I just bought a purple 1996 GMC Jimmy from a friend. It snowed 5 inches last night and I never got to test the 4 wheel drive till now. Well it dosen't work............ I have no clue what to do. I have 3 little girls and spending a lot of money this month on them. Don't really have the money to see a mechanic. Can anyone help me????? Give a few ideas? ? ? I have friends that will fix it, if they knew what to do....Please help me

 

Welcome to the site.

Congrats on the kids, they are the best mod.
I hear ya on money this time of year. I have 2 sons. The older they get the more expensive the presents cost. My oldest is 21, and the youngest is 17. I spent a fortune on Christmas this year. Close to 2K by time its all said and done.

What happens when you push the button, anything? Does the light for 4hi come on, flash, etc?

 

Thanks.....Yes the light does come on for hi and low. But no 4 wheel drive

 

That is good news.
The next thing to verify to see if it is working is the front driveshaft.
Push the button to put it in 4hi, put the truck in park, kneel down and grab the front driveshaft, try to spin it. If it doesn't spin then the modulator is locking the t-case in gear and the drivesahft is engaged. If not then it could be your modulator. However more then liekly since the light is coming on, I suspect your modulator is working and it should be engaging the front driveshaft. All good.

IF were all good to this point, its getting cheaper to fix. Plus you are getting to the more common problem and easiest to repair.
It could be a vacuum line that is cracked, broken, or fallen off.

There should be 3 vacuum lines that run to the top of the t-case.

One is a vent line and it goes into the engine compartment by the battery.

The second should be a vacuum line that supplies vacuum to the system and it goes intot he engine compartment behind the engine on the passengers side, although it ties into the vacuum ball in teh engine compartment too. I am guessing your heater controls work fine, in other words when y ou move the levers the air comes out of the different vents, like all up top by the windshield, or split between the windshield and floor, etc. If not it helps determine where the vacuum leak is.

The third line goes to the vacumm actuator which controls the front axle engagement. Teh actuator is located under the battery. The battery and battery tray need to be removed to gain access to teh actuator. Make sure it holds pressure. You can take the hose off, assuming it hasn't fallen off. IF it did fall off, more then likely that might be your problem. As I was saying, you can take the hose off, put your thumb over the hole and push on the diaphram with your fingers. If you feel resistence it is holding vacuum. To verify, leave your thumb off the hole and move the diaphram with your fingers, place your thumb over the hole, and let the diaphram go, it should stay in place, if not it is leaking and needs to be replaced.

If the diaphram (vacuum modualtor) is in good shape, I would suspect one of your vacuum lines is cracked, broken, or has fallen off. Replacing the hose is cheap (less then a $1 per foot normally) The hose just pushes into place, can be a pain to trace around the engine compartment, and it usually breaks off at the fittings, like behind the intak manifold where it is hard to see. You cna listen for the leak with some of the new vacuum hose. Cut off about a 4' piece, place one end next to your ear, and move the other end around the engine compartment. Carefull of moving and hot parts. The hose doesn't need to be straight to work, so you can turn your head and watch where your hand is. Just don't kink the hose. When you hear a sssssss sound, you found your leak. Repalce with new hose.

There is a temp fix, if your interested.
When ever you need to engage the front axle, push the button. Open the hood and pull the cable out and attache a pair of vise grips to hold tha cable out. As long as the vise grips are there your front axle is engaged, even if you push the button for 2wdr, the front axle will still be engaged, make sure to disengage it before driving on dry pavement. The cable is attached to the other end of the vacuum actuator under the battery, you might have to temporary disconnect it and place it somewhere where it won't get damaged.

I have attached a few pics of the actuator so you know what it looks like, and a pic with the actuator removed so you can see what the cable looks like. These pics are from my '87 S10-Blazer. The actuator on teh '87 is next to the battery, not under it like the newer models. Please ask if you don't understand or have more questions. There is a lot of info here and most won't make sense until you see things in front of you, hence the pics. IF the cost of repairs gets over $50 you should seriously consider replacing the vacuum

 

That diaphram (black piece does have slits in it. Not sealed completely. All the vacuum lines seem to work fine. What do you think is the cost of replacing that?

 

Not sure, someone said it was like $70. Although I bet you could get one from a junk yard pretty cheap. They either work or don't.