I just went through the process of diagnosing a problem where the symptom was "the front end won't lock when I push the 4WD button" and I thought it might be helpful to list out what I did to get to the problem.

The setup:
I headed out with a couple of folks for a night run up a local trail. Got to "The Hill Climb" and it was clear that I didn't have any "go" on the front end. The 4WD light was on.

And...

• One of the guys out that night got under the truck and we confirmed that the transfer case was indeed locking up and spinning the front driveshaft.
  
  We messed around a bit trying to get it to lock, and I decided to go up on a winch line and look at it more when I got "up top".
  
  
• Up top, I was able to:
  • Check the fuses (the light was on, so this was kindof a known, but just to be sure...)
    
  • Check that "pushing the button" activated the vacuum solenoid
  • "Releasing the button" activated the other solenoid.
  • There was associated vacuum present at the diaphragm on the actuator.
  • Check the filter in the vacuum line, thought it might be too restrictive, patched around it and ... no change in the root symptom.
  • Put the truck in some gravel, pulled the E-brake, jumped on it and verified that there was no spin at the front wheels.
  
  At this point it was pretty dark and I figured I'd bundle things back up and work on it at home later in the light.
  
  
• So, back at home, I continued on:
  • The FSM states that you should have about 20in of vacuum to activate the actuator. To test this, I T'd the vacuum/boost gauge from my supercharger into the main vacuum line. 20in, no probs.
  • I moved the T to after the "A" solenoid - this is the one that pulls the actuator and disengages 4WD. 20in, fine.
  • I did the same on the other solenoid, which pushes the actuator and engages 4WD. No probs.
  • I went under the truck and used the gauge from a hand vacuum pump to verify that there was vacuum _at_ the actuator. No probs pushing or pulling.
  • The diagram in the FSM shows that the dash light should only come on when the actuator actually moves. I verified this by pulling the connector from the associated switch at the actuator and noting that the dash light didn't come on. From this I can conclude that the actuator _is_ moving.
    
    
    I moved on to the diff/wheel mechanical connection.
    
    
  • Doing this is tricky, since you can't disconnect the rear axle with the flick of a switch. You can pull the driveshaft, or get all 4 wheels off the ground. I opted for getting all 4 in the air.
    
    With all four up, things happened quickly...
    
  • Drop it in drive - rear wheels are turning.
  • Look under truck, front driveshaft is turning.
  • Catch movement out of the corner of my eye... It's the right front axle turning, but the wheel's not.
  • I walk around to the wheel, spin it by hand... no probs. But sure enough, the axle's spinning.
  • There's _NO_ sound, so I figure it can't be a CV.
  • I had replaced a CV boot a while back, and though that maybe I had a brain fart and forgot the cotter pin in the hub so the axle was spinning free in the hub. (read that as "fingers crossed").
  • Three wheels back on the ground, pulled the right front... Nope, the pin is there and the nut is tight.
  • I reach out for the axle, and... spin... spin... spin.. I pull up on it, and it moves. argh. I spin the wheel and... funny, NOW it sounds like a bag of rocks!
    
    Yeup... I did bust a CV - the outside joint on the right side. ARGH!
  
  A call to Schucks finds a new loaded driveshaft (loaded = with boots ready to go) for $105 + $100 core, and they can have it in three days. NAPA wanted $130 and 4 days, AutoZone had NONE in their system and couldn't order one.

Hindsight points...

• Check the obvious first
• That 4WD dash light means more than you may think it does
• Remember what you have with you for diagnostic tools (like the vacuum gauge from an S/C install)
• Have various sizes of hose with you in a scrap bag

(side note: I got a lot of help on the trail from Rob, Anthony, Ben, Brad and Shannon. THANK YOU!)

 

this happend to me when i busted my cv axle.... says i was in 4wd but no power to the weels. this is whats was remaining of my axle.

 

Careful analysis thats for sure.....But wasnt if obvisously a CV when you saw the front DS locking in but getting no tire movement? btw your camera takes good pics....

 

Nope, not when you're not thinking clearly, and it's getting dark, and a bit cold, and people are kinda' waiting on you, and and and...

The thing about it was that we started thinking down the line of "since there's no "bag-o-rocks" sound from the front end, and the wheel seems fine, then it MUST be that the actuator isn't pushing the sleeve across the shaft". It made sense at the time.

I know that I didn't actually break it that night, and for the life of me I can't think of when I DID break it, but my best guess is that I put at LEAST 500+ road miles on that CV, including driving at 70+mph. VERY weird.

Only once I totally unloaded the front end, and tried to spin the wheel backwards did the axle give and start clanking around in my hand. Good thing that happened in my driveway.


Yeah, that's a nice shot... I'll get a pic of mine after I pull it before I return it for the core charge.

 

ohh oops...i thought that was you that took the pic...i really need to start paying attention to whats going on around me hehehe