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2003 gmc envoy_slt_xl
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Discussion Starter #1
I have had this vehicle for half a year. Winter is just ending and now we get a few muddy/slippery spots when parking or pulling out. When stuck I engage the 4WD - Hi and the light goes on and it seems to drive normally. However when stuck the rear wheels spin and the front ones don't move. Is it possible that the 4WD is not working even though it doesn't seem to give any error codes or warning signals?
 

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My 4wd failed except in automatic mode, where 4wd engaged as it should.

Test yours by going into 4 high, and turning a fairly sharp corner - like pulling into a parking space. If 4wd is engaging it will bind up and it will feel like the brakes are applied. If it feels like you are turning in 2wd, you have a problem. Don't do this too much - it is quite hard on u-joints.

The dealer took me for a ride in the repair as well, but that's a story that can be found on another post.

-- Dan Meyer :coffee
 

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Discussion Starter #3
ok, so 4WD not working. What could it be?

There are no messages indicating a problem with the 4WD. The light on the switch shows that it will engage if you turn to any selection (even 4LO). What parts could possibly be causing the problem?



:confused:
 

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Welcome! This system is my specialty. There are two motors that work to engage 4WD. A front axle actuator that engages a splined collar on the passenger side CV shaft, and an encoder motor that moves clutches and gears in the transfer case. Each of them has a distinctive sound that can help diagnose failures.

But not yours. That trick only works if you get the Service 4WD light on, because the TCCM (transfer case control module) is looking at feedback signals on each of those motors to confirm that they moved to where the controller commanded them.

Your failure is much more likely to be mechanical. The cheaper of the two is the front axle splined collar that the actuator presses on. To confirm this, jack up the front of the vehicle, both sides on jack stands, put it in 4HI and listen to the motors whine. Confirm it's in 4HI and the light on the mode selector switch goes steady. Then turn the driver's front wheel. If the opposite one turns backwards, then the actuator is working and the problem is in the transfer case. If you can spin the passenger side front wheel freely, then the problem is in the actuator.

Some folks have done this work themselves - some pay mechanics. Let us know what the experiment tells you and we can advise further.
 

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Discussion Starter #5
Thanks, Roadie

I have read a lot of your posts but I couldn't find anything quite like this. I was hoping you would post. Fortunately the school where I teach has a full mechanics shop, so I can use the shop and check it myself. I will let you know what I find out.:thx
 

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Discussion Starter #6
What advice can you give to go from here?

Your failure is much more likely to be mechanical. The cheaper of the two is the front axle splined collar that the actuator presses on. To confirm this, jack up the front of the vehicle, both sides on jack stands, put it in 4HI and listen to the motors whine. Confirm it's in 4HI and the light on the mode selector switch goes steady. Then turn the driver's front wheel. If the opposite one turns backwards, then the actuator is working and the problem is in the transfer case. If you can spin the passenger side front wheel freely, then the problem is in the actuator.

Some folks have done this work themselves - some pay mechanics. Let us know what the experiment tells you and we can advise further.
The wheel spins freely. What are the options for replacing the front axle splined collar?
 

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A professional mechanic "Shawn d" posted in this thread:

http://forums.trailvoy.com/showthread.php?t=47365

Shawn d Shawn d is offline
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Join Date: Nov 2008
Posts: 10
Quote:
Originally Posted by q65js View Post
I'm pretty sure it's in the shift fork /splined collar assembly . I'd previously jacked up the right front and went through the motions , in 4hi the front driveshaft is engaged but the right front driveaxle turns ( it shouldn't) when it turns I hear a clicking sound coming from the driveaxle housing where the actuator/shift fork /splined collar assembly's are housed. I didn't see anything shiftfork wise for the acuator to push against when I unbolted it from the housing which is why I was wondering what I should see with the actuator removed.

You wont see much with the actuator removed (back side of the fork). With it removed all four wheels off the ground and someone in the drivers seat. Use a long 3/8 extention or something similar, push this into the actuator hole with the truck in 4 wheel, selector in drive and your foot off the brake. Apply moderate pressure, you should be able to lock up the front axle. If not you may indeed have a busted fork. If it does lock up you have a very common alignment problem (more common than I originally thought) my guess is that there are alot of people driving around thinking there in 4 wheel drive and there actually not.. You can also try this. With the actuator installed and all four wheels off the ground, put the truck in 4 wheel drive. With a pry bar or stick lightly pry up the passenger side inboard CV shaft knuckle. You should hear the clicking sound of the actuator trying to lock the axle. Once this slight pressure is applied the axle will almost imediately engage. If this is the case you have alignment/gear issues. Remove the CV shaft, remove the back side housing bolts that attach the gear box to the oil pan, next (on the bench) remove the gear case bolts, take the gear box apart being careful of the spring keep track as to what goes where, clean out all the grease in the parts washer and get things dried up for analyzing. Look closely at the CV shaft mating gear. Typically this will have a wear ring where it seats in the needle bearing if it does replace it. This is putting slop into the system causing things to cock out of whack. Next look at the shift fork. It will likely be worn at the tips where it rides on the synchronizer, replace it if it does. Most of the time these parts are always worn. Next you have a choice on the syncronizer gear and back side mating gear (thinner of the two) you can either replace or if you have a small pencil die grinder and a small carbide bit put a slight taper on the mating face of the gear and syncro. Your trying to eliminate any restriction in the gear and syncro from meshing without getting carried away. (dont even attempt this with a file, you will ruin your file these teeth are hard!) I typically do the beveling then I buff the face gear teeth to a mirror finish. Buy new parts or modify, either option works. Just depends on what you have for equipment. With either option I always replace the cv case side needle bearings, gear if worn even slightly, both front and back seals, and almost always the fork. Lastly I DONT use the recomended GM chasis lube. I've been using a molybdenum grease. Pack the gear housing lightly, dont get so much in there that nothing moves. Put a small bead of flange sealer or silicone, reinstall the box and shaft and your good to go. I have noticed that when its cold you may have to wait a few minutes for the grease to warm up with the engine for good engagement. I have often thought about drilling and tapping the case to accept pipe plugs and then use gear oil which would eliminate the cold weather issue. Only problem is that its such a small gear box if a leak were to appear and not get caught in time it would run out of oil quick and you would have some major problems.
If anyone has any questions let me know. I've done a few of these and its always the same issue weather you have 7k or 80k. Very crappy design...Many customers have had it to the dealer many times before it comes to me, I dont think GM is seeing it as a problem...


You can try to buy the parts one at a time, call junkyards, or buy the assembly from GM.

It's assembly #56 in this exploded view. GM part # 15884292, available at parts4chevys.com for $412.

 

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:ugh:Would it be a good idea to remove the Actuator and get some Moly grease in there as a preventative measure? ...sounds good to me, I mean better than nothing.:undecided
 

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Discussion Starter #9
Salvage - what models are interchangable?

We have really good salvage service here for parts, as it is government controlled. What models are interchangable for the front assembly? I may not be able to find an '03, but what years would be acceptable. I think in 05 they ended the XL, but it shouldn't matter for the front axle assembly, should it?

Also, I need to match to my 4:10 ratio when I get parts, right?
 

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The front differential is entirely separate from the part with the splined disconnect. Of course, your entire differential could be in fragments and have the same result of no front drive action, but you'd be hearing noises and your differential oil would be full of metal.

The differential is on the driver's side of the oil pan. An intermediate shaft (item #15 in the drawing) goes THROUGH a tube case into the oil pan, and the splined disconnect assembly is on the passenger side of the oil pan. Take that out and see what's broken inside.

Are you proposing to do the work yourself, or do you have a favorite mechanic? You only need to match gears ratio if you're changing the differential. That's a lot more work than the splined disconnect. Buy a factory shop manual or at least the Haynes if you're going to do the work.

I think every year uses the same hardware here. The only different version is the one without the disconnect, used on the AWD vehicles like Rainiers.
 

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Discussion Starter #11
I am not sure. I think that since I have access to a mechanics shop in the building where I work I can put it on a hoist and take it out and see what is actually broken. I can always call on a couple of my coworkers that teach in the mechanics shop if I run into trouble. If I get it into my local mechanic shop before verifying where it is broken I run the risk of an escalating bill if it is misdiagnosed. I can start a new thread when I actually get some time to take it off and look at it, so that I can post what I have found out.

Besides, if I am going to own this vehicle for a while I may have this happen again.
 

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Discussion Starter #12
How can I rule out the front differential as the problem?

I thought that I should rule out the differential as the problem before tackling the axle assembly. I thought about draining the fluid in the differential and checking for metal. No, I never heard a grinding noise before, but I wasn't the one doing most of the driving with it. The last long trip my wife took, the weather was bad and she said she put it in 4HI, but she was still on pavement (although there was snow and slush).

How do I know it didn't rip up when she was driving?

There doesn't seem to be an easy way to directly check the front differential by taking the cover off like you can on the rear.

Any advice?:confused:
 

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have you solved this problem yet? i am experiencing the exact same thing with my trailblazer. I was just wondering what you found out.
 

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Discussion Starter #15
I did check underneath and had my son work the 4WD controls. It sounds like the motor which controls the actuator is working. I changed the oil in the front and rear differentials. I didn't find any metal fragments at all but it did look like the previous owner hadn't changed it in a while.

I agree with Roadie - if the front diff were the problem, it would be noisy and I would have found chunks in the change, but it is quiet when I drive either in 2W or 4WD. So it has to be the actuator or the axle assembly, as I was previously advised.

My mechanics instructor at the school where I teach pointed out that he actually had a student with her mom's vehicle with the same problem, and it turned out that the actuator wouldn't move fully and lock the front axle. So while the acutator sounded like it was working, it was the problem. He advised that I should pop the actuator off and check it before dealing with the axle assembly.

I think that is good advice to rule out the actuator first, just in case. I will have to wait until they have a free hoist in the school to check it our.
 

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Discussion Starter #16
Sorry this took so long to follow up as I know that a couple of you were reading this thread with the same problem. I finally got around to putting it on the hoist and checking things out. Definitely a clicking sound when in 4Hi and trying to spin the wheels. I popped the actuator off and it seemed to work correctly when asked to engage. I use an extention in the actuator hole as described in the post above, but couldn't lock the front axle.

Roadie is correct - I can hear it in the splined collar assembly. I don't know if I am brave enough to tackle this alone - I'll ask the mechanics teacher at school if I can steal some shop space or I'll get my friend who owns his own auto repair shop to tackle it. The second option will be expensive, though.
 

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Same problem I have. I would like to get it fixed before winter, but I dont really have the funds right now. let me know how much it cost you, or if you did the work on your own.
 

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Discussion Starter #18
Same situation here. My wife wants it fixed before winter - no one wants a RWD vehicle like this in the winter in the Canadian prairies without the option of 4WD. I am going to at least try to get the assembly on the passengers side off to take a look at what is broken inside. If it isn't too hard for someone like me who is not a mechanic I'll post a few pics so you can see what you might be up against.
 

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Hey Roadie - Thanks for the link up above the pic of the passenger side is great - Todays mission is to pull that box and see what's ugly....
I will try and pic as much as I can .....linn your not on your back in the garage - get at it man LOL....
 
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