I just spent several days doing a clutch job and repairing everything not working with my Hijet 4WD system. I searched all over and pieced together lots of posts and now feel I have a very solid understanding of the system and hope to share so that others can benefit. The system is controlled by: High/Neutral/Low Lever with 4WD Button SU Diff Lock Diaphragm on the rear axle and associated switch. Transfer case Diaphragm at the back of the transfer case transmission assembly Axle / Hub Engagement Diaphragm at the front Diff accessible under drivers seat ahead of radiator 4 Vacuum solenoids under drivers seat 2 Vacuum solenoids just above and ahead of fuel tank The 4WD button switches the Black and White solenoids (electrical connector color) and the successful operation of that diaphragm/transfer case switches the Green and Blue solenoids to engage the front axle. Checking diaphragm operation: Disconnect vacuum hoses to each diaphragm Use a manual vacuum pump to suck on each hose, you should notice a smooth operation forward and backward of each diaphragm and the attached linkages, you may have to rotate the wheels or drive shafts to get engagement. For me my transfer case diaphragm linkage was badly rusted and wouldn't move, so I had to disassemble every pin, sand and lubricate with anti-seize and reassemble. Checking solenoids: Applying 12 volts to the contacts should produce an audible click. The solenoid pairs act on each side of the diaphragm, so its possible to isolate and test them with a vacuum pump individually. The 4 solenoids in the front of the truck operate sequentially. The front diff axel engagement and 4WD light only work after the transfer case diaphragm operates, allowing power to the forward drive shaft. What should work and when: In HIGH In first gear or reverse, Diff lock switch will lock the rear diff. 4WD button will engage the front wheels and the light will light up In LOW It appears 4WD is automatic/default once in Low Diff lock switch locks rear in first or reverse General Advice: The system operates of vacuum from the engine, so ensure no vacuum leaks, good hoses, and good engine operation. I'm not a huge fan of this system because issues with the engine can manifest as issues with the 4WD. Ensure all the linkages operate freely and easily and are well lubricated. Hope this is helpful to someone.
