excursion 4x4 ?

i have a 2000 excursion when i turn on the 4x4 switch on the dash, light comes on and you can here a solonoid click, but the 4 wheel drive does not engage. if i manually lock hubs all is fine. any ideas on the problem?

Sounds like a vacuum application/leak problem for the hubs.. Gas or diesel?

this is a diesel.

Up on the right/passenger inner fender well is the systems vacuum pump.. You 'should' be able to hear/feel it run with the key cylced to ON or if a vacuum line is removed.. It supplies vacuum to the hub locks through the vacuum resevoir and PVH (Pulse Vacuum Hub) solenoid.. It supplies heater/defrost functional vacuum as well, and if the pump is bad, the heating vents will default to Defrost..Check to see if the pump runs, and check for any vacuum line leaks or if the hoses have come off/leaking on the spindles AND on back up to the PVH solenoid..The hubs need high vacuum to lock, a a lower vacuum to unlock..  If you have a vacuum gauge, you can test for a good vacuum at the reservoir, and to test for PVH functions you can plug off 'one' side/spindle of the vacuum lines (on the hub, one per hub) with a gauge installed in the other hose/side.. Turn the key on and wait 15 seconds or so, then turn the MSS (mode select switch) to a 4x4 range while monitoring the vacuum reading.. Hub lock vacuum will be applied for 45 seconds, then drop to zero.. If all the system checks out, then the hub/spindle seals may be leaking.. A simple vacuum test off of the hose connection will determine a leak, and which side..

Here's a simplified picture of the system.. Doesn't show the vacuum pump but is noted..


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thanks!! for all the info. will work on it.