4 wheel drive problems

i am having problems with my four wheel drive. It just suddenly stopped engaging 4hi and i can only get 4 low. its stopped letting me engage during a very bad snow,i was out in it most of the day and i have alot of snow and slush and ice built up on my vehicle so i wonder if the four wheel drive is sensitive to temp and ice. i have four low but it takes a long time for it to engage into 4low. if anyone knows whats going on with my four wheel drive i would love any imput.

Check your vac hose to the diaphram under the battery, check the vac diaphram for tears and make sure it's still connected to the cable. After that check the vac switch on the transfer case. If all these things are good, it's most likely your encoder motor, but I would say it's vac related from the sound of it...

I'm going to suggest that the vacuum system can be affected by temperature/moisture. I only have problems with my front diff trying to disengage itself while in 4HI when the temperature is -10 celcius or below and after extended highways speeds. It will start to humm rather loudly. If I throw it into two hi, and pull over, let the truck idle for a minute or so and then put it back into 4hi, I'm good to go again. I had the vacuum switch replaced as the last time this happened it chewed up my front diff. Still happens on occasion.

Ok i will have to wait for the foot of snow that has fallen before i can do anything with it but now my four low is not engaging it trys to but it waits till i hit like 2k rpm and then it kicks in really hard and disengages when i slow down this is really starting to suck cause like i said its snowed like almost a foot and i can't really go anywhere

sounds like a leaking vac system when you say that, when you rev it up the vac overides the leak until you let go... Check yer hoses..

If you have 4 wheel low then your vaccumn system is most likely working correctly. Though a bit slow due to the weather. If you don't have 4 wheel high I would suspect the TCCM (transfer case control module)

The way the system works in the 99 and up (possibly earlier but I don't know for sure). Is that the Transfer case is in 4 wheel high at all times unless in 4 wheel low or neutral ( which in both cases not very often).
Now 4 wheel high becomes effective when the front differential is engaged, when the front is dis-engaged you are in 2 wheel high. The front differential is engaged via a vaccumn pod and cable conector found under your battery tray on the passenger side. There is a control solonoid located on the firewall lip above the distributor. This sometimes goes bad. It is available from GM for about $43.00. Something else to check before you replace the vaccumn solonoid is the vaccumn line itself. I have had one that was burned trough and leaked all of the vaccumn. Replaced the line with a new lenght of 5/32 vaccumn hose and the system was back in service. If engagement is slow I would first check for a vaccumn leak or a sticking solonoid.

Hope this hepls,
Heli

I found out what the problem is, both motors that engage the four wheel drive have shorted out. this has happened to me before about 2 years ago and the motors only have less than 20000 miles on them so i am hopping they are covered under a waranty i had it done at my dealer .