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I HAD A BLAZER WITH THE SAME SYMPTOMS REPLACED THE TRANSFER CASE MOTOR AND SOLVED PROBLEM. TRANSFER CASE MOTOR SHIFTS FROM HIGH TO LOW GEAR IN THE TRANSFER CASE AND TELLS THE FRONT ACTUATORS TO INGAGE FRONT HUBS . MOTOR WON'T MOVE TO 4 HIGH AND GRINDS WHEN TRYING TO MOVE TO 4 LOW, TRANSFER CASE MOTOR FAILING . CHECK VACUUM LINES AND ACTUATORS ON FRONT DIFF THEY FAIL, CAN CHANGE TO CABLE ENGAGEMENT INSTEAD OF FAILURE PRONE VACUUM ACTUATORS.
I had the same problem on my 90 K1500, it was the 4 wheel drive actuator unit itself. Bout it at the local parts store for 30 bucks and installed it in 10 minutes or less - and vavoom! 4 X 4 back in business.
Having the same exact problem but with my 1999 escalade. Thought it was the actuator bought a new one installed it transfer case would engage but front end wouldnt lock. Took it out of housing left it plugged in got a friend to go through all the 4wd for me and the plunger never moved out the actuator. Checked my wiring harness which is a four prong ann it had one hot prong like it is suppose to. Traced wires even replaced dash switch same problem. Taking it to local dealership tom to have it checked for codes. I was told it could be the transfercase actuator even though its workin that actuator is what sends power to the front to engage the actuator to lock the differential.
Hi: 4 High and 4 Low button should be flashing, If no 4 low either problem will be in the vacuum actuator located below the battery tray. 9 out of 10 times the actuator is not getting vacuum. Battery acid at some time dripped down on the actuator or vacuum lines and created a vacuum leak.