4x4 engages in 2wd at start. 1990 k1500 5-speed manual.

[Second Series]

I pulled out my trailer and attempted to back up hill on gravel, no-go. I shifted into 4x4, but the light didn’t come on. I did put a deep rut in the road. I got out my test light and crawled under the truck. Power to the transfer case switch, no power to the diff. I jumpered the connector at the T-case switch and got the trailer moved. I finally got the T-case switch out, and found a replacement at a local store. I decided to test it before putting in. The old one tests open at all times. The new one tests closed at rest, push the ball in some and it tests open, push the ball in all the way it tests closed. So this new switch seems to be a normally closed, closed-open-closed switch, single pole-three throw. Not what I would expect. I put it in, and the truck shifts into 4x4 as it should. Now it’s not shifting out of 4x4. With truck off, but key on, I take the test light to the diff connector and see voltage. I can grab the t-case linkage and move it through several positions, and power remains on the harness. Is the switch bad, or is something inside the t-case bent or broken?

I went to my flaps and bought another switch, tested it with ohm meter there. Same brand switch, this one is n/c and reads open when pressed. I just noticed the drawing shows it closed in the schematic I posted above. So I put it in and the 4x4 works as it should. I checked the first new one and it is closed at all positions, so I'll take that back to the other parts store. Now it's time to drive!