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

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[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