97 Blazer. I've replaced actuator, some vacuum lines, actuator switch, actuator cable, encoder motor and selector switch. Here's the symptoms: 4x4 will not engage at all. If I put the truck in neutral, the 4 hi light flashes, then stays in 2. It sounds like the encoder motor tries to turn, but is stuck. Pushing four low, I hear the solid "clunk". No 4x4 lights are on. Trying to drive, truck in 4lo barely moves and revs out.

When I disconnect the battery for a few minutes, the 4x4 works perfectly for a short time. Then, the light randomly flickers back and forth between 2hi and 4hi. Eventually, it goes back to dong nothing when button is pressed. My 4x4 issues began with a blown TRANS fuse. Haven't blown a fuse since replacing the old one.

Any thoughts on where I should begin my next troubleshooting step?

 

Most common failure mechanisms for Blazer 4wd systems off the top of my head and as I have gathered on the forum:

• leaks in vacuum lines that goes to the vacuum actuator that engages the front axle
• leak in vacuum actuator for front axle engagement
• leaks in vacuum system that control the transfer case ("3-button" but not "4-button" transfer cases)
• bad or failing transfer case control module (TCCM)
• rounded over coupling gear for front axle engagement
• transfer-case engagement motor

Are there others? I'm sure there are. Perhaps other people will add to the list.

 

Thanks Christine_208. An update: I went for a drive and pretty much the only thing the truck will do is to flicker back and forth between 4hi and 2hi. This flickering is random; there's no discernible pattern. It's also very fast, kind of like a strobe effect.

 

I went through nearly the same problem. It ended up that the big plug block for TCCM had corrosion. Also some of the wire terminal in this block became unlocked and wasn't connecting properly after cleaning. Make sure all the terminals are locked into plug blocks after cleaning