4 Wheel Drive Actuator for all 6.5 equipped K series P/U trucks, Suburbans and Tahoes using the 2 wire actuator, 1996-1997. (1996 trucks build a build date after 1/96) Only the harness connector differers between the ssd-012 and ssd-019.  If you have a 96 truck, please compare harness connectors in the photos or check your build date.

This is usually the reason why your 4 wheel drive front axle won't engauge. It is also known as the TLA Thermal Linear Actuator. When you shift the transfer case into 4x4, this actuator engages the front axle. Its located on the front axle facing the rear of the truck. It installs in minutes. Has 2 wire harness connector.There is a switch located on the transfer case that supplies power to this actuator when in 4x4. Check and replace that switch also. Its p/n ssd-016 also in this category.

Check to make sure your actuator looks like the one pictured below. If not, your's may have already been upgraded to the ssd-600101. The ssd-600101 is a better/longer lasting actuator, but is NOT a direct replacement. It requires the use of an adapter/harness kit ssd-6317 or ssd-6319 depending on model.