Your truck employs what is called a Central Axle Disconnect to disengage the front differential from the front driveshaft (otherwise your tires would be forced to turn the front driveshaft in 2wd)
When you pull your t-case lever to 4wd, two seperate things happen. The first is that the transfer case engages the front driveshaft. The second is 12V sent to the thermal actuator (which, it sounds like, you have already correctly located and identified). The thermal actuator contains a small 12V heater and a gas the expands (under heat) to engage the front axle. A system like this replaces the function of good old bullet-proof manual hubs.
These acutators are extremely prone to failure. They wear out over time, and also do not engage very well at extremely low temperatures.
Take out the actuator (its as easy as it looks). Hook the wires to a guaranteed 12V source, and see if the actuator moves (it likely wont, form what you are describing). If it doesnt move, its toast. If it does, you could be looking at more serious problems with the internal workings of the system (the system also employs a spring, engergizer, and "syncroninzer" for lack of a better word) to complete the engagement process.
If it is toast you have some options for repair;
http://www.4x4posi-lok.ca/ (canadian site, not sure on the american one) has a cable system to replace this, but the cable looks prone to mud/salt/sand/ice etc.
You can also retrofit (if your truck is pre 96) to the post 96 soleniod system. This involves changing some wiring harnesses and is more expensive than the third option below.
Replace the factory part. It takes two minutes, the original lasted for how ever many miles are on your truck, and it will be under $100.
Choice is yours.
My 92 k1500 developed the same problem. I bought a new acutuator and it still didnt work. My problem lies somewhere in the wiring from the t-case to the actuator in the front diff. (With a test light it only provided 12V intermittently at the actuator).
I solved this by rewiring the actuator to a reliable 12V IGN+ source and a lighted toggle switch in the dash. It works ok now, but I am still having some issues with the actuator (very slow, sometimes incomplete engagement). I am going to install the posi-lok cable so i can at least get some "feel" for the engagement via feed back from pulling the cable. I am aslo going to install a rubber boot from an e-brake cable over the posi lock cable at the actuator to ensure it stays road-salt free. |