2000 4x4 troubles

I have a 2000 s-10 4x4 4.3 v6. Last week during an ice storm the truck was working just fine. This week during a snow storm the truck started having some kind of harsh engagement just after take off. It doesn’t do it every time and is only noticed in 4 wheel high and maybe in low. The truck will take off from a stop and after a second or two is when the jolt is felt. It feels like its shifting gears but I know its not. I don’t understand much about four wheel drive systems. It goes in and out of the different settings just fine.

Is it possible for the system to have a vacuum leak that can provide enough vacuum during idle but not during acceleration?

I have never heard of that happening before but I guess it is possible. Except you may have it backwards. If that is what's happening, It should be easy to test.

Jack up one front wheel, leave the truck running in park and in 2wd. The front wheel should spin by hand and the front driveshaft should spin by hand, each being independent of the other.

Put the truck in 4wd, with the truck in park this should lock the driveshaft. If the driveshaft still spins by hand your transfer case is not engaging.

If the driveshaft is locked then try spinning the front wheel. If it spins by hand but the driveshaft is locked then the front axle is not engaging. If this is true then rev the motor while someone is turning the front wheel, If it engages then something is keeping the front actuator from getting enough vacuum at low speeds.

That's the best I can come up with.

James

...Jack up one front wheel, leave the truck running in park and in 2wd. The front wheel should spin by hand and the front driveshaft should spin by hand, each being independent of the other.

Both drive shafts are "live" at all times. The two drive shafts are linked together with a chain in the transfer case. If they ever move independently of each other, there is a problem in the transfer case. This thread explains basic transfer case operation: http://www.automotiveforums.com/vbulletin/showthread.php?t=665369

Both drive shafts are "live" at all times. The two drive shafts are linked together with a chain in the transfer case. If they ever move independently of each other, there is a problem in the transfer case. This thread explains basic transfer case operation: http://www.automotiveforums.com/vbulletin/showthread.php?t=665369
That is the same description that I've seen many times. I have personally quoted the same information several times. However, as myself and several others have discovered, the actually operation is quite different. When the vehicle is in park and 2wd, the front driveshaft can easily be turned by hand. It doesn't become engaged until 4hi or 4lo have been selected. I have rebuilt several transfer cases, but unfortunately this style isn't one of them, so I can't say with 100% certainty the internal operations. However, being that my front driveshaft and many others can be easily turned by hand and the 4wd still works as it should it seems that this is normal. It also seems to be confirmed by looking at the exploded view and GM disassembly and assembly instructions, one can see that the transfer case has a "mode" shift fork in the chain assembly that goes from synchronizer hub on the mainshaft to the front output assembly, and a "range" shift fork on the range shift hub assembly. The range shift assembly is on the input shaft and ahead of the "mode" shift fork and synchronizer assembly. Due to its positioning the "mode" shift fork, and synchronizer assembly could only affect the correlation of the mainshaft (rear output shaft) and the front output shaft assembly. If the front driveshaft was indeed "live" at all times and 4wd was only acheived by the encoder motor activating the vacuum switch for the front diff actuator, then this "mode" shift fork and synchronizer hub would not be needed, because the chain would already have the mainshaft tied to the front output assembly.

Both drive shafts are "live" at all times. The two drive shafts are linked together with a chain in the transfer case. If they ever move independently of each other, there is a problem in the transfer case.

My bad, I should have specified that this works on the NP 233 (3 button) transfer case like is in my 2001 S10 Crew Cab and 2003 S10 Blazer. I have no idea how the autotrac transfer case works if that is what you are working on.

James