03 Expedition Vacuum Hubs
 

Hello all, I have been lurking for some time. I have an 03 Expedition 4x4 E/B with a 5.4l.
Recently it has developed a rattling sound at around 80kmh (50mph) that appears to be coming from the drivers side front, it also will not engage 4wd. The transfer case is shifting but the hubs do not engage. I jacked the front and spun the wheel and it also spins the axle when not running (no vacuum), after starting the engine the hubs disengage and the wheel spins freely without spinning the axle (vacuum). I removed the hose at the Vacuum and Emissions purge solenoid on the passenger side firewall and the hubs appear to engage while running. When I check the voltage at the plug to the solenoid it is around 12 volts when set to 2wd, when switched to 4hi it drops to around 8 volts.
Should the voltage at this plug be dropping to 0 volts when switched?
I don't know if the rattling sound is related to the hub engagent issue, anyone else experience this problem?
(I also posted this in the Expedition area)