Transfer case not working?

TMC8732

MEMBER
2002 GMC SONOMA
4.3L
V6
4WD
AUTOMATIC
155,300 MILES

My front axle will not engage. I checked fuse, vacuum actuator, and front axle cable. All checked out okay. I don't know how to check shift motor, TCCM, or transfer case vacuum switch. The lights work on the dash and I can hear a click when I push buttons. The rear end locks in both wheels but nothing up front. How can I check the last 3 items. This is my first 4wd truck and when I bought it the guy said 4wd worked. Well it doesn't. Can you shed some light on this problem? Thanks

1 Reply

JACOBANDNICKOLAS

MECHANIC
109,708 POSTS

Hi,

If the front axle isn't engaging, it could be related to a vacuum leak, the front axle clutch solenoid, or the front axle switch. Since the light is turning on, it appears as though the actuator motor on the transfer case is working.

What I suggest is this. I have provided the diagnostics for a front axle not engaging condition. Take a look through everything and let me know if it is something you are comfortable doing or if you have any questions.

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2002 GMC Truck S15/T15 Sonoma P/U 4WD V6-4.3L VIN W
Front Axle Will Not Engage
Vehicle Transmission and Drivetrain Transfer Case Testing and Inspection Symptom Related Diagnostic Procedures Transfer Case - NVG 236-NP8 Diagnosis By Symptom Front Axle Will Not Engage
FRONT AXLE WILL NOT ENGAGE
Front Axle Will Not Engage

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Circuit Description
The front axle control circuit consists of a Front Drive Axle Clutch Solenoid which is electronically controlled by the transfer case shift control module. This solenoid opens and closes a vacuum control valve allowing vacuum to apply and release a clutch pack inside the front axle housing. The transfer case skim control module determines whether or not the clutch pack has been applied or released based on a signal from the front axle switch mounted on the front axle housing assembly. The front axle is normally locked or engaged during AUTO 4WD, 4WD Hi, and 4WD LO modes. The front axle is unlocked or disengaged only during 2 HI or neutral modes.

Test Description
The numbers below refer to the step numbers on the diagnostic table.
2. Helps determine whether the transfer case shift control module has electronic control of the front axle actuator.
3. Tests the operation of the front axle switch by determining whether the state of the front uncle switch is correct.
4. Determines whether correct vacuum is supplied to the front axle actuator.
5. Tests for proper voltage supplied to the front axle solenoid.
6. Helps determine continuity through the front axle control circuit, testing the wiring and module driver.
7. Tests the front axle circuit for an open.
8. Tests the front axle switch for an internal open.
9. Tests the front axle switch feed circuit for proper voltage.
10. Tests the front axle switch circuit for an open.

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Let me know what you find.

Take care,
Joe

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