What causes the 4 wheel drive to partially engage without being selected. The problem is intermittant. When it happens, there is a grinding noise of gear on gear. Slowing down or selecting 4hito completely engage and then seleting 2hi usually fixed the problem. I thought that the system was spring loaded to the off position. Is it possible there is a spring broken inside the mechanism, or am I missing something? The truck is a 1997 4.3 Auto.

 

It is possible the spring broke or that the vacuum switch on the top of the transfer case is supplying vacuum pressure to the actuator when it isn't supposed to be.

You can test the vacuum switch by disconnecting the vacuum line at the front axle actuator (under the battery) and starting the engine with the transfer case in 2HI. If you have any vacuum, the switch is bad.

You can then further test the system by plugging the end of that line and driving around for a week or so (providing that you do not need 4wd) with this disconnected. If the problem persists, then you know it is either in the actuator itself, the cable, or the mechanism in the front axle.

The cable can be worked back and forth, feeling for rough spots in its travel that may partially hang it up when attempting to return (disengage). It can also be lubricated with any light oil spray and worked back and forth to push the lubricant into the cable jacket.

 

Thanks for the information about the 4wd vacuum switch. I went to Autozone and for $15 bought a new one (it's called a transfer case switch) and replaced it. It took longer to find where it was than toinstall it.No more grinding gears.