<font face="Comic Sans MS">Diagnosing the vacuum actuated front axle engagement system
Tim "The Sandman" Specht
The first step in diagnosing the vacuum actuated front axle engagement system is to start the truck and observe the vacuum actuator as someone moves the transfer case lever to either 4x position from the 2wd position. You will find the vacuum actuator either on the passenger side inner fender, or below the battery tray.
If the vacuum actuator seems to be pulling in and moving the cable, then you can skip down in this document to Step 4.
If the vacuum actuator does not move, proceed to Step 2.
Hook a vacuum guage onto the vacuum hose from the transfer case to the vacuum actuator. Note the vacuum reading. If there is no vacuum reading (as you shift from 2wd to 4wd, and then holding constant), you can proceed to Step 3.
If there is vacuum getting to the vacuum actuator, you will need to inspect the diaphragm to ensure there are no large tears in it. Also while youíre inspecting that area, check to see whether the cable that is attached to the actuator will move freely. To do this, manually pull on the cable in the direction of the diaphragm. If the cable doesnít move freely, you will need to inspect the differential itself. Trace the cable down to the differential, and inspect the internal mechanism.
At this point, there are 2 vacuum lines you should be concerned with inspecting. One leads from the vacuum switch on top of the transfer case to the vacuum actuator. The other leads from a port on the intake manifold (typically) to the vacuum switch on top of the transfer case. If they seem even the slightest bit worn out, rotted, or otherwise damaged, replace them and re-test to see if the diaphragm on the vacuum actuator pulls in or not. If it does pull in as it is supposed to, but your problem still doesnít go away, look to see if the little clip that holds the cable to the diaphragm is still there. This little clip is the cause of much grief.
If the vacuum actuator and cable mechanism has been working the entire time, then the most likely problems are either a malfunction inside your transfer case or a malfunction in your differential. As of this time I donít have the technical information on which parts itís likely to be. I will update this document as the information becomes available.
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