My 1998 Isuzu Rodeo 4wd unit will not engage.? ///what could be the problem.?

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

its not for dry pavemen
the Part time system is for OFFROAD (dirt) or on ICE.

so what are wheels (tires) touching, we cant guess that.... ever.....
or is it TOD system..
cant guess that either.

four wheel drive has an actuator on axel under the guard has to plugs and some vacuum hoses connected to it around maybe 6 hoses I picked one up at the junk yard

check if the automatic hubs are engaging ( vacuum or electrical wiring fault) Check that the 4wd actuator is actually engaging (vacuum or electrical fault)

no problems before the mud deal but problems after the mud deal means that the problems started in the mud
check for mud in connections , wires disconnected , harness problems, vacuum hose to actuator broken or missing
run the fault codes to help point to a problem area
the transmission is sealed so there will be no need to change the oil or filter--just clean and check everything that is mud affected

well could be faulty coil packs could be bad grounds(corroded)

try a pcv valve

If you hear the click in the floorboard then the problem is either the vacuum switch on the top of the transfer case, the vacuum actuator under the battery tray or the cable from the actuator to the front differential lockout. Changing all three will cost less than a hundred bucks if you shop around for the parts. The vacuum switch is the part where the three hose bundle attaches on top of the transfer case. The actuator will pull the cable when engaged, you will feel it pull when you engage the 4wd. It will move slowly. The other thing to check is the vacuum hoses between the source on the back of the motor, the hose from the switch to the actuator and the vent attached to the trans dipstick. If the vent is clogged it won't engage or disengage. Good luck

Hahahaha!... Seriously....!?!?! First off don't try to use a scan tool. The 4WD system isn't integrated into the OBD system. You won't get any codes related to 4WD.

Most likely the problem is defective shift solenoids. There are two shift solenoids which control the engagement and disengagement of the 4WD (locking the front axle gear into the the driveshaft input). These solenoids are located on the front axle. They have their own small skid plate. One is blue, and one is gray. One controls engagement, and one controls disengagement. One is a normally closed electrically controlled vacuum solenoid, and one is a normally open electrically controlled vacuum solenoid.

I'm not blindly recommending changing these, but usually one of these are the culprit of the problems you described. I'm even more confident in those being the problem due to your explanation of the the intermittent problems. At any rate test first.

The other components of the 4WD system are the vacuum lines and the actuator which is also located on the front axle and covered by the forementioned small skidplate. Moderate to serious rock-involved off-roading with a stock suspension usually results in this component being damaged.

It is possible that the vacuum actuator that engages the 4X4 is faulty. It will have to be tested with vacuum pressure tester.
This is usually the case when 4X4 doesn't engage.
Other case is the module inside tranny is defective and may need rebuild