What turns?

[99TanHoe]

As most of you know I just had my 99 re-geared. After the swap from 3.42 to 4.56 there was some vibration issues. The guy that had it before me installed the lift and didn't lengthen the rear drive shaft.

I had a custom drive shaft made for the rear and that got rid of most of the vibration but I still had some vibration in the front end. I pulled the front shaft and the vibration is totally gone.

What I can't understand is why do I get vibration from the front shaft when the Hoe is in 2 wheel drive? What is turning the front shaft.

My Hoe has the auto-4wd option on the transfer case. There are 4 pushbuttons, 2wd, auto 4wd, 4Hi and 4Lo.

With the front shaft off and rolling the truck in the driveway with someone watching the front diff yoke, it doesn't appear that the front diff yoke is turning. The transfer case shouldn't be turning the shaft in 2wd should it? I'm thinking maybe the Hoe is actually dropping into the auto 4wd mode and turning the front shaft because sometimes the 2wd light goes out for no reason.

The front shaft goes to the machine shop tomorrow to see if it needs balancing. More than likely I'm looking at a custom drive shaft to help flatten the shaft angle like on the rear.

I'm really curious whats turning the front shaft in 2wd.

 

[iwaslowr]

If I'm not mistaken the front axle is always engaged and "turning" the yoke/shaft when the front wheels are in motion(i.e. 2WD driving), and it is the transfer case which engages/disengages power being sent to the shaft which drives that axle. If I understand correctly during your testing neither the axle yoke nor the transfer case yoke turned as the vehicle was moved in 2WD mode?

 

[Donnie Yukonie]

maybe i got it backwards because i thought since the transfer case is always turning your front drive shaft will turn only when 4X4 is engaged does the front driveshaft slip into the yoke?? but yes your front drive shaft always turns . inspect the shaft itself for worn Ujoints or dents , also drive shafts tend to sometimes have a weight kinda like a wheel weight to balance it , if it came off it may be unbalanced

 

[99TanHoe]

If I'm not mistaken the front axle is always engaged and "turning" the yoke/shaft when the front wheels are in motion(i.e. 2WD driving), and it is the transfer case which engages/disengages power being sent to the shaft which drives that axle. If I understand correctly during your testing neither the axle yoke nor the transfer case yoke turned as the vehicle was moved in 2WD mode?

During the test we could only see the front diff yoke. There is no yoke at the transfer case, the drive shaft is splined on that end and slips into the transfer case, so I'm not sure if the transfer case is turning or not.

---------- Post added at 05:10 AM ---------- Previous post was at 05:02 AM ----------

maybe i got it backwards because i thought since the transfer case is always turning your front drive shaft will turn only when 4X4 is engaged does the front driveshaft slip into the yoke?? but yes your front drive shaft always turns . inspect the shaft itself for worn Ujoints or dents , also drive shafts tend to sometimes have a weight kinda like a wheel weight to balance it , if it came off it may be unbalanced

Hopefully its just a missing weight, the shaft will be tested today. See my answer to iwaslowr for how the shaft attaches. The shaft was definately being turned, I'm just not sure from which end or if it actually should be turning in 2wd because when the shaft is removed there is no vibration.

 

[Donnie Yukonie]

Sorry thats what i meant its always turning this i know but i thought it turned because of the transfer casr

Sent from my DROIDX using Tapatalk 2

 

[BattelWagun]

The transfer case always spins the shaft. The only thing you can change from the transfer case is whether it's in high or low range. The actuator that engages 4WD is on the front diff case, and that controls the 4WD. When you're in 2WD, the front driveshaft is still spun by the t-case, but isn't connected to anything in the front diff case. Then, when you engage 4WD, the actuator on the diff case engages the yoke at the front of the shaft. So yes, it should spin, everything is fine. It probably just has bad yokes or is unbalanced.

 

[99TanHoe]

The transfer case always spins the shaft. The only thing you can change from the transfer case is whether it's in high or low range. The actuator that engages 4WD is on the front diff case, and that controls the 4WD. When you're in 2WD, the front driveshaft is still spun by the t-case, but isn't connected to anything in the front diff case. Then, when you engage 4WD, the actuator on the diff case engages the yoke at the front of the shaft. So yes, it should spin, everything is fine. It probably just has bad yokes or is unbalanced.

Thanks, that makes sense, but that raises a couple more questions.

When you say bad yokes do mean bad u-joints in the yokes or that the yokes themselves may be damaged? The u-joints seem to be in good shape.

You explained the other modes well but what is physically happening when the the 4wd auto pushbutton is selected? I know that the Hoe is able to go in and out of 4wd as needed but what part of the 4wd engagement is not made up when its not automatically in 4wd and then what does engage when it auto selects 4wd?

 

[Donnie Yukonie]

i believe AUTO 4wd AKA autotrac locks up the front diff to the front axle shafts but the transfer case cluth at this point is unlocked until the rear end slips. All it is locking is the transfer case clutch when the ABS sensors sesnes a slip

 

[99TanHoe]

i believe AUTO 4wd AKA autotrac locks up the front diff to the front axle shafts but the transfer case cluth at this point is unlocked until the rear end slips. All it is locking is the transfer case clutch when the ABS sensors sesnes a slip

If the front diff is locked up to the front axle shafts, would that mean that the front diff is turning the front driveshaft when the transfer case clutch is not engaged?

 

[Donnie Yukonie]

If the front diff is locked up to the front axle shafts, would that mean that the front diff is turning the front driveshaft when the transfer case clutch is not engaged?

thats where i got confused but i look at it this way i did alot of research and GM hasnt released how it works at least to my knoweldge its one of their secrets , i think it works like a old 4x4 remeber manual locking hubs? i think when Auto4wd is selected its like locking the hubs but not the transfer case

 

[bowtiefreak]

auto 4wd requires a CV front drive shaft. Also the vibration could be coming from rear drive shaft, make sure the U joints are phased correctly

 

[992door]

^ what he said.

 

[bowtiefreak]

when swapping gears to a lower ration a vibration that you typically might have only seen at 90, will now show up at a much slower speed. Been there

 

[99TanHoe]

auto 4wd requires a CV front drive shaft. Also the vibration could be coming from rear drive shaft, make sure the U joints are phased correctly

Right now with the new rear drive shaft installed and the front drive shaft removed, there is no vibration. I'm going to have the same shop that built the rear shaft check my front shaft for balance, if its OK then I'll have them build a new front shaft, if they think that it will cure the vib problem.

I was mainly wondering what was turning the front shaft while in 2wd, the front diff or the transfer case?

I wanted to make sure the front shaft was supposed to turn when in 2wd, because if its not supposed to then I might have another issue. If it is supposed to turn in 2wd and I could find out whats turning it, then I would know which direction to look if the vibration persists after the front shaft balance/replacement.

---------- Post added at 06:14 PM ---------- Previous post was at 06:12 PM ----------

when swapping gears to a lower ration a vibration that you typically might have only seen at 90, will now show up at a much slower speed. Been there

That makes sense, what did you do about the vibration you encountered?

 

[992door]

Right now with the new rear drive shaft installed and the front drive shaft removed, there is no vibration. I'm going to have the same shop that built the rear shaft check my front shaft for balance, if its OK then I'll have them build a new front shaft, if they think that it will cure the vib problem.

It is not that the drive shaft is bad. It is getting in a bind because of the angle it is at. You can get one from rough country for 425.

 

[99TanHoe]

It is not that the drive shaft is bad. It is getting in a bind because of the angle it is at. You can get one from rough country for 425.

Thanks, I'll check out the Rough Country one and compare it to what the shop will charge to make one. The rear shaft they built is heavy duty and really well made. By going to them directly and cutting the Off-Road shop middle man out of the picture I might get a pretty decent price. The rear shaft cost $550, but I'm sure the Off-Road shop added a little something on to the price for themselves.

 

[992door]

If i am not mistaken you can get them to cut off the end by the transfer case and put the right one on and balance it.

 

[bowtiefreak]

I added a rear CV shaft, then had to shim the rear a bit, still gets some vibration but minimal. 2 doors get it bad when lifted due to steeper angles in the drivelines

 

[99TanHoe]

I have a bad habit of over-thinking things most of the time. The answer was really right in front of my eyes the whole time. I had actually already seen the shaft on the Rough Country site but had forgotten about it.

Thanks Cameron, for pointing me back in the right direction. The Rough Country site strongly recommends the Dual Cardan Shaft for the Autotrac transfer case that my Hoe has. They don't have a picture but I did find another site with a photo if anyone else is curious.

http://performancelifts.com/cgi-bin/cart/GMDRVSHAFT.html

It's pretty much just a smaller version of what I now have in the rear. Their price is right in there with the Rough Country one.

I still want to get a bid from the shop that did my rear shaft. I'm just kind of "Old School" that way. I regularly buy off the internet but I like to deal with a real person, face to face when I can.

 

[99TanHoe]

I'm reviving this thread to tie up all the loose ends. First Thank You, to all that commented and offered suggestions. I think every post had some accuracy to it with at least a couple that were "spot on" in reference to my problem.

The same shop that built my rear shaft (USA Driveline) modifed my front shaft. They cut off the transfer case end and installed a dual cardan CV similar to the one on my rear shaft. Just installed it yesterday afternoon and test drove the heck out of it.

There is ZERO vibration now, I took it up to 85mph and held it there for a while just to see if higher speeds would adversely affect it but it was OK.

I got a away about $60 cheaper by having USA Driveline modify my existing shaft over buying the one Rough Country sells. I'm sure the RC one is good but I felt alot more comfortable dealing face to face on this kind of deal.

---------- Post added at 12:02 PM ---------- Previous post was at 10:36 AM ----------

Now, for part two of my thread revival. By talking with local mechanics and doing alot of googling, I was able to verify many of the comments that were made about "What Turns" my front drive shaft.

To sum it all up for anyone new to this thread that may need to know, the following is how an AutoTrac system works:

2WD - the transfer case turns the front driveshaft but the front driveshaft is not turning the front Diff because the front diff is not engaged to the front driveshaft until one of the three 4WD modes is selected. I don't remember which, but one of the front axles is alway engaged to the front diff even in 2wd but the front diff is still not engaged to the front driveshaft until a 4WD mode is selected.

According to one of the local mechanics I talked to, when any of the 4WD modes are selected, there is a fork in the front diff that engages the other front axle. Once the other front axle is engaged, the ring and pinion is engaged with both axles and the front driveshaft is also engaged to the front diff.

4WD Auto - As in 2WD the transfer case turns the front driveshaft and clutches in the transfer case apply as much bite to the front driveshaft as necessary to compensate for slippage from the rear diff. I think it was the owners manual that stated: Fuel economy would be lower in 4WD Auto I am assuming due to the front diff being locked up in that mode and the possiblily of some amount of 4WD being applied at any given time from the transfer case clutches. I would think that this mode was developed for driving in snow/ice conditions so that some level of fuel economy could be maintained but 4WD was still available if needed.

4HI - As with most 4WD systems the transfer case is turning the front driveshaft at a 1:1 ratio and the front diff is locked up and pulling.

4LO - Same as 4HI but with a 2.72 transfer case gear ratio applied.

I am by no means an expert on this subject but I do like to know as much as I can about the equipment that I own. If anyone see's anything that I have blatantly misstated or if you have something constructive to add please feel free to comment.