1997 Chevy Blazer LT 4X4 4.3L AT. Today, I got into a spot with my rear tires on a patch of ice and front tires on solid pavement, slight incline. I engaged 4HI (light came on just fine and can hear t-case engage). Rear tires spun, but front never pulled. So, I put into neutral and tried 4LO (can hear t-case engage into LO and light comes on). Again, rear tires spun but front never engaged. I finally went back to 4HI and spun back and forth until I melted the ice beneath the rear tires. No blinking lights, lights come on in both 4HI and 4LO and you can hear t-case engage, just no front pull.

Side note....This may have absolutely no bearing on the subject, but I am aware all of this is vacuum controlled (t-case and heat)....Recently, my vent heat began blowing out the defroster only on inclines and it seems like my damper between vent, defroster, floor, and hot/cold never seems to fully work. Anything related here...mostly, what's going on with the 4WD?

 

Bear with me here......the t-case is electronically controlled (engages the front driveshaft) but when the t-case is electronically shifted it opens a vacuum valve which applies vacuum to the front axle actuator (under battery tray) which pulls a cable going to your front axle which engages the front axle. So instead of having twist lock hubs like on the older vehicles it is vacuum/cable controlled. Your problem can be in a couple of places. Either vacuum hose going from t-case to actuator, the actuator itself, cable is broken/frozen or the axle itself. Now let’s find the problem

1.the first thing to do locate the vacuum line going to underside of the battery tray then disconnect it, start the car and push your 4HI button then put your finger over the vacuum line and see if its got vacuum to it. If not fix broken vacuum line, if it does then go to next step.

2.pull battery/battery tray out and you will see the actuator, visually inspect for broken rubber boot, if boot is broken replace actuator if not then detach cable from actuator then try to move cable by hand if you can move cable is a bad axle(highly unlikely) if you cant move it, its most likely a frozen cable. Hope this helps.


actuator - http://www.napaonline.com/Search/Det...+50014+2014003

t-case vacuum valve - http://www.napaonline.com/Search/Det...+50014+2014003

 

Just had this same problem man after changing cap and rotor, Dash Vent actuator and 4x4 actautor under battery tray, get vaccum from back of intake right above distributor. I left that hose unhooked and had defrost only position, wouldnt change and no front diff lockup. Check that line and where it runs up to the ball on the hood, and then splits into the firewall. I'd say you have a vaccum leak bud.

Thanks for those links ryan.

 

yep i have the same problem sort of. My heat doesnt work but thats the blower motor. And my vacume lines and actuator/cable are all good. I diagnosed it to b thew front driveshafy u joint.
(the one closet to the t-case) But theres like a weird metal cup around it and i dont no how to remove said cup to get to the u joint. cuz before my frnt shaft didnt turn at all. And if i twisted the shaft and the cup the twist different ways.

95 chevy blazer 4.3 vortec lt 4dr 4x4 (w)

 

Got caught up with some other projects...will try to diagnose it this weekend. Will post my results.

 

Found it. Bad vacuum switch on tcase. Replaced. Checked the actuator under battery tray. Bad. Replaced.

 

Sounds good! Glad you found the problem!