Pulsing vibration at around 55-65 mph

[GMC Guy]

Originally Posted by Rodms

Check tires and then front calibers could be freezing on you and the brake lines will have to be replaced also.the rubber ones,to the calis

would the calipers only freeze at highway speeds?

[Cue-Ball]

Originally Posted by 76/85 Truck

I have this exact same problem. At around 55 it starts pulsing (vibrates, then pauses for just a SPLIT second, then vibrates, and this continues until i go below 50). And when I let off the gas and let it coast it gets worse, then stops. I've had the driveline spun and the tires rotated/balanced. Nothing helped.

Like I said earlier in the thread: You should visually and physically check your tires for roundness and your wheels for straightness. Balancing alone is not enough. It's very easy to balance a tire/wheel that is out of round, defective, or bent. The computer will say it's balanced, but if it's not round and true, it can cause vibration. This kind of thing is pretty common, and very easy (and free!) to check.

[76/85 Truck]

I'll have to try that. I will have to borrow a friends shop because it's full time four wheel drive, so i can't jack up the front/rear alone, and I don't have any concrete at my house to put it on jackstands... could be tricky

[soule64]

Check for a bent rear axle. Mine was doing the exact same thing at same exact speed. I had it in the tire place 4 times within a month when one of the guys working on it noticed that the hub had a slight wobble when it was rotated.

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[irishman999]

Does it make a kinda romp romp romp romp romp romp sounds?

[Keith Seymore]

Originally Posted by 76/85 Truck

I'll have to try that. I will have to borrow a friends shop because it's full time four wheel drive, ...

Ah - then you have an advantage!

You can take the front shaft out to run the rear wheels up on jackstands - then - you can take the rear shaft out and run it at the problem speed with just the front shaft in! If it doesn't do it with the rear shaft out then you know it's driveline related.

Problem diagnosis is all about "divide and conquer".

K

[76/85 Truck]

Ohhhh very good point! I never thought of that. However, I was thinking (before your idea) that i could put it on jackstands in the rear, because the NP203 transfer case (when out of the "LOCK" position) will let the power go to the driveline of least resistance, so if one axle is in the air, that is the only that would spin. (At the moment, I can't get it in lock.. something is wrong in the shifter mechanism, and i can only get it from Hi to Lo). But I think I will try your idea before I rely on ALL the power going to one side... Knowing my luck 80% would go to the jacked up rear and the other 20% would go to the front, and I would have myself a problem Thanks for the idea!!!

[GMC Guy]

after researching the symptoms of a bent or out of round wheel, i'm thinking my problem lies in on of my aluminum wheels. i'll be taking the truck to the shop this weekend, hopefully they will be able to verify that that is my problem. cuz i want to put a set of steel rallies on anyway. this vibration crap is really driving me nuts...

[willett]

dude balance your wheels!!!!

[GMC Guy]

Originally Posted by willett

dude balance your wheels!!!!

i already have. see the thing is i lost control and hit a curb about a year ago. it was raining and i wasn't aware of a recent oil spill in the intersection. slid out and as i was correcting, the tires caught traction and shot me into the curb...i slammed the front left tire up the curb and thought for sure i messed something up. but i put it in reverse and away i went, not noticing any ill effects..until now. back then i really only drove the truck in town so i never felt the vibration as it only occurs over about 50 mph...
and since i've had the tires rotated a couple times, i have no idea which tire hit the curb.

so that's why my guess is that it's a bent wheel

[texasbigbear_wink]

I've had the same problem before and spent hundreds trying to fix it and it wound up being the brand new tires I got..... Only other thing is the trans mount.
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[GMC Guy]

this weekend i had the truck in the tire place twice, the first time they checked all my wheels for roundness and turns out all four aluminum wheels were a little out of whack. two of them being worse, and they put those on the back. then this morning i took a pair of factory rallies to them and had them mount my tires on em. then balance and swap them with the backs. driving back up to school today (3 hours) i still had the same vibrations...starting from about 50 and continue up to around 80. but its at it's worst from approx. 50-70 mph. the next thing i'm going to check is the driveline. hopefully a shop around here will be able to check it out for free...

[irishman999]

If you take a standard business card and stick it in between your wheel and the hub of a static balancer it will throw the machine out of whack bad, just that little bit. The aluminum wheels get this weird caked on oxidation where the wheel mounts to the steel on your truck. What do the mounting surfaces look like on your wheels?

[Cue-Ball]

Good luck finding a place to check out your driveshaft for free. To balance a driveshaft you have to remove it from the vehicle. Nobody is going to do that work without charging you for the labor.

[GMC Guy]

true, but the thing is i just had the driveshaft balanced and new u-joints put in...so that's why i'm having trouble imagining it's the driveshaft. maybe something in the rear-end? transmission? i'm running out of ideas for practical causes...everything else is sounding expensive to fix i appreciate all the help, but i gotta figure this thing out cuz this is my daily driver and i'm on the highway everyday

[Cue-Ball]

You said you had the wheels checked, but never mentioned if you checked the tires for roundness/bulges/separating belts. If you smacked a curb sideways you could have also bent an axle, though the wheel usually tends to bend before the axle does. It's unlikely to be the driveshaft if you already had it balanced.

I still think you need to jack up the truck and check the tires - by hand, with your own two eyes. While you've got it in the air you can check for bent axles. That covers pretty much everything behind the transmission. I think you can pretty much discount anything from the transmission forward as being the problem since those parts are very small, relatively light, and precision machined. They're unlikely to be bent or out of balance, and even if they were you probably wouldn't feel it due to the low rotating mass.

[Cue-Ball]

Originally Posted by irishman999

If you take a standard business card and stick it in between your wheel and the hub of a static balancer it will throw the machine out of whack bad, just that little bit. The aluminum wheels get this weird caked on oxidation where the wheel mounts to the steel on your truck. What do the mounting surfaces look like on your wheels?

It can't be the wheels since the problem still happens even after trying two different sets of wheels (one set aluminum, one set of steel rallies).

[GMC Guy]

thank you for the advice, i never really thought about the axles. the unfortunate thing is that away at school in chico. and my tools/resources are very limited when it comes to working on the truck. i'm working on getting a jack and some stands so i can at least get the truck up in the air

[white82]

Originally Posted by GMC Guy

really? the tires are only a couple years old with like 85% tread. 275/60/15 bfg radial t/a's

if they sit at any point for long time.. the tire can create flat spots.. thats why u see people putting there classics on jack stands in the winter..

[white82]

heres another idea? find a frind with a truck like urs.. ask hime to swap tires to see if the viberation goes away with his tire rim combo.. to see if it transfers the viberation to his... if it does then its the tires/wheel.. if it dont its in the drive line /axles

[white82]

heres another idea? find a frind with a truck like urs.. ask him to swap tires to see if the viberation goes away with his tire rim combo.. to see if it transfers the viberation to his... if it does then its the tires/wheel.. if it dont its in the drive line /axles

[datguysaL]

do a visual inspection of your driveshaft. check for missing weights, usually indicated by a clean spot. you said u just got it balanced. did the person who did the repairs, did they tell you they balanced it? or did they just removed it, did the ujoint and reinstalled it?

it could also be that that when it was reinstalled that they didnt "clock-it". everytime u remove a driveshaft, you should mark its position to the trans and to the rear end. i dunno about your truck, but with all the swaps done lately, it is possible that your driveshaft doesnt have the aligning groove, and that letting it be set-in in any position to the trans and the rear end.

does your truck have a drop/ been lowered. if it has, then it could be your pinion angles that are off. this is really common on lowerd trucks with the homemade kits. i believe it should be +/-1degree max, but not sure, u might wanna check that.

what else does u truck feel like when it beggins vibrating? does u steering wheel shake?

oh and ive also heard this happen when your torque converter is tryin to lock-up, but thats only if u have a lock up converter.

[GMC Guy]

Originally Posted by datguysaL

do a visual inspection of your driveshaft. check for missing weights, usually indicated by a clean spot. you said u just got it balanced. did the person who did the repairs, did they tell you they balanced it? or did they just removed it, did the ujoint and reinstalled it?

it could also be that that when it was reinstalled that they didnt "clock-it". everytime u remove a driveshaft, you should mark its position to the trans and to the rear end. i dunno about your truck, but with all the swaps done lately, it is possible that your driveshaft doesnt have the aligning groove, and that letting it be set-in in any position to the trans and the rear end.

does your truck have a drop/ been lowered. if it has, then it could be your pinion angles that are off. this is really common on lowerd trucks with the homemade kits. i believe it should be +/-1degree max, but not sure, u might wanna check that.

what else does u truck feel like when it beggins vibrating? does u steering wheel shake?

oh and ive also heard this happen when your torque converter is tryin to lock-up, but thats only if u have a lock up converter.

well i removed the driveshaft myself and took it to the shop for u-joints and balancing. they did a nice job and i could tell they balanced it because i saw the new weights on there. but when i re-installed it, i had no idea that the driveshaft needed to be installed the same way it came out...rookie mistake

the truck is lowered 3 inches front and back. could a 3 inch drop really throw it off that much?

[GMC Guy]

would it be safe to out the truck on some sturdy jack stands and run it up to 55? see if it's still vibrating?

[Cue-Ball]

You just drove it last weekend and it was vibrating. You haven't changed anything, but you want to see if it's still vibrating? Why wouldn't it be?

And yes, a 3" drop is more than enough to screw up your pinion angles.