The 4WD free wheel air pump on my 98GV failed - symptoms were no 4WD and no green 4WD light when 4WD was selected.
I checked the pump, it was turning on and running for 10 seconds before turning back off - hmmmm - sounds like a problem internal to the differential - one way to verify this is to disconnect the airline and plug it, which should get the light to work, but, of course, not engage the free wheel clutch.
Tried it - still no light - put a gauge on the pump, it's delivering just under 5 psi, and the FSM says 5.40~8.25 psi, so my pump is defective.
Here's a tear down.
The pump removed from the vehicle
And from a different angle
The rectangular metal cap comes off - note the rubber seal - revealing a small circuit board and the "dump valve module" which closes when you're in 4WD and opens in 2WD to dump the pressure, and allow the freewheel hub to disengage, there is a pressure switch module under the circuit board that senses when the system is "up to pressure".
Remove the two screws and gently slide the innards out, the wires connected to the board slide through the rubber grommet and are long enough, that they do not need to be disconnected.
This is the same picture as the last one - but the use of flash allows you to clearly see the ports in the bottom and the O ring seals.
In this picture you can see the pressure switch module - when the pressure is below roughly 6psi, the switch is closed, connecting one end of the pump motor windings to chassis ground (the black wire), and also sending a "low" signal to the 4WD controller on the pink wire, +12V on the red wire turns the pump on, and when the pressure rises above 6psi, the switch opens, effectively switching off the motor, and sending a "high" signal to the 4WD controller.
This one shows the "dump valve module" - essentially a solenoid with a rubber tipped plunger - energize the solenoid and the plunger pulls in so that the rubber tip seals an exhaust port. Note there is light rusting on the tip of the dump valve module - this can be polished away with a wire wheel in a Dremel or Black & Decker rotary tool.
Here we've removed the motor, you can see the eccentric that causes the diaphragm to move up and down, and we've cut the top off of the diaphragm chamber and taken it out. The little red thing is a rubber "outlet" valve.
Here's the diaphragm disassembled - the little flapper in the middle is the inlet valve.
A good cleanup and reassembly with new O rings (came from a Harbor Freight metric o ring kit), the lid of the diaphragm chamber was gently clamped into place with a vice and sealed in place with Devcon plastic steel.
My 4WD works again, and without shelling out for a new (or good used) pump.
Edited for clarity & spelling corrections.
