4 Wheel drive (Manual trans) not engaging
First, IF your vehicle has a true manual-engage for the 4-wheel drive, then you either have a linkage that needs adjusted, or there is a problem inside the transfer-case (ie- broken shift-lever, broken chain, or loose gear, depending on which metod of drive it has (they WERE gear-to-gear, in my 1981 Isuzu, which means no chain to break...)
OR- if your vehicle has the same system of activation on the 4-wheel drive, as the Chevrolets of that era (Isuzu and Chevrolet were together and may still be), it's probably a vacuum problem. See, there is a vacuum-pot that pulls the cable to actuate (lock-in) the front-axle, using vacuum from a line on the transfer-case (vacuum that comes from the engine). IF there is a hole in any of the lines &/or if the vacuum-pot itself has a hole in it, then your 4-wheel drive linkage (on the front-axle) WON'T be pulled-in, so it won't engage, at all. This happened to me in my 2000 ZR2 during a snow-storm! It clicked, but wouldn't engage.
Go online or get a reference-manual from Oreilly's, Autozone, etc. to see how your system is actually actuated.
If it is manually actuated, then look for a linkage or a line that needs adjusted (HOPE it's not inside the case!) or IF it is vacuum actuated, then you can test it by starting the engine and spraying all the lines and the vacuum-pot with WD40 or other aerosol, to see if the engine revs- which will tell you where the problem (leak) is.
Either way, GOOD luck!
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