4wd light stays on after put back to auto and it is still in 4wd

SOURCE: My 98 chevy blazer won't shift into 4wd.

Most common problem is the vacuum diaphragm that is so inconviently mounted UNDER the battery tray.The cable that locks in the front diff is attached to it and vacuum pulls it in/out.

SOURCE: 1999 Blazer AUTO 4WD stays lit up in ALL transfer case modes! WHY

the transfer case module is bad it is bolted to transfer case above front drive shaft

Harvey N Tawatao

• 3854 Answers
• Posted on Jan 07, 2010

SOURCE: Had 2001 GMC Sierra in 4WD Auto and went to put in

On the full size, the front actuator is electric, not vacuum.

When in 4wd mode, a switch on top of the transfer case conducts 12 volts out to the actuator. Wire #50 (brown) is the 12v feed, and it conducts to the light blue wire (#900) to the actuator.
When shifted out of 4wd, the switch opens up and no voltage goes to the actuator.

Check voltage at the connector for the front actuator (at the front differential - it looks like a large bullet that threads into the front diff.) If it has 12 volts, the actuator is the problem. If it does not have 12 volts, check the transfer case switch and the power feed to the transfer case.

I'll assume it is the actuator, if the plug to the actuator is getting 12 volts when in 4wd.
The way this actuator works, is it is a sealed chamber that gets heated by voltage. When it heats, the pin in the end swells and pushes a shift fork in the front differential. The shift fork engages a spline on the passenger side of the differential, and boom! you have 4WD.

GM offered an upgrade to this actuator that is a motor instead of a heated device. However, it requires a small harness addition, a spacer, and a new actuator. It is about a $150 to $200 option.

Good luck. The actuator is usually the problem 90% of time but recheck all of the wire harness just to maker sure there is no short before replacing any parts.