Wednesday, June 22, 2011

2001 F150 4x4 vacuum line explanation?

The vacuum line:--

The vacuum line hose:---

The 4wd fuse.

First off, check the 4WD fuse. That has to be working before you can check out anything else.

The two solenoids behind the battery are always connected to battery voltage through the 4WD fuse, so there should always be voltage on them. The 12V is fed to each solenoid through a white wire with the light blue stripe. (W/LB)

There are relays built in to the GEM module (located above the fuse panel above the driver's left knee). When you move the floor lever, or twist the dash knob, you activate these relays built inside the GEM module. They will ground one of the solenoids and un-ground the other. Grounding the solenoid completes the circuit and causes it to provide vacuum. Un-grounding the solenoid causes it to shut off the vacuum. The ground path for the 4WD solenoid is via the red (R) wire.. The ground path for the 2WD solenoid is via the grey wire with the black stripe (GY/BK) wire.

To check the operation of the GEM module, you can unplug the connector and connect a 12 volt test light across the 12V terminal and the ground terminal on the plug you just pulled off. The light will come on if the GEM is providing a path to ground. Test each solenoid connector and make sure the appropriate solenoid gets activated as you switch between 2WD and 4WD. If the test light isn't coming on for one or both of the solenoids, then suspect a faulty GEM module.

If they're working then you might have a vacuum problem. Make sure the vacuum lines are all OK. Pull out the battery and make sure none of the lines got crushed, cut or cracked by the last person who installed the battery. Also check the vacuum reservoir. It's a thin black square thing with a vacuum line going to it. It's fastened to the battery tray on the side closest to the fender. As a note.. A bad leak in this area can cause the heater controls to malfunction or not work 'quite right'.. If your heater controls aren't switching normally, then pay close attention to the vacuum lines.

If the vacuum is OK, then you need to check the actuator. You need to take of a plastic shield to see it. Watch the arm to make sure it's working.... If the actuator is OK, then the lever system that disconnects the axles inside the diff is not working. They do break from time to time. If that's the case, then the front diff has to be pulled apart.

Look at the diagram of the actuator below and try visualize how the rod will move depending on which side of the diaphragm is receiving vacuum. Vacuum from the 4WD solenoid will push the rod out... Vacuum from the 2WD solenoid will pull the rod in.

Below is a diagram of the solenoid system to hopefully help you understand what I'm talking about Note that the vacuum lines are shown as hollow lines in the diagram and the electrical wires are shown as solid lines.


1. The solenoid on the right side (passenger side) that has the blue hose is the one that activates the vacuum actuator to engage the front differential.

2. The solenoid on the left side (drivers side) that has the pink hose is the one that activates the actuator to disengage the front differential.


problem solved - when i shift the switch i can hear the transfer case "clunk" and engage, but i have no power at the front wheels - i can kinda of hear a slight grinding noise coming from the front end, and every now and then it will actually engage into 4x4

( WARNING do all of this with the vehicle safe and secure, ie in park, park brake on, with wheels chocked, well vented area, and all other safety stuff - cause really if you dont know this muchh then STOP - you can't fix your truck - duh! )

do this step by step

Step 1 shift to 4x4, can you feel the "clunck" or the transfer case engage? did the light 4x4 turn on? then go on to #2

Step 2 check the little vacumm relays (by the battery tray) by switching from 2x4 to 4x4 - see that they work (one will allow vacumm through on 2 wheel drive, the other will allow it on 4 wheel drive - the vacumm from here goes to the vacumm actuator, which will either locks the front diff or unlocks the front diff). (have vehicle stared in park with park brake on to do this) are they working properly? if so go onto #3

Step 3 remove the little plastic drip tray thing (3 10mm bolts) from the front of the front diff (it is the oild filter drip shield). under that sheild is the vacumm actuator that either pulls the shift fork into 4x4 or pushes the shift fork back into 2x4 (have vehicle stared in park with park brake on to do this) shifting into 4x4 will move the actuator one way and hold it there, shifting into 2x4 will move it the other way and hold it there (it will either pull the rod in (at 4x4) or push it out (at 2x4)) is it freely moving back and forth like it is suppose to? if so go onto #4

Step 4 if it is moving and the shift fork linkage is moving, you probably need to replace the little nylon pads on the shift fork in the transfercase - they break and fall off, when they doe this there is too much slack between the shift fork and the sliding sleave. it took me about 3.5 to 4 hours to take it appart, go to ford and pick up the part and put it all bagk together working on the ground - here is the biggest problem - You have to buy the whole shift fork, whith the new sleave (about $350), you cannot just buy the little nylon parts (that are worth about $0.25 each)

hope this helps someone - i had to figure it out the hard way

You'll have to dig up a volt meter and check to see if there is voltage on the solenoids.. You'll also have to climb under the truck and make sure the electric actuator is moving the control arm on the transfer case... (That is assuming that you have a dash mounted 4WD switch and not a floor mounted lever).


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