Test A: Selective Four-Wheel Drive Does Not Engage (S4WD)

If you were sent here from SYMPTOM INDEX (BLAZER - JIMMY CANADIAN, JIMMY, SONOMA & S10 PICKUP) table, go to next step. If you were not sent here from the symptom index table , go to DIAGNOSTIC STARTING POINT .
Check transfer case shift control module for DTCs. See appropriate TRANSFER CASES article in TRANSMISSION. If any DTCs are present, see appropriate TRANSFER CASES article in TRANSMISSION. If no DTCs are present, go to next step.
Inspect vacuum hoses for kinks, plugged line, damage or disconnected. If vacuum hoses are damaged, go to next step. If vacuum hoses are not damaged, go to step 5.
Repair or replace vacuum hoses as necessary. If system operates correctly, system is okay. If system does not operate correctly, go to next step.
Press the 4HI button to attempt to engage from drive axle. Disconnect the vacuum hose from vacuum actuator. Inspect for vacuum at vacuum actuator. If there is vacuum at vacuum actuator, go to next step. If there is not vacuum at vacuum actuator, go to step 7.
Replace vacuum actuator. See ACTUATOR ASSEMBLY under REMOVAL & INSTALLATION.
Replace vacuum actuator switch. See appropriate TRANSFER CASES article in TRANSMISSION.
Inspect clutch cable for damage, binding or poor connection. If clutch cable is damaged, disconnected or binding, go to next step. If clutch cable is not damaged, disconnected or binding, go to step 10.
Replace clutch cable. See CLUTCH CABLE REPLACEMENT under REMOVAL & INSTALLATION.
Disconnect clutch cable from clutch fork. Inspect operation of clutch fork using Pinion Oil Seal Installer (J-33798). If front drive axle engages, system is okay. If drive axle does not engage, go to next step.
Replace clutch fork. See CLUTCH FORK REPLACEMENT under REMOVAL & INSTALLATION.
The cause of concern may be in the front axle. Disassemble front drive axle. Inspect for damaged components and repair as necessary. See DIFFERENTIAL CARRIER under REMOVAL & INSTALLATION.