The other day my 4wd stopped working so I climbed underneath to test the vacuum lines. The black one sucks with the engine on and with the transfer case shifter in 4hi, however the red one doesn't do anything. Is this how it's supposed to operate? Where should I look next?

 

Vacuum is supplied to one of the fittings on the differential to engage the axle, and then supplied to the other hose to disengage.

 

Oh, alright. So I have to have someone shift it from inside while I'm laying underneath to test it then.