1994 GMC 4X4

I have a 1994 gmc ext cab 4x4 and the other day while driving at a crawl speed in 4x4 i heard a clunck and my 4x4 stopped working.I can engage the shifter into 4 hi however the 4 wheeldrive lights dont come on and i have no 4 wheel drive.

any suggestions,

thanks  

is it auto locking hubs or manual... i dont know gms to well. but i had a jeep with auto hubs and it went clunk and had about the same symtom. it turned out to be the actuator shift fork snapped  
Nothing Better Than A Fullsize Ford

its auto  

i would check out the fork. its most likely snapped. if its set up like the jeep i had it wont be hard to get at. and not to hard to replace.  
Nothing Better Than A Fullsize Ford

ok, thanks so much i will check it out  

leavy20:
is it auto locking hubs or manual... i dont know gms to well. but i had a jeep with auto hubs and it went clunk and had about the same symtom. it turned out to be the actuator shift fork snapped

There is no auto of manual hubs on a GM 4x4 that year. Axles are bolted right to wheels. The 4x4 light is controlled by actuator on front axle. That year has a Thermo one that is unreliable with age. First thing to check here is action of that actuator.  

leavy20:
is it auto locking hubs or manual... i dont know gms to well. but i had a jeep with auto hubs and it went clunk and had about the same symtom. it turned out to be the actuator shift fork snapped

SnoMan:
There is no auto of manual hubs on a GM 4x4 that year. Axles are bolted right to wheels. The 4x4 light is controlled by actuator on front axle. That year has a Thermo one that is unreliable with age. First thing to check here is action of that actuator.

I hate to tell you this but GM did have auto hubs that year.I'm not sure how many they made with them but I have been in both jimmys and full size chevys with auto locking hubs(push button 4x4,no need to get out and lock them in)  
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leavy20:
is it auto locking hubs or manual... i dont know gms to well. but i had a jeep with auto hubs and it went clunk and had about the same symtom. it turned out to be the actuator shift fork snapped

SnoMan:
There is no auto of manual hubs on a GM 4x4 that year. Axles are bolted right to wheels. The 4x4 light is controlled by actuator on front axle. That year has a Thermo one that is unreliable with age. First thing to check here is action of that actuator.

Finder:
I hate to tell you this but GM did have auto hubs that year.I'm not sure how many they made with them but I have been in both jimmys and full size chevys with auto locking hubs(push button 4x4,no need to get out and lock them in)

Boy you are dead wrong here. Dead wrong!!! (I can tell you have never had one apart!!!) Half shaft are splined DIRECTLY to hubs and wheels. What you are calling automatic hubs is a thermo actuator that couples and decouples half shaft on passenger side and lets other axle spin out in spiders and stops ring gear and drive shaft from being driven by wheels in 2wd. These thermo actuators where used up to 97 and can have a never style servo drive one retro fitted in its place which is a lot more reliable.  

yeah it sounds like the shift fork the actuator pushes on in the frt diff broke so its not engaging the frt diff.
On the K1500 bodies from 88-98 pick ups there was never an auto locking hub like the ones you find on the k10-v10 bodies of the late 80s, and subs/k5 blazers of the early 90s, that type of auto hub was only on the solid axle and not on the ifs frt ends.the push button 4x4 (auto) system uses the same frt end as the manual shift 4xs.  

snoman you are saying that on of the axle shafts is cut in half has splines on each end and a sleeve that slides to lock those to shafts as one and that sleeve is driven by a fork generally by a vaccume. correct that is what i am calling an auto hub. if its like dodge jeep its call a central axle disconnect... i believe. i am not a dodge or gm expert. and will admit to my limited knowlege on these vehicles.  but i do believe that we are all talking about the same thing here a central axle disconnect. and he should check his fork as it requires limited amount of effort.  
Nothing Better Than A Fullsize Ford

ther is no vacuum on the gms 1/2 tons-1 tons frt ends they are electric. vacuum was used on the s-10 bodies.  

but its the same concept correct only its driven by a electricity rather than vacuum correct?  
Nothing Better Than A Fullsize Ford

leavy20:
but its the same concept correct only its driven by a electricity rather than vacuum correct?

Yes, the is a dog clutch in diff that couples and uncouples passenger side half shaft from diff, inside diff case. It is thermo electric thru 97. A "pill" is heated and it expands and extends fork on dog clutch to engagement. Power must be continuously applied for this and it will decouple with truck is shut off/power removed even though 4wd drive is still selected. It is also effected by cold weather and in extreme cold or cold water in can cool it and cause it to disengage even with power applied. Starting in 98 it was servo driven in and out of engagement meaning that once engaged, it stayed engaged even when power was removed (there is a limit switch on servo that removes power from it when at end of travel and power is reversed to drive it to other limit switch to decouple)  

sounds to me he should still check his fork and/or the electricals  
Nothing Better Than A Fullsize Ford

leavy20:
but its the same concept correct only its driven by a electricity rather than vacuum correct?

SnoMan:
Yes, the is a dog clutch in diff that couples and uncouples passenger side half shaft from diff, inside diff case. It is thermo electric thru 97. A "pill" is heated and it expands and extends fork on dog clutch to engagement. Power must be continuously applied for this and it will decouple with truck is shut off/power removed even though 4wd drive is still selected. It is also effected by cold weather and in extreme cold or cold water in can cool it and cause it to disengage even with power applied. Starting in 98 it was servo driven in and out of engagement meaning that once engaged, it stayed engaged even when power was removed (there is a limit switch on servo that removes power from it when at end of travel and power is reversed to drive it to other limit switch to decouple)

Sorry sno...but I have to disagree w/you on the fact that the thermo needs constant power to be engaged.my 93 k1500 has the old style thermo actuator and I drive in 4wd pretty much every day because of the ice and snow and when I get home from work I shut the truck off w/it in 4wd and when I go to work the next day I fire it up and its in 4wd. no waiting, its there.