15 Lml vibration not factory issue

[2006coalroller]

So truck is far from stock so I’m sure it’s not the factory issue. Problem started when I re geared truck 3:73 to 4:30. Truck is on 37’s originally had bfg km3 on it well road forced them twice no fix. Had new driveline built no fix so I bought new tires road forced no fix. It’s a high speed vibration about 63-72ish. I can power through with no vibration. Also coasting through that speed with motor slowing truck down no vibration. But if I’m coasting in that speed range and give it slightly any throttle it vibrates. So I don’t think it is tires or drive line. Any ideas on what it could be I’ve heard of fuel surging with tuning? It is ppei tunes. I’m assuming it’s something speed related or rpm rated since it started after gearing. Also has a hard clunk when I lock the brakes up and let off them like a kick in the ass. Thought maybe slip yoke but not it lubed it up still there. Also doesn’t do that at high way speeds or when I take off only braking really hard and letting ass sit back down. Both started around same time. I thought maybe trans mount? Don’t see how that would vibrate

 

[gmduramax]

Transfer case or driveline

 

[2006coalroller]

Brand new driveline balanced twice. What In Transfercase would cause it

 

[2006coalroller]

I’m going to pull drive line this weekend and run it up to speed if it’s that it should vibrate then right?

 

[LBZ]

I’m going to pull drive line this weekend and run it up to speed if it’s that it should vibrate then right?

It might freak out if it sees gear and no wheel speed or torque slip. One way to find out I guess.

I’m going to guess it’s something in those gear sets. I’d pull the rear cover and have a look and see if anything looks funny and recheck backlash and tooth pattern.

 

[2006coalroller]

I’ve done that a few times but every One keeps saying gears won’t vibrate

 

[LBZ]

Well it was fine before the gears were installed and not after right? Either a driveline is out of phase or a u-joint is bad which you’ve already checked. Common denominator here is the gear swap. That’s where I’d start.

I doubt it’s the t-case unless it was bad before and you just didn’t notice it.

 

[JoshH]

Do you have a way to datalog the truck?

 

[Chevy1925]

gear swap speeds up driveline speeds. So if the vib was there prior, it would had this same vib around 80+ mph prior to the swap.

I would make sure the front driveshaft is not turning from them not assembling the front diff back together correctly. you should be able to get under the truck and turn the shaft by hand.

give us a little more info on the "new rear driveshaft". why, what was done, joints used.

what gear mfg was used for the install

 

[Mike L.]

I would be inclined to watch what the TCC is doing when this happens.

 

[2006coalroller]

I’ll look at data log. Truck is straight axles nothing in front moves.

 

[2006coalroller]

Truck is 4 linked in rear and old driveline was 1.5 inches to short so I figured gearing lower caused driveline to vibrate so built new one longer and used spicer joints. I’ve never seen gears cause a vibration also backloads is 5 thou where I set it wear pattern looks great

 

[2006coalroller]

Nitro gears and there locker. All spicer u joints timken bearings

 

[Chevy1925]

I’ll look at data log. Truck is straight axles nothing in front moves.

so this thing has locking hubs? its kind of hard to know what you did when your very short with information

Truck is 4 linked in rear and old driveline was 1.5 inches to short so I figured gearing lower caused driveline to vibrate so built new one longer and used spicer joints. I’ve never seen gears cause a vibration also backloads is 5 thou where I set it wear pattern looks great

i would start looking at rear pinion angle and working u-joint angle at both ends

 

[2006coalroller]

Truck has a Dana 60 it is straight axled. Pinion is 8.1 degrees up trans is 8.1 down perfectly parallel u joints have a 2 degree working angle. I built a rear 4 link cantilever on this truck nothing has changed for 100,000 miles except gearing which was all new parts except yoke

 

[2006coalroller]

I’m down to either transfercase or rear yoke or converter it’s a go rend converter so I don’t think it is it plus it doesn’t do it in any other gear at that rpm. But I’m not sure how that works

 

[NC-smokinlmm]

When my transfer case broke a snap ring it bucked and vibrated like crazy at 65 mph.

 

[2006coalroller]

What you mean bucked?

 

[NC-smokinlmm]

It felt like the front would speed up and slow down while rear end was constant if that makes sense.

 

[Mike L.]

I’m down to either transfercase or rear yoke or converter it’s a go rend converter so I don’t think it is it plus it doesn’t do it in any other gear at that rpm. But I’m not sure how that works

I don't believe it's the converter. I have had a couple of defective TCC spring in the valve body that caused a similar problem.