Anyone ever had a pulsating vibe at highway speeds?
 

It may just be the nature of the beast but my truck has a faint pulsating vibe only at highway speeds. THe vibe or slight rumble comes and goes every few seconds. I can usually start to feel it around 60 mph. My truck is an 88' 4x4 crew dually 454/th400 & NP205 with 4.10 axle ratio.

Here's the funny thing.....About 5 years ago I owned an 87' GMC 3/4 4x4 w/350/th400 4.10 axle ratio. I can't recall the t-case model but it was a factory t-case. Anyway, the 87' had the same exact same pulsating highway vibe around 60-70mph. When the trans went out I had it rebuilt and the vibe was still there after the rebuild so I assume its not the tranny.

Anyone else ever experience this. I always thought maybe a t-case vibe on the 87' but now since I have it on my 88' crew it makes me think maybe its an axle or driveshaft problem since the t-cases are different models but the vibe is identical.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Check your u-joints first. Then check your carrier bearing.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

That makes sense but my 87' always had good u-joints in it and still had the vibe. I owned that truck from 120k to 185k miles. Put 2 sets of ujoints in it. Also replaced tranny and replaced motor at about 170k. Still had the vibe.

I just wondered if anyone else has ever experienced this because 2 completely different trucks I have owned have the same vibe. Maybe its just a coincidence?

Re: Anyone ever had a pulsating vibe at highway speeds?
 

In the biz, a "pulsating vibe" is called a "boom/beat".

It results from two disturbances, very close in frequency but slightly out of phase, such that the amplitudes gradually add and subtract over time. That's what results in the pulsating effect. The closer the two frequencies are the faster the boom/beat.

At any rate - it means you have two disturbances going on, like 4th order engine firing being very close to second order driveline, for example.

Fix either one of the disturbances (or dramatically change the frequency of one) and the pulsating effect will go away.

K

Re: Anyone ever had a pulsating vibe at highway speeds?
 

That's deep!
I once had an issue like that and I couldn't find it.
Ended up being a leaky door seal that somehow let in air pulses.
It went away when you would crack another window.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Mine gets it every so often. One thing that helped was cleaning 20+ years of greasy dirt buildup off of the drive shaft. My rumble, when it shows up, starts around 67, peaks around 70 and fades after about 75. Oddly, it doesn't always show up. If Keith is correct, could be something like the exhaust rumble interacting with the drive train, modified by the ambient air temps changing the length of the drive shaft and exhaust pipes.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Check your pinion angle.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

I had a 88 crew cab srw 454 with a 4 speed manual that also did this but I sold the truck before i fixed it. And I now have a 90 crew cab dually 2wd 454/th400 and it also does this same exact thing. I'm almost positive that it's the carrier bearing but haven't chance to fix this one yet either

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Okay well at least I know now that others have felt this same pulsing vibe. It is very faint but on a smooth highway you can definitely feel it. I've always just learned to live with it because everything appeared to be in working order.

I know these trucks weren't meant to ride or handle like Cadillacs especially the 1 tons. Maybe it is just a vibration combination of tires, exhaust pipes, driveshaft and engine.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Yeah same here, it hasn't bothered me enough to fix it yet

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Check your front axle seals.......... If they are worn and water and grime has gotten into the axle, one of your axle shafts could be worn and causing the vibration. This was the case with my 79 1/2 ton 4x4. The seal was shot and when I pulled it apart the short axle shaft had a groove worn into it.......... I replaced it and the seals and then I could drive the truck at 70 mph plus with NO vibrations whatsoever.

RIZ

Re: Anyone ever had a pulsating vibe at highway speeds?
 

An unbalenced drive line can cause some strange stuff at varying speeds. ?

Re: Anyone ever had a pulsating vibe at highway speeds?
 

It may just be the earth pulsating when that 1 ton crew cab 454 Dulley hog rolls down the freeway that fast.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Sure. Driveline imbalance is a "first order" of propshaft rotation disturbance (one "kick" per rev). U joints are a "second order" of shaft rotation (two "kicks" per rev).

Imbalance by itself usually results in a cabin "boom" (low frequency sound; almost more like a pressure that you sense) or a vibration that you feel, and normally is pretty steady. Large cabin volumes, like a crew or a Suburban, definitely are more sensitive to this. The boom/beat, or the phasing in-and-out, is a result of the driveline finding some other disturbance that is close to it in frequency.

LOL! That's probably it. I thought I felt the earth move earlier today.

K

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Lol! Probably so! The thing is I don't typically drive that fast anyway. I only live a 3 miles to town so it doesn't see many highway drives that often. I'm happy with 55 mph in the beast but my wife wants to see me crank out 70 mph. She thinks I'm a grandpa when I drive slow, lol. I told her when these trucks were built speed limits were 55 mph and the truck wasn't intended to go much faster, lol

Re: Anyone ever had a pulsating vibe at highway speeds?
 

When I need the u joint replaced I will take it and have the shaft check/balanced with new joints and carrier bearing.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Just curious...Have you put new tires on it since it's you've notice the pulsating? I've seen a lot more educated response above than that one posted here but I bought a burb once that had the oddest rythmic thumping noise that had no rhyme or reason to when it would start and stop or so I thought. Took it to the tire shop first cause they looked good but were like 10 years old. Short story someone locked up the brakes and put matching flat spots on either front tire which led to total loss of balance but only on certain surfaces at certain speeds. Like I said there are a lot of more knowledgeable guys than me chiming in but whenever I have a problem with the way my truck rides I check connection to the asphalt first.

I've done the carrier bearings on every FSC in my family in the last few years it seems like. I always hang the carrier then take the shafts down to be balanced because the bearing is cheap and not too too hard to install, one of those things I feel like you overpay a service mechanic for like doing disk brakes, ok maybe a bad example but a carrier is between like $15 and $50 depending on what brand you use, but it'll get a lot more spensive once the grease monkey touches it. This doesn't sound so much like a carrier though. They tend to go out a little more violently. EDIT: That was effed up of me. I said that bearing is easy peasy but you either have to redneck pull/press which is a bear if you aren't used to it, or have a real press (no idea why i don't own one).

Another question that may lend some help, do feel a chunk out of the drive train when you are accelerating rapidly then completely let off the pedal? Do the driveshafts make any noise when you apply heavy throttle to them? I'm not even sure if the way i'm describing it will make sense to you but usually the carrier effs the U joints when its worn out so you hear the slop in the shaft catching up.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

CABNCHASSIS....I understand what you are saying. I bought this truck and the tires that were on it were about 50% tread and the vibe was present. I installed 6 new tires and balanced them and the pulsating vibe still exists. There was no change. I didn't expect the tires to fix the problem becasue like I said my old 3/4T single wheel had the same pulsating vibe and it always have good tires on it also.

The pulsating vibe doesn't bother me that much. It comes and goes. It may have something to do with the type or condition of the road itself. Like others who have experienced this...it comes and goes? Kinda hard to determine a cause and fix that type of problem. The truck is going on 25 years old so I can't complain much.

I have noticed as another member mentioned above if I go past the "vibe sweet spot" and accelerate through it then the truck does smooth out. But I'm running 70-75 MPH at that point and I wont run a 454 with 4.10 gears like that for long.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

That typically indicates a driveline disturbance.

The driveline doesn't have enough mass by itself to upset the vehicle without the synergy of resonance. As a result it builds, and then decreases, as it passes through the resonant frequency.

Contrast that with tire/wheel disturbances, for example. They have enough mass that once they start rockin the boat it continues to build the faster you go.

K

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Thanks Keith. That makes sense

Re: Anyone ever had a pulsating vibe at highway speeds?
 

I figured I'd ask. I think Kieth nailed it though, something drive-liney. When I changed the 14ff on my c+c I went from the trans back to the diff with new u-joints carrier and had the shafts balanced. After I did that, well you know how good a 3+3 feels out on the highway when it's right. I didn't have too much vibe that I noticed but I had a ton of play between the tranny and rear-end. My fault really. the 14ff had been sitting and all the seals failed by the time I bought it, and of course being a lazy 20 something I said Meh it'll last. It did for about a year but when i finally cut it out some of the internals were failing pretty bad. The magnet was a sight to behold.

I can feel your pain on trying to outrun the noise. My 5.7 k2500 ain't beating your fuel mileage by too much if at all. 500lbs of tools and these beat up old M/Ts keep me right in that prius type fuel economy range you know 9-10 city and a whopping 11 or so on the highway.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

I figured I'd ask, the other day I was about to do a gasket set on a honda small engine when my dad asked me "you mean you couldn't even make it stay running by tuning the carb?" Boy did I feel stupid, a couple quarter turns later all my "air leaks" went away..

I think Kieth is correct, seems like maybe he knows a thing or two about the engineering of these vehicles ;). Seriously though Kieth thanks for the science lesson.

When I changed the 14ff on my c+c I went from the trans back to the diff with new u-joints carrier and had the shafts balanced. After I did that, well you know how good a 3+3 feels out on the highway when it's right. I didn't have too much vibe just a lot of slop on the front yoke of the rear end. My fault really. the 14ff had been sitting and all the seals failed by the time I bought it, and of course being a little younger and little lazier than I am now I said Meh it'll last. It did for about a year but when i finally cut it out some of the internals were failing pretty bad. The magnet was a sight to behold.

I can feel your pain trying to outrun the noise. Had a vacuum leak on the intake manny gasket on my 5.7 and the truck would run like trash sometimes unless you put your foot through it. At least your engine makes some torque if it is going to be so bad on fuel, my 5.7 k2500 is pretty impressive in the mileage dept. I get about 9 city and a prius worthy 11-12 mpg hwy. The M/T tires I run and 500lbs of tools don't help that any.

I'm real picky about the way my trucks run and I really hate those little noises, clicks, whines, rattles, etc. My old man will see me after it and he tells me the same thing every time. All you have to do to fix that noise is turn up the radio.

Re: Anyone ever had a pulsating vibe at highway speeds?
 

Any updated? I'm chasing the same issue

Re: Anyone ever had a pulsating vibe at highway speeds?
 

I had a 86 cucv and it was having a slight vibration in the driveline it turned out my transmission mounts were worn out . so I would get some new mounts and change them out there are not hard to replace and not a lot of money to replace

Re: Anyone ever had a pulsating vibe at highway speeds?
 

No updates. I really haven't had time to worry about it. I hope I figure it out someday