Bought a used 2014 Expy. Tested 4x4 when buying by locking it into 4x4 and making a slight turn in pavement to feel it bind up, was some gravel so it slipped a little and I locked it in 4low and 4hi back to 2hi several times with no issue. I towed a trailer today and had to back it into some muddy area. Locked it into 4x4 low and tried to pull the trailer out(5x10 landscape trailer) and I noticed the front would catch and as soon as it had traction it would disengage/re-engage/disengage/re-engage you get the picture it acted more like it was in Auto rather than locked in Low or Hi and Im pretty sure I heard the traction control trying to work even though it showed as off. Dash showed 4x4LOW with traction control off. The passenger tire finally started to dig in and I was able to get the trailer out. Purposely got into some mud and all 4 wheel would spin but if one got traction it would start unlocking from 4x4 like it did when trying to get the trailer out. Again it acted like it was in Auto and not locked into 4x4. There was a sound coming from under the truck about midway, same sound as you'd normally hear when the 4x4 binds on dry pavement plus I could hear traction control working even though it was showing off I'll try to post video tomorrow of what it's doing but anyone have this issue? Or have a starting point to diagnose the issue?

 

Jack the rear tires up off of the ground, put it in 4x4 and see if it will drive off of the jack stands.

 

Your front hubs may not be fully engaged and disengaging under load.

 

Dangumbit I was gonna do that but can't find my jackstands. Might have to run to harbor freight.

 

Another person asked the same thing. I'm not super familiar with how these guys activate the 4x4 as I'm used to manual hubs I'll have to investigate.

 

The backside of the hub bearing has a sliding splined actuator that works off vacuum. When there is no vacuum they are engaged by a spring, vacuum applied disengages them so that if the system loses vacuum it defaults to the engaged position. They can and do fail, but are generally reliable. If they do fail Dorman makes a replacement part that is engaged all the time, with no vacuum.

 

Thank you! I'll get under and do some checking.