, before you start repalcing items.

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:
•Check for vacuum on the lines while shifting between modes. Pink vacuum line is 2WD mode, Light Blue vacuum line is 4WD.
•Check the fuses for the 4WD system and the solenoids.
•Check the vacuum lines from the vacuum shift motor to the solenoids on the firewall.
•Check the vacuum lines from the solenoids to the vacuum reservoir box behind the battery.
•Check the vacuum reservoir box for cracks.
•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.
•Could be the shift plate contacts in the transfer case are corroded / not making good contact, and not activating the solenoids.


- 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.

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.

I would suspect that the front drive shaft will not turn, and the issue is with the vacuum shift for the front axle.

The vacuum disconnect motor is on the front axle ( item #10 in the picture below )


Close up of the vacuum disconnect motor ( this is in process of being repaired )


The vacuum solenoids are mounted on the firewall, and take vacuum from the vacuum reservoir to move the shift motor when the GEM ( Generic Electrical Module ) calls for a mode ( 2WD or 4WD ).
The Pink line will have vacuum on it when in 2WD mode, the Light Blue when in 4WD.
Use a vacuum gauge to verify that the vacuum is on the line when selecting the mode.

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.

I am going to guess the drive shaft will not turn, and the problem is the shift disconnect motor on the front axle, or the shift solenoids.
The shift motor looks like this


The shift solenoids look like this :


You can check the shift solenoids with a vacuum gauge. The blue vacuum line is the 4WD operation, the pink is the 2WD operation. Pull the vacuum line at the shift disconnect motor, and install the vacuum gauge. Shift into 4WD and the vacuum should be on the line, and hold. If not could be the disconnect motor or the vacuum line to the motor.

Could also be the shift motor in the transfer case, but with the 4x4 light on the cluster lighting up, the transfer case should have shifted, the confirm comes from the transfer case to illuminate the 4x4 indicator on the cluster.

I would suspect the vacuum disconnect motor or the vacuum solenoids for the disconnect motor ( or the vacuum lines ), but best to confirm
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.

With the engine running ( and wheel chocked, parking brake on ) have someone shift into 4WD, and watch the shift motor to see if it operates
It is on the front axle, looks like this


If the disconnect motor does not shift, you can check the vacuum on the lines with a vacuum gauge. Blue line is 4WD operation, pink line is 2WD operation.
The vacuum solenoids are on the firewall, and look like this


This is the block diagram for the ESOF vacuum operation of the disconnect motor.


This is assuming you have checked the fuses for the transfercase shift motor, and the vacuum solenoids.

shift into 4wd and move truck a couple of feet.
if light is still off, park truck block the wheels set the park brake jack right front wheel and try to hand spin the wheel if wheel doesn't turn 4x4 is functioning properly but there is a wiring problem use an ohmmeter to check the 4x4 switch wiring fuse and bulb
if wheel turns freely the front axle or transfer case is not engaged
attempt to hand spin the front driveshft if the driveshaft will spin the transfer case is not engaged check the transfer case shift linkage, problem is in linkage or transfer case internals
if the driveshaft will not spin the transfer case is engaged properly
with trnsmission in neutral or park start engine measure vacuum on the two vac lines at front axle shift motor . one line should have approx 20" vacuum. other line should have none
if no vacuum measured look for vac leak or bad vac switch
if both lines have vac look for a bad vac swithc or ruptured cad actuator diaphragm
if one line has 20" +- VAC
move transfer case lever to 2wd measure vac again lines should have swapped vac signals
if no vac switch bad or 4wd shift likage needs adjusting
if yes vac motor bad or stuck or shift fork stuck
remove vac shift motor from the axle attach vac lines to shift motor engage disengage 4wd
does shift motor plunger move at least 1/2 inch
no shift motor bad
yes check axle shift fork and vac motor for binging

check the wires on the transfer case mine got melted by my exhaust somehow and i just have learned to deal with it lol.