Squatting Dog
FRF Addict
It's the wire that leads from the solenoid to the vacuum system. I think it's the IWE solenoid (solenoid vacuum valve). Ford once thought it was the wiring, but didn't know which wire. Then they gave up on that idea. After further failed attempts and more new parts, they figured something wasn't right with the command to activate 4wd. They overlaid a wiring harness with a new wiring harness and... tada, everything was fixed. So they replaced the harness.
Someone posted this on another FRF thread:
The vacuum from IWE module holds the diapham against return spring until the driver switches to 4wd. At which point the IWE solenoid changes state and the vacuum is removed. The spring in the IWE presses the diaphram the opposite direction while engaging the cv shafts to the hubs. This movement pulls unfiltered air and everything else into the IWE.. Which in time degrades the IWE diaphram and causes grinding and eventually loss of 4wd.. Once the diaphram is compromised, the crap travels up the vacuum line and trashes the IWE solenoid..
That was me, I believe...
Yes, there is a connector on the IWE solenoid that can rub on the cluster cluck of stuff on that side and will short to ground causing issues..
-Greg (aka squatting dog)