2001 F150 5.4 4wd supercrew

When the truck isn't moving I can switch into 4wd, engages fine, drives fine, and I can switch back to 2wd while driving.

The problem is that I can not switch into 4wd on the fly.

When I switch over to 4wd while the truck is moving I get a very loud high pitch grinding type sound. It is horrid. I can't pinpoint the location, so I don't know if it is coming from the axle or the transfer case.

As soon as I switch back to 2wd it immediately stops.

Everything seems to be working except being able to shift into 4wd on the fly. For the past 3 years I have been only engaging while at a stop, but I have got to fix this, it really is starting to get inconvenient.

I took it to the dealer and they first thought it was the forks on the front axle.

After they looked at it they said everything appears to be working fine, it wasn't the shift forks and they then told me the problem was the transfer case itself and they wanted to put in a rebuilt unit. I talked to the tech personally and he basically told me he wasn't really sure.

I told the service advisor no thank you and here I am. Maybe they are right, but since the 4wd drives fine once engaged I am finding it hard to believe the transfer case is bad.

I did some searches to see if this was a common problem, but I haven't turned up anything yet.

Any suggestions on how to trouble shoot this, what to look for, parts that might be causing this?

 

Disconnect vaccuum from front actuator and try . if dont get noise. Thats not it. Cant just unplug solenoid i dont think, computer may see that and not shift.

 

You did say switch, so I am assuming you do not have the shift on the floor but the turn style switch on the dash correct?

 

By 4wd I'm assuming you mean 4H not 4L?

 

Yes. 4H

 

Yes. Dash switch.

 

You mean the actuator on the front axle?

Will that cause a vacuum leak, should I plug the hose after I disconnect it?