92 GMC Sierra 2500 4WD not engaging

[bashmanx]

Hey all, I have a 1992 GMC sierra 2500, 5.7 liter automatic, with 4WD engaged by a lever and 110,000 miles. About 90% of the time, the truck will not go into 4wd drive, hi or low. It worked all last year, and this winter it started doing this. When it engages, it works normally, but getting it to engage is the issue. The indicator light works because when the 4wd works, the light is on, but when the 4wd doesn't engage, the light is off, as it should. Any ideas on a possible solution? Thanks, Brett

[grampadirt]

It should go into 4hi in gear while moving. Do you give it a good solid pull into 4hi?All the way? I found I need to shut my engine off to get it to go into 4lo so the gears in the transfer case won't grind.Remember the gears for 4lo are spinning at a much different rate.

[bashmanx]

Oh, believe me, I pull it all the way. And, it works some of the time when I move the lever, but not other times.

[sputtle]

Seems silly, but crawl under there and see if there is something mashed up in the linkage. Also, make sure there isn't any rust impeding the movement from the lever. This happened to me once.. When I was 13, I took the old farm truck mudding in our neighbor's field and got stuck. Dad was pretty pissed, so was the neighbor. I got stuck because it wouldn't go into 4x4. I mashed so much mud up into it, the lever wouldn't fully move.

 

If that's not it, sounds like a transfer case problem.

[Friz2]

Is the problem physically getting the lever into the 4hi or 4lo position or is 4wd not engaging in the front hubs?

[bashmanx]

Thanks for all the responses guys. The lever moves into gear just fine, and when I put it into 4low the rear wheels do go into 4 wheel. I thought it was 4x4 lamp switch, and replaced it. This did NOT solve the problem. When I was under it today I noticed that while in park and not running the front drive shaft can be spun. Is this normal? I still do not have 4wd working, so any suggestions would be helpful. The guys at the part store thought it may be the actuator. It's about 90 bucks, so before I spend the money I thought I'd see if you guys would do the same.

[Friz2]

The front axle actuator is thermal and they do go bad.

If the Xfer case is engaging, the front drive shaft should spin in 2wd and be fixed in 4wd if you are under the truck trying to spin it while the truck is stationary.

I'm trying to remember way back when the actuator went bad on my '95. I do believe that the 4x4 light would not come on when the actuator was bad.

[grampadirt]

Consider this...before spending the money on a thermal actuator investigate the upgrade to an electric(12v)actuator like the later model GMT400s.I got tired of waiting for the gas in the thermal actuator to get hot enough to extend the plunger to engage the diff.With the electric actuator it's almost instantaneous.Also it will not disengage until you shift out of 4hi or 4lo,the thermal one might disengage when it gets cold out or going through cold water or snow.I used Dorman parts.

[oldscsc]

If the system is vacuum actuated, check all the lines for cracks or other abnormalities. A leak can prevent 4x4 engagement.

Edited June 20, 2015 by oldscsc

[Super 8]

4WD 25 amp fuse in panel by left knee, if this is popped your actuator is bad.

[chip lambert]

ok ,my 4wd in my 1988 k1500 will not engage ,i  have one hot wire to the actuator in the diff , if i have current there can it be anything but the faulty actuator ? i replaced the actuator once probably 15 years ago ,i heard they have updated it recently ?the two switches on back of transfer case is one a speed sensor ? rear most and what is the more forward one ? no current to the rear most and one hot wire to the forward one ?