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Can’t get 4x4 to Work

gto0188

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gto0188 Enthusiast

gto0188

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Posted

2001 GMC Sierra SLE 4x4 4.8. It has the push button 4x4 setup with Auto 4wd.

 

Owned the truck almost a year. 125,000 miles. 4x4 worked great last winter. Not working this year.

 

Couple days ago during snowstorm I selected AUTO 4wd and was about to start my journey. Truck was slippin and sliding and clearly not in 4wd. Just as I started to recognize the condition, the Service 4wd System light came on. I pulled over to a side road. Shut the truck off and restarted. Service 4wd light went away and this time I selected 4 HI.

 

Still not working in 4hi even though the light is illuminated on 4hi. Ok great. This time I try 4 LO. Light is illuminated on 4 LO and the truck is clearly in low range because the truck is much slower but torquier. BUT, still no traction, front wheels not engaging.

 

I can hear the transfer case doing its motor thing and engaging between all the 2wd and 4wd options. I removed the front diff actuator and that is functioning fine (retracting and moving). I decided to put a new one in (Dorman) after removing the original because the plastic threads got kinda messed up.

 

Still no 4wd. I jack up the front of the truck. In 2hi each front wheel is independent from one another. I engage any of the 4wd options and yup, the diff is definitely engaging because now the front wheels react to one another when the truck is jacked up.

 

I had a friend carefully drive the truck while I observed the front drive shaft. It spins although I also notice it spins in 2wd so not sure if thats a good test. 

 

All 4 wheels on the ground if it’s in 2wd I can spin the front driveshaft. In the 4hi and lo modes it definitely locks in.

 

Ok on to the transfer case fluid. Drained it out. Black and BURNT like real burnt stunk up my garage. I replaced with new Auto Trak II and tried to cycle thru the modes a few times and drive. Still no 4x4.

 

Is it safe to say my T case is screwed? Is it possible the T case motor although it seems to function that it may be broken? I would think the selector lights would be blinking and not going into the different modes if that were the case?

 

Think I bought a lemon :(

gto0188 Enthusiast

gto0188

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18 minutes ago, 75z28 said:

Sounds like the front hubs aren't engaging.  Are you sure someone didn't convert it to a manual hub lock?

Has not been converted.

 

As mentioned, with the front wheels off the ground and in 2hi, the wheels are independent from one another (spinning one wheel has zero effect on the other). Once I engage any of the 4x4 selections and I spin a front wheel, the other wheel spins opposite of it. That tells me the front diff is certainly engaging.

Supreme Pizza Senior Enthusiast

Supreme Pizza

Posted
Posted

Bad encoder motor? 

Bad encoder ring? 

 

To test the t-case, put the tuck in 4x4 mode, and then shut it off. Jack up the front end. Turn the front driveshaft by hand. It should not turn. If it does turn, then your t-case is not engaging power to the front shaft. 

gto0188 Enthusiast

gto0188

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Posted
14 minutes ago, starman8tdc said:

Bad encoder motor? 

Bad encoder ring? 

 

To test the t-case, put the tuck in 4x4 mode, and then shut it off. Jack up the front end. Turn the front driveshaft by hand. It should not turn. If it does turn, then your t-case is not engaging power to the front shaft. 

Thanks starman8tdc

 

I am going to revisit this... the encoder that is. Based on some further research I gathered, *usually* if there is something wacky going on with the encoder, the drive selections will not be successful which would be noted by blinking selector lights and failure to go into the selected mode.

 

However, I did read a few cases where all *seems* OK on the indictator... such as in my case.... where the encoder motor makes the audible noises during selection followed by a nice solid indicator light. BUT, the encoder motor isn’t holding in the clutches therefore it isn’t actually engaged.

 

Going to do the very test you just mentioned by jacking up the front.

 

I also have access to what seems to be a pretty elaborate Snap On scanner. Not sure if it will read GM 4wd codes but I will give it a shot. As mentioned, when the issue first started, the “Service 4wd System” light came on. It never returned but I’d love to see what the code was.

gto0188 Enthusiast

gto0188

Posted
  • Author
Posted
14 hours ago, starman8tdc said:

Bad encoder motor? 

Bad encoder ring? 

 

To test the t-case, put the tuck in 4x4 mode, and then shut it off. Jack up the front end. Turn the front driveshaft by hand. It should not turn. If it does turn, then your t-case is not engaging power to the front shaft. 

Thanks starman8tdc

 

I am going to revisit this... the encoder that is. Based on some further research I gathered, *usually* if there is something wacky going on with the encoder, the drive selections will not be successful which would be noted by blinking selector lights and failure to go into the selected mode.

 

However, I did read a few cases where all *seems* OK on the indictator... such as in my case.... where the encoder motor makes the audible noises during selection followed by a nice solid indicator light. BUT, the encoder motor isn’t holding in the clutches therefore it isn’t actually engaged.

 

Going to do the very test you just mentioned by jacking up the front.

 

I also have access to what seems to be a pretty elaborate Snap On scanner. Not sure if it will read GM 4wd codes but I will give it a shot. As mentioned, when the issue first started, the “Service 4wd System” light came on. It never returned but I’d love to see what the code was.

gto0188 Enthusiast

gto0188

Posted
  • Author
Posted

Scanned it today.

 

Code C0374 - General System Malfunction

 

After 10 or more consecutive slips are uncorrected, this code is activated.

 

Says if transfer case is shifting normally... which mine is.... problem may be internal.

 

The burnt fluid doesnt lie. Sounds like I need a new T case.

 

 

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