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Tech Mod
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Discussion Starter #1 (Edited)
First, i will say i am no differential expert, but i am going to share my explanation of the operation. if there is any incorrect info please let me know.

So guys, this is a common issue and really i have not seen a break down thread on how the front differential works on these trucks. a lot of people are unaware of how it works and if a problem occurs they don't know the seriousness when it arises. so this thread is to educate people on how the front differential engagement works and if you happen to have a grinding noise coming up front.

here is the PDF on the specs of the front differential and rebuild info, more info can be found in the service manual sticky up top. as well as the vacuum sticky for an explanation of how that works.
https://skydrive.live.com/redir?resid=5429672B1FB043B7!143&authkey=!AC4sS6HZL2AbQdg

here is a video i found on youtube and demonstrates the grinding noise. it generally gets louder as you slow down and will stop when you stop, or when you put it in 4WD high. if this is happening to you, STOP DRIVING IT!

you can "fix" this by unplugging the vacuum going to the actuator and capping the vac line until you get a new vacuum switch for the transfer case, or you do a posi-lock or bravada shaft swap.

This is my old aluminum differential. i removed it when i upgraded to a cast iron unit so its perfect for this demonstration. now, lets get started!

to the left you can see the vacuum actuator, all this simply does is pull on a cable which engages the slide collar in the front differential (more on that later) the red line shows how the cable connects.


you can pull on the actuator and manually engage the front differential to check for a seized cable. you may have to have someone under the truck turning the front drive shaft to align the gears while you pull the cable.
here is a picture of what the end of the cable looks like. my old one was rusty and fairly hard to engage.


you can remove the engagement sensor from the plastic triangle housing and look inside. when the differential is engaged it should look like this.

when you remove the 3 15MM bolts for the plastic cover (which holds a 4x4 engagement sensor and the cable) you see this. the cable is inserted to the end of it via metal clip. to remove the cable you simply lift the strait part of the clip and the cable should come out. its a bit tricky when the cable is installed though.


pulling on this shows the amount of travel needed to engage the 2 part axle shaft. a bit more than 1/4" of pull.


remove the other 2 15MM bolts, that metal clip, and lift off the axle tube and you see this. it may or may not be here. just to note, there is a thrust washer on the axle tube, it sticks on there with the diff fluid but it is suppose to be against the gear. just a FYI.


here is what you see when the axle tube is removed. neat right? lets tear into it further :)



so lets pull up on that part where the cable is inserted. the GIF below shows how the front axle engages. the slide collar simply moves over and meshes both gears. when this happens the differential is able to transfer power to the ground, otherwise it will essentially be like having a tire off the ground.


lift up on the axle shaft to remove it and this is what you get. you can see where the gear is a bit chewed up from improper engagement due to faulty vacuum switch on the transfer case. if your front end randomly grinds when on the highway or whatever this is what is causing the noise.




next lift up on the shift fork assembly. Note, the thrust washer. if you remove the axle shaft with the differential installed on the truck, this thrust washer can "fall" out of place and the end of the axle shaft wont be able to go back in. when this happens you have to remove the axle tube and put a dab of grease on the thrust washer to hold it in place.


the 2 pieces just slide on to each other.


pic of said thrust washer.


then lift up the clutch shaft


there is a pilot bearing inside for the axle shaft to ride in.


inside of the differential housing you should see this. just a washer.


finally this is it. nothing else. if you were doing a bravada axle shaft/tube swap this is what you want left before installing the shaft and putting everything together.


parts laid out.


engagement GIF.


so what causes the noise?

first off, when the vacuum switch fails, it generally supplies 100% vacuum to the actuator, which pulls the cable. so the front driveshaft ALWAYS turns and well, its like a normal open differential, no harm done.

lets say you are going up a steep hill, the engine load is pretty high, which looses vacuum through the system (since there is no check valve, or due to leak) and the actuator looses vacuum and disengages...again no big deal...BUT! when you reach the top of that hill, vacuum again starts to build, and the actuator pulls the cable, which pulls the slide collar and since the 2 halves of the axle engagement are moving the opposite direction it grinds. why opposite you say? you ever had the rear tires off the ground and tired turning one of them? one spins backwards while you move the other forward, that or the driveshaft turns while the other tire is stationary. its just the way the open differential is designed.

so again these 2 parts are going opposite directions and the slide collar is trying to engage it while you are going 70 MPH or whatever, not good....you'd be surprised how hard the vacuum actuator can pull too.

here is a crappy drawing of what is going on when its unlocked. when you select 4x4 the front driveshaft turns the same direction you are going, and syncs the inner shaft to the outer one, so no grinding!


so there you have it. i believe the rebuild kit for this is ~$100 or so from GM if you wish to repair it. that or you can make a home made posi-lock cable to eliminate the vacuum operation so its a manual pull cable, or you can do the bravada shaft upgrade which does the same thing, but without the option of disengaging the axle shaft. hope this thread helps people out who are having similar issues.
 

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Nice info.

This is also why you will hear it grind if you try to engage 4x4 when the shafts are spinning different speeds.

I'm not sure if it's easy to do with an automatic transmission, but the front differential doesn't seem to like it when you attempt to go into 4x4 when the front tires are not spinning (or close to it) and the rears are- usually with the RPMs above idle.

Such as, I was stuck in the snow I tried reverse then 1st the engine was around 1500RPM back tires were spinning- still 2wd fronts were not spinning, I pressed 4hi and it did the dreaded grind.
 

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Funny thing is, I hopped onto this dust covered* part of the forum to research and if needed post about a whining noise I get when in 4hi. Looks like home made posi lock is happening when I get paid again.
 

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what is the name of that yoke piece? The section where the spring holds the actuator cable to that shaft has snapped off on my blazer and i went to the parts store but nobody could figure out the name of the part. i really want to get this fixed before much more snow falls
 

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That YouTube video is exactly what my truck did yesterday. I've already replaced the vacumn actuator under the battery. Did I not get it installed correctly or is there another vacumn switch that needs addressed?
 

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Jeeps have this too. and a few didn't, just a solid shaft instead of the split gear coupling. I like the solid ones better, one less thing to go wrong.
 

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That YouTube video is exactly what my truck did yesterday. I've already replaced the vacumn actuator under the battery. Did I not get it installed correctly or is there another vacumn switch that needs addressed?
There is a vacuum switch on the T-Case that gets gummed up and can stick open causing the vacuum actuator to operate erratically.







 

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i already replaced the hub assys, it is definitly in the front diff, and it still whines in 4 hi too, i haven't tried plugging the vacuum line yet, i will do that tommorrow, but i believe the damage is already done
 

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I have no clue what all this means (here for answers, I know nothing about cars) but my 2000 s-10 LS makes this same grinding noise. However, it only does it about 50% of the times I drive it and stops when I engage 4wd-hi. Also the 4wd is currently not working just had it out in the snow and I was really upset when it didn't kick in. My mechanic just "fixed" my 4wd but apparently not. What does that mean in non automotive talk? And I basically want to know how much it is to get fixed and how much damage am I causing by still driving it. Any information would be greatly appreciated. I'm just lost. thanks
 

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Discussion Starter #18
Thanks cknaffle! I need to post this on zr2usa as well.
 
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