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Why Won't My 4wd Work?

sahls01gmc

By sahls01gmc
in Ask A GM Technician

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sahls01gmc Senior Enthusiast

sahls01gmc

Posted
Posted

I can't seem to figure out why my truck doesn't have 4wd anymore. When I push the 4hi button, it lights up normally, and I can hear the solenoid engage but the front wheels are not spinning with the rear.

 

 

What else should I be looking for? I'm assuming the solenoid is ok, since I can hear it working. I also have not thrown a code or seen "Service 4wd" yet either.

 

 

Thanks for any advice

 

 

James

NitroCherokee Enthusiast

NitroCherokee

Posted
Posted
I can't seem to figure out why my truck doesn't have 4wd anymore. When I push the 4hi button, it lights up normally, and I can hear the solenoid engage but the front wheels are not spinning with the rear.

 

 

What else should I be looking for? I'm assuming the solenoid is ok, since I can hear it working. I also have not thrown a code or seen "Service 4wd" yet either.

 

 

Thanks for any advice

 

 

James

 

You may want to add Year, Make, Model of the truck, does it have Auto 4wd? How anyone could come close to a diagnosis with the info you provided is beyond me INFO INFO INFO

NitroCherokee Enthusiast

NitroCherokee

Posted
Posted
its right in his signature below the post

 

So I should "assume" that is the one he is talking about? I still don't know which T-Case it has

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Vehicle is the one in the sig below. Yes, it has the Auto 4wd as it is not an HD.

 

My bad, I thought that would be easy to figure out if I didn't give any other information than what is listed in my entire post.

 

 

James

NitroCherokee Enthusiast

NitroCherokee

Posted
Posted

I would be looking into the front axle acctuator on that setup, the reason I asked for more info was on some of the non auto 4wd t-cases there was a problem of encoder motor's not holding all the way into 4hi. Just for giggles put it in 4lo and see if the front grabs, most likely will not and will ensure you do have an acctuator problem

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Appreciate the info Nitro. No, nothing happens in 4lo either.

 

You're saying that there are 2 actuators on my set-up? 1 on the T case, and another on the front axle?

 

 

 

James

NitroCherokee Enthusiast

NitroCherokee

Posted
Posted

There isn't an "acctuator" on your T-Case it is an Encoder Motor yes there is a acctuator on your fron differential if you look under your truch at the right side of the front diff runnin parralell to your right axle housing you will see a black accuator with a wire harness going to it............... If you didn't know it was there I would not recommend trying this repair yourself leave it to the pro's

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Ha, just looked and saw it! All these years, and I never noticed it. Although, thankfully I haven't had to spend hardly any time under it. I can handle fixing it, it only fits 1 way. I'm pretty handy under the hood, just gotta make sure to do it right. What is the reason for having 2 different "actuators or motors" for 1 4WD set-up?

 

 

If that is the problem, does someone know a part # for it? I've only been able to find motor on the T case, but nothing yet on the front axle.

 

James

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Awesome, thanks Hawkeye! I think I have the right one now. Can anyone else confirm part # 600-101 or TCA-22 for the 4WD axle actuator is the right one?

 

Any way to test the current one to make it works/doesn't work? Does it make a sound, or a clunk, or anything in between?

 

 

James

NitroCherokee Enthusiast

NitroCherokee

Posted
Posted
Ha, just looked and saw it! All these years, and I never noticed it. Although, thankfully I haven't had to spend hardly any time under it. I can handle fixing it, it only fits 1 way. I'm pretty handy under the hood, just gotta make sure to do it right. What is the reason for having 2 different "actuators or motors" for 1 4WD set-up?

 

 

If that is the problem, does someone know a part # for it? I've only been able to find motor on the T case, but nothing yet on the front axle.

 

James

 

The Motor on the T-Case just took place of the floor shifter I guess for sissies that can't grab a lever and need to push a button, the actuator just unlocks the front diff so your not spinning the whole drivetrain all the time, I assume this was done in the intrest of gas mileage

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Ah, that all makes sense now. I know other "brands" have the hubs you have to get out and manually engage.....it all makes sense now. I'm no sissie, just bought the truck that way. The extra floor space is nice though!

 

Thanks Nitro for your quick responses, I think I'm close to having it all fixed now!

 

 

James

sahls01gmc Senior Enthusiast

sahls01gmc

Posted
  • Author
Posted

Ok, here's what I got so far.

 

I jacked the whole truck off the ground, and went through the gears and 4WD options.

 

I messed with the actuator on the front axle, and I believe that it does in fact work.....partially. With the truck in 4hi, the 4wd seems to work ok. Sometimes switching it in and out of 4wd (always with brake on either in D, N, or R) I heard a slight grinding noise when it is in 4wd and I let off the brake. But when I go to R for a sec, and put it in D, it then works correctly. Sometimes the front axle would not engage at all, and the R for a sec and back to D process fixed this. I didn't go through the same symptom tests with 4lo, but I did get it to work when I did engage it.

 

 

Should both front wheels be turning when in 4wd? The handful of times I got out and checked all 4 corners, the passenger's side never spun. It also appeared that the front was spinning faster than the rear, is that possible? Or was I seeing the driver's front at the correct speed, and the driver's rear was "slightly" engaged as the posi had not locked fully?

 

 

 

Quick tech notes (all taken off compnine.com with my vin entered):

 

4L80E Transmission

NP8 Transfer case

3.73 Gears

G80 Posi rear end

 

 

James

treefort Enthusiast

treefort

Posted
Posted

My 4WD stopped working last week too. The front wheels wouldn't spin but everything else looked good. In 4 low the truck stayed in low gear as well.

 

If you turn the key on and turn off the radio so it is quiet you can hear the motor, then the actuator. They might still be bad but if you don't hear one of the two it will help in isolating the problem.

 

Brought my truck into GM today and they found a broken shift collar and fork. And the splines on the front axle were shot. Thankfully it was covered under powertrain warranty. Otherwise it would've been $1600.

 

I will still be updating my thread with this info as it will assist someone with the same issues. Good luck, hopefully you have a nice warm shop and some time on your hands.

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