4wd will not engage vacuum pump fine - 2001 Ford F350 Super Duty SuperCab

The problem may be caused by the central axle disconnect actuator (CAD). If you have or can get a handheld vacuum pump, you can test the function of the CAD. Here's how:

1.) Raise the right-side front wheel off of the ground.
2.) Pull the right hose from actuator and, apply vacuum to the actuator's right side.
3.) First make sure 4WD isn't engaged, and turn the front drive shaft by hand.

A.) If the right wheel turns when the driveshaft is turned, the actuator is GOOD.
B.) If the right wheel DOESN'T turn, the actuator isn't engaging the clutch in the axle.

If A): Turn on the ignition. The 4wd light should be on.
If B): (and the 4WD light is ON), remove the actuator and check the central axle disconnect.

Most common problem is a vacuum leak to the hubs. Check the lines from the vacuum pulsating solenoid all the way down to each hub connection. If the lines are good,use a vacuum pump to apply vacuum to the hub with the line removed. You should hear a slight click at about 5 inches of vacuum if the hub seals are good(test both hubs individually). Just one bad seal in either hub can cause the entire system to malfunction.

When my 96 Ram 2500 4wd would not engage the front drive, I traced it down to a loose vacuum line. the drive is engaged with a vacuum unit. The hose came off when I pressure washed the underside of the vehicle. Check to see of your 4wd is engaged with vacuum; then check for loose hose or no vacuum. If no vacuum, the vacuum pump might be out. I put a vacuum gauge in my truck to help diagnose any problems like these.

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

Use a hand held vacuum pump and see if it engages. Sometimes the shift fork gets bent in the axle and this will cause it to not engage or it could be a bad vacuum motor.

There is a vacuum line that goes to the transfer case.if this line is ruptured it will not engage the 4wd.Next there is a switch on the t-case that may not be working.In 4wd it supllies vacuum to the 4wd actuator that is under the battery tray.next the applied vacuum pulls the cable engaging the frontaxle.

Lift and support the front of the truck so the front wheels are off the ground.
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.

There are the two lines that operate the diaphragm on the axle. Facing the truck, the right solenoid (pink & black lines) is activated to disconnect the axle and go back into 2WD. The left solenoid (blue & white lines) lock the axle into 4WD

you'll have to get a vacuum source like a Mighty-Vac hand pump and apply vacuum to the pink and blue lines, one at a time. If one leaks down you'll have to check the line, including the diaphragm, to locate the leak. If vacuum was held, reconnect the bottom half of the line "connector" (black & white lines) to the solenoids and crank the truck. Switching from 2WD to 4WD and back verify each time that there is vacuum on the top nipple of the activated solenoid. If you're in 2WD the right solenoid (pink/black) should be sucking, in 4WD the left (blue/white) should be. If these operate as described then you axle shift fork may be bad. If you're not getting vacuum when you should be, replace the solenoid.

The Vacuum shift motor ( item #10 in the diagram )

Close up :
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Vacuum shift solenoids



The other source of the vacuum leak ( if the lines to the solenoid test good ) is the vacuum box behind the battery.

a quick fix to solve the problem with the 4wd not working on the sportage is fitst check the vacume soionoid is working lockated by the suspention turet on theL/H side of the engine bay, then, chech hoses going to the hub, if there is still a vacume leek from the hub remove the 6 bolts from the outer hub locker place in vice remove the circlip, remove the spring, remove the spline gear then take the spring and swap it around so that it gose under the gear so that it is in conatant mesh/ lock, build back up and fit, it means the front axle will be live all the time but you will still be able to select 2 /4wd,,

Most of the time it’s a Fuse, PVH,(Pulse Vacuum Hublock) ,Vacuum line or Vacuum pump.
ESOF 4WD Trouble shooting It is easier to trouble shoot the system if you have a “Mityvac” vacuum pump. Turn the ignition switch to run (you don’t have to start the truck) and the air controls to vent and the fan control to low. If the truck’s vacuum pump comes on wait until it stops running before each test (open the hood and you’ll hear it better). First you need to remove both lines (left and right vacuum lines) from the hubs. Plug the passenger’s side line and attach the vacuum pump to the driver’s side line. Switch the dash controller to 4WD high. The vacuum gauge should go to 10 in.-Hg. or higher. (It should hold the vacuum at 10 in.-Hg. or higher for 45 seconds on engage and 15 seconds on disengage.) Repeat the test plugging the drivers side and testing the passengers side. If you have the right amount of steady vacuum the problem is probably in the hubs. If you do not have the right amount of vacuum or it is bleeding off, plug the lines back on the hubs and go to the next step. Second remove the vacuum lines from the PVH (Pulse Vacuum Hublock). This is the solenoid located on the passenger side fender well behind the battery and in front of the vacuum pump. Plug the vacuum tester into the lower hose and wait for the vacuum pump to build back up and shut off. The vacuum gauge should read and hold steady at 10 in.-Hg. or more. If it does, the vacuum reservoir should be fine and not leaking, so go on to the next step. Third, plug the bottom line back to the bottom of the solenoid and hook the vacuum gauge to the top barb of the solenoid (not the vacuum line but to the solenoid itself). Try switching between 2WD and 4WD again. The gauge should read 10 in.-Hg. for 45 seconds on engage and for 15 seconds on disengage. If it doesn’t it is probably the solenoid (PVH) on the fender that needs replacing. This was my problem so this was as far as I have gone. If all these things check out, THEN you need to go to the hubs and check them for leaks with the vacuum tester. Most of the time it’s a fuse, PVH, vacuum line or vacuum pump and not the hubs. Does your ac/heater work properly when you switch modes (IE linger before going in?) or sometimes stay on defrost? These trucks have a vacuum pump that controls it and the auto hubs.If the vacuum pump or hose,fails those are easy to see symptoms.


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Most common problem is the vacuum diaphragm that is so inconviently mounted UNDER the battery tray.The cable that locks in the front diff is attached to it and vacuum pulls it in/out.