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Vibration diagnosis

Discussion in '1973-1991 K5 Blazer | Truck | Suburban' started by foxman, May 14, 2002.

  1. foxman

    foxman 1/2 ton status

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    I know there's got to be other guys out there that have been here before. The damn thing vibrates only while in drive, rooling with the foot on the gass, let off the gas, vibration stops. I've changed all u-joints and run in 2 wheel drive 95% of the time. My ideas are this=
    if it was tire ballance, it would vibrate all the time weather or not foot on gas.
    if it were motor mounts it would bibrate during a brake tork, which it doesn't. same with trans mount. the engine does have a miss in it I think due to valves, but the vibration is everywhere. in the seat the dash the steering wheel. Can I pop the rear driveshaft off and run it in 4wd to determine a bent driveshaft or orther rear related drive problem? Please any suggestions are welcome, I can't take it much longer!!!
    thanks.
     
  2. SkulzNBonz

    SkulzNBonz 1/2 ton status

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    Try turning the driveshaft 180 degrees. I replaced both my rear ujoints a few months back, then took it for a test drive, and it vibed like crazy. Took it home, spun the d-shaft 180, put it back together, no more vibes.
     
  3. foxman

    foxman 1/2 ton status

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    do you mean disconnect the rear and turn the driveshaft so that the joint slips in the other way in the yolk, or flip it so the rear end of the driveshaft is now at the front (output shaft of trans)? iknow stupid question but I had to ask.
     
  4. shane74

    shane74 1/2 ton status

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    you can also get speed specific vibrations. It has to do with the frequencies and harmonics of whatever is rotating at different speeds. I'd ry the driveline thing first, but I had a real bad vibe problem with my 35's at 35-40 mph. Anything under or over was fine. I got the tires rebalanced and the vibe went away. Now, bigger meats are a totally different story. I have 38 swampers and I only had the fronts balanced so my steering wheel wouldn't shake all the time. I left the rears alone and have no problems.
     
  5. SkulzNBonz

    SkulzNBonz 1/2 ton status

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    Sorry for not being clear. Just unhook the driveshaft at the rear u joint and turn the shaft over 180 degrees.

    John
     
  6. Blazinaire

    Blazinaire 1/2 ton status

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    What t-case do you have (slip yoke or not)? Is it lifted? What speed is it occuring at? A wild guess tells me that it is just axle wrap, but it is hard to say untill we get more info. I get vibes from all sorts of stuff like axle wrap, bad u-joint, bad u-joint angles (and pinion angle) all of which have been or are being solved.
     
  7. foxman

    foxman 1/2 ton status

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    I've got the stock 241transfer case. i heard the chirping of a u joint this morning, it must be binding or something... This has happened two times already. it just keeps eating u joints. it does have a helper spring set in the rear but that only adds about 1/2 inch to the spring pack. could this offset the pinion angle enough to keep eating up u joints? where can i get axle shims and how do I know which ones to get?
     
  8. SkulzNBonz

    SkulzNBonz 1/2 ton status

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    Sounds like a pinion angle problem, or perhaps a beat up yoke. To get the d-shaft balanced is pretty cheap. I don't think any more than $30-$40. Still need more info. Lifted? Stock? That helper spring may be enough to throw the angle out, but I don't think so.

    John
     
  9. foxman

    foxman 1/2 ton status

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    it is stock no lift (other than the helper in the rear) with 32" tires.
     
  10. Blue85

    Blue85 Troll Premium Member

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    If it eats U-joints and you are only lifted 1-2" in the back, something is wrong with the angles. I would say that you either have mismatched angles or a loose pinion.

    Do your self a favor and get an angle finder. They are only $6-$8 at Home Depot or other places. Otherwise you are only guessing at the problem and don't even know which direction to adjust in. Then measure the angle of the pinion (usually there is a flat surface in the casting to the side of the pinion that is parallel to the pinion) and the angle of the t-case output shaft. If they differ by more than 1 or 2 degrees, that is your problem. You can by shims for the spring pack at any local suspension shop (even semi-truck shops sometimes stock them) or adjust the height of the T-case crossmember. If you can only get the 3" wide shims, no problem, just cut them off.

    A loose pinion is pretty easy to find with the rear U-joint disconnected.
     

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