4x4 No Worky.... Maybe this will help....
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4x4 No Worky.... Maybe this will help.... - 12/6/2006 11:21:09 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3479
Joined: 5/11/2005
Status: offline
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The S-Series trucks (GMC too) have several options for the Transfer Case. The Transfer case or t-case for short takes power from the engine/transmission and splits it to the front and rear axle. Thus driving both axles under the truck.
The 2 common versions of t-case are;
1) Manually shifter via a floor shifter.
2) Electronically shifted via a push button on the dash
Regardless of t-case they all have one thing in common. A vacuum controlled engagement of the front axle. When you push the button, or shift the lever the vacuum switch in the t-case provides vacuum to the actuator which in turn pulls a cable that locks the front axle. Vola 4x4.
To rule out the t-case as being broken, start the truck, leave the tranny in park, put the truck in 4 hi mode, reach under the drivers-side and grab the front driveshaft. Try to turn it by hand, if it turns your t-case is not in 4x4 mode. If it doesn't turn then your t-case is in 4x4 mode.
If the t-case is locked in 4x4 mode, then the problem lies in the vacuum system.
The vacuum system consists of a switch on the t-case, vacuum hoses, vacuum actuator, cable and front axle.
Lets take them one at a time.
The vacuum switch is located on the t-case between the front driveshaft and tranny. It has three hoses attached to it.
The three hoses- one provides vacuum to the vacuum switch. One is a vent line and has a tee fitting on top of the t-case that goes to another vent line on the back of the tranny, and then up next to the battery and has a little filter on the end to keep debris out. The last hose provides vacuum from the switch to the vacuum actuator.
The vacuum actuator is located under or next to the battery. Older trucks have it next to the battery tray. Newer trucks have it under the battery tray, you have to remove the battery and battery tray to access it. Battery tray bolts are above the tire in the fender well.
There is a cable that runs from the vacuum actuator down to the front alxe. The vacuum from the t-case pulls the cable and it engages the front axle. Rare for this cable to break or get stuck, but it happens. A quick easy check to make sure the cable works and the vacuum system is broke. Pull the cable about 1/2"-3/4" and attach a pair of vise grips so it wont retract. Drive the truck and see if the 4x4 is working.
You keep saying vacuum, where does this vacuum come from?
Good question. It comes from the engine. I wont go into how the engine creates it, just know that it does.
There is a vacuum port on the back side of the intake manifold next to the distriburator. This provides vacuum to more then just your 4x4 system, it also provides power to your HVAC (heater and A/C controls), and a few other things. There is a tee fitting shortely behind the distriburator, look for it. It splits the vacuum, one hose goes down to the t-case, one hose to the center of the firewall and feeds your HVAc system, and one hose oges to the big black ball. A leak in any of those hoses will cuase your 4x4 to stop working, and you might have trouble with your HVAC controls.
The vacuum hose runs down to the t-case, remember the three hoses attached to the t-case I wrote about earlier? The vacuum switch sends that vacuum up to the actuator by the battery. To test the actuator you have to pull the vacuum hose off. Then depress the rubber diaphram with your finger and place your thumb over the hole the vacuum hose was on. The object is to stop the air from going past your thumb. If the diaphram moves, its defective and needs replacing. It the diaphram holds vacuum, then your trouble lies elsewhere. More then likely in the vacuum hose.
All three hoses run from the vacuum switch on the t-case, over the tranny, and follow the tranny dipstick tube into the engine compartment. There is a bracket that holds them in place. The bracket is bolted to teh top tranny bellhousing bolt hole on the passengers side. This bolt is a real bear to get too.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: 4x4 No Worky.... Maybe this will help.... - 12/8/2006 2:58:38 AM
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kakn
Posts: 57
Joined: 12/2/2006
Status: offline
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what if you cant manually shift it into 4lo? i have a shifter on the floor and cant get it to go into 4 lo but i can get it into 4 hi. as much as i pull it wont go and the button on the top of the shifter (if it even is a button) doesnt work.
i had to take a friendhome tonight and i got stuck in his drive way with about 3"'s of snow. took an hour to get me out. does this sound right? i know the 4 hi works b.c. we tested it like in the posts ive read. ive got brand new tires on my blazer but they arent by any means an agressive tread. they look like car tires or would be good for summer driving. ive got 3 bulged disks in my back and i figured a 4x4 blazer would be nice but ive done more shoveling of snow b.c i got stuck in a drive way than id like.
_____________________________
1991 2 door 4x4 s10 blazer auto 4.3
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RE: 4x4 No Worky.... Maybe this will help.... - 12/8/2006 11:12:43 PM
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Hanr3
Wheels & Tire Moderator
Posts: 3479
Joined: 5/11/2005
Status: offline
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If memory serves me well, yes the button on top is important. You need to press it down in order to shift into 4lo. The tranny also must be in nuetral. The button releases the shifter so it can be shifted into 4lo. Its a safety feature so you dont accidently shift from 2wdr to 4lo at 45 mph. If you did, you could do some serious damage. I suggest you wiggle the shifter and push the button. Wouldn't hurt to spray some PB Blaster around the button and let it soak in for a while. There is a linkage under the button that runs down to the base of the shfiter. If you remove the rubber boot you can see it. Spray the rubber boot with WD-40 around the base of the shfiter knob. It should and will slide out of the groove under the knob. Then you can pull the whole upper rubber boot off and get a better view.
4hi should be enough to get you out of 3" of snow, unless you got crossed up. One tire on each axle is free spinning. The way to fix that is to lightly apply the brakes. That makes all the tires gain traction, somewhat.
Tires play a huge role. Street tires are great for street driving, but in the snow and ice, you need something more aggressive. All-Terrain tires is more like it. They make a smidge more road noise then street tires, however the performance in the slippery stuff is hands down better.
I run a set of BFG All-Terrains and I was pushing people out with my grill guard during this past snow storm. We got 2" of slush that turned to ice, and 8"-14" of snow on top of it. Even on unplowed streets I was doing alright, even while pushing a car down the street. I was having trouble in the 14" stuff. Heck, tonight while out Christmas shopping with the wife I pushed a car off a huge ice pile. He got a chuck of ice stuck up under his car and was stuck. I had all fours on ice and pushed him right off at a little over idle.
And your positive that all four tires are getting power when your in 4hi?
4x4 means at least one tire on each axle is getting power. The trouble with open dfferentials is that hte one tire with power is the one with no traction. Hence the lightly applied brake trick. It slows the free spinning tires down, and provides power to the other tire on the axle, not 50/50 split, but usually enough to get you moving.
Another trick is to turn the front tires back and forth. This helps the edge of the tires grab something, anything. Another reason All-Terrain tires are better then street tires in the slipery stuff. The All-Terrains have lugs on the sides to grab something, while the passneger tirs are rounded and smooth for better cornering at higher speeds.
Hope that helps.
_____________________________
"Some people spend an entire lifetime wondering if they made a difference.
The Marines don't have that problem."
-President Ronald Reagan
'87 S10-Blazer - SOLD
'93 S10-Blazer 4x4x4 - Sons DD, engine swap is done.
'00 S10-Blazer 4x4x4 - My
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RE: 4x4 No Worky.... Maybe this will help.... - 12/10/2006 10:29:45 AM
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John Boelte
Posts: 7
Joined: 6/13/2006
Status: offline
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Great Technical post on how the 4x4 system works. I wish I would have read this before I fixed my problems, would have saved me some time. :)
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