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How Does The 4wd Work On The Gmt900s?

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I never feel when the truck engages 4 hi and only hear an audible cluck when going into 4 lo. In my brother's 01 Yukon, you can feel when it shifts into 4 hi. Does the GMT900 use a CV joint or something to attach the 4wd?

 

 

Anyone have the schematics of the 4wd system?

 

 

I don't know why, but stuff like this makes me curious. :lol:

Snow White Enthusiast

Snow White

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in mine, you know when it's in 4lo and it doesn't go easily..... from what I've read, there are clutches that engage the front end.

gnet158 Senior Enthusiast

gnet158

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You should hear the clunk in 4HI, but to get your truck into 4LO you need to:

 

1. Have your truck rolling (under 5 mph)

2. Place the shifter in N

3. Move your switch to 4LO

 

It will not engage unless your truck is moving, you should see it blinking if your not moving. Or at least that's how my truck works.

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I know how to operate the 4wd ;)

 

 

I'm curious as to how it actually functions. Is it more like a true AWD system in which it uses clutches for the front end or does it still use strictly gears in the transfer case?

AgDoctor Senior Enthusiast

AgDoctor

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Most likely it has an actuator(electronic) as the past two platforms have had. Although I have not heard of any going out on anything later than a '99 classic silverado. I am fairly sure this is the way the 4WD works though.

marine_devildog801 Enthusiast

marine_devildog801

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Question on engaging 4wd. I was taught that you stop the truck, put it into 4wd (no need for Neutral for high, only low), move backwards in reverse briefly to set it in, and then take off. I have never had problems doing it this way. Is it better to be slowly rolling (except for low of course) to engage it? And what have you heard about rolling backwards? Any need for it?

AgDoctor Senior Enthusiast

AgDoctor

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I shift into neutral, stopped or not, and then re-engage Drive(or limit the gear by chosing) and go. however there are many times when i simply do not have enough time to do these steps and i press 4 HI and put it to the floor. i have engaged at 20 mph. hope that helps.

Wolverine350R Enthusiast

Wolverine350R

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In my 99 Sierra (Autotrac) there is no real indication of the change to 4 hi at any speed and I've gone into 4auto at 50+. I always completely stop to engage 4lo, in neutral. The only time it didn't lock in within 2-3 seconds was when a rat chewed the wires to solenoids.

nickvrebel Senior Enthusiast

nickvrebel

Posted
Posted
Question on engaging 4wd. I was taught that you stop the truck, put it into 4wd (no need for Neutral for high, only low), move backwards in reverse briefly to set it in, and then take off. I have never had problems doing it this way. Is it better to be slowly rolling (except for low of course) to engage it? And what have you heard about rolling backwards? Any need for it?

this is not necessary on any GM trucks as far as I know, GM has "shift on the fly 4wd" - I know on the early 90's Fords you had to stop, put it in neutral and all that jazz, and to unlock their auto hubs you had to back it up sometimes. You can engage 4Hi and back to 2wd at any speed at any time, auto trac or manual shift, doesn't matter. The only exception is to not do it while the drivetrain is under load (during excessive throttle), in that case the auto transfer case will wait until you let out and the manual transfer case may grind gears.

cmh2001 Enthusiast

cmh2001

Posted
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the truck does not have to be moving to go in 4lo but it goes in faster if you are

 

 

I hope not, if I were stuck and wanting 4Lo, this would make me very unhappy.

music Senior Enthusiast

music

Posted
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Question: Where are the front CV axles physically locked? Does that happen at the hub, or in the front diff? I'm assuming it locks in the differential and that there is a slip clutch in the transfer case, but this is my first GM 4wd, and I haven't worked on (or studied) the system at all.

nickvrebel Senior Enthusiast

nickvrebel

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Question: Where are the front CV axles physically locked? Does that happen at the hub, or in the front diff? I'm assuming it locks in the differential and that there is a slip clutch in the transfer case, but this is my first GM 4wd, and I haven't worked on (or studied) the system at all.

it's locked in the front differential. the hubs are never disconnected. The front diff is a open diff and uses an actuator to engage it. the transfer case is gear/ chain driven - the only GM transfer case to use clutches is the AWD systems.

redvett Senior Enthusiast

redvett

Posted
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The Autotrac transfer cases have a clutch to engage the front drive shaft. When in auto the front actuator is engaged and upon wheel slippage the transfer case module engages the clutch until slippage stops. In 4hi & 4lo the front axle is engaged and so are the x-fer clutches.

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