Trying to locate a vibration

[monster50iii]

I've honed in on a new vibration. The signs are making me question the trans. Here are the details:

-built trans with alot of miles. Around 170k on build, 270k overall on hard parts
-i make around 670hp at the tires and the trans holds it just fine, tho I rarely use even 500hp..
-the vibration occurs at 65-68mph only.
-it also only happens in 6th gear.
-when vibration comes on my edge cts2 shows 15-35rpm of tcc slip
-when vibration occurs if you give it some throttle vibration dissappears
-if it occurs and you manually drop yo 5th it goes away
-when it occurs it gradually comes on and goes, lasting maybe 4sec total
-is not constant, happens maybe every other mile
-I have yet to experience it in 4wd

The fact it is gear sensitive makes me believe it is in the trans. The tcc slip also has me thinking maybe it's valve body related if it is truly tcc slip? Maybe a shake within the trans will somehow make it think there's tcc slip when there isn't? Hopefully someone can help me narrow ut down!

Thanks

 

[JoshH]

I'm guessing this is on an LMM? What you are describing sounds like a problem I have fought with before. I can't tell you exactly why they do it, but I have seen it a few times on a few different trucks. Starting with the LMM, GM started leaving balance rates active even while driving under a light load. The vibration you feel starts as a fluctuation in the balance rates, and as it tries to correct itself, the balance rates keep going up increasing the vibration. If you do a datalog of the balance rates, you can usually see it when it occurs. I'm not sure if it is caused by an injector issue or some sort of other vibration in the engine/drivetrain, but I know it can be fixed with tuning. Once fuel quantity exceeds 50 mm3, the balance rates freeze. All you have to do is adjust the pulse width so the fuel quantity is not below 50 when the vibration starts. From what I've seen, the fuel quantity is very close to 50 when it happens, and it doesn't take much adjustment to resolve the problem.

 

[juddski88]

I've honed in on a new vibration. The signs are making me question the trans. Here are the details:

-built trans with alot of miles. Around 170k on build, 270k overall on hard parts
-i make around 670hp at the tires and the trans holds it just fine, tho I rarely use even 500hp..
-the vibration occurs at 65-68mph only.
-it also only happens in 6th gear.
-when vibration comes on my edge cts2 shows 15-35rpm of tcc slip
-when vibration occurs if you give it some throttle vibration dissappears
-if it occurs and you manually drop yo 5th it goes away
-when it occurs it gradually comes on and goes, lasting maybe 4sec total
-is not constant, happens maybe every other mile
-I have yet to experience it in 4wd

The fact it is gear sensitive makes me believe it is in the trans. The tcc slip also has me thinking maybe it's valve body related if it is truly tcc slip? Maybe a shake within the trans will somehow make it think there's tcc slip when there isn't? Hopefully someone can help me narrow ut down!

Thanks

It could be your output shaft bearing, or the bearings in your transfercase as well. As bearings wear and tolerances increase, their resonant frequency changes.

 

[1FastBrick]

I'm guessing this is on an LMM? What you are describing sounds like a problem I have fought with before. I can't tell you exactly why they do it, but I have seen it a few times on a few different trucks. Starting with the LMM, GM started leaving balance rates active even while driving under a light load. The vibration you feel starts as a fluctuation in the balance rates, and as it tries to correct itself, the balance rates keep going up increasing the vibration. If you do a datalog of the balance rates, you can usually see it when it occurs. I'm not sure if it is caused by an injector issue or some sort of other vibration in the engine/drivetrain, but I know it can be fixed with tuning. Once fuel quantity exceeds 50 mm3, the balance rates freeze. All you have to do is adjust the pulse width so the fuel quantity is not below 50 when the vibration starts. From what I've seen, the fuel quantity is very close to 50 when it happens, and it doesn't take much adjustment to resolve the problem.

That's an interesting find!!! Thanks for sharing.

 

[N2BRK]

If Josh is right and it’s an LMM, I had the same problem once I added larger injectors. Watch your timing and VSS when it’s happening. If the timing starts climbing and the vss reports speed fluctuations up and down during true steady speed, then that is what happened with mine.

Mark called it, had me check the timing and vss, and then tuned my truck and fixed it

 

[monster50iii]

I crawled under the truck and everything is tight. My trans mount moves quite a but, so I'm replacing it.

I did notice during the shake, balance rates do jump around, but from say +2mm3 to -1mm3. Not a ton of movement but maybe it doesn't take much. Trying an old known good tune file, and will see if that cures it. I'm more curious if the balance rates jumping are cause or effect.

 

[monster50iii]

Josh, u think you're correct with the balance rates jumping around causing the vibration. I narrowed it down to "something " within the tuning. I figured it was me doing something that ultimately pissed the ecm off. I made sure I'm following stock torque values for the pedal, same with fuel delivery via pedal. Made sure I'm close to commanding the same total mm3 with the regulator as called for. Whenever I ask for less pilot than stock it'll shake. Recently I lowered my 10mm3 -40mm3 significantly as the thing likes to take off kind of hard from stop signs. Now it shakes the worst it ever has. Went from no shake, to shaking like crazy just from lowering those pulse values.

Kinda has me scratching my head. So far calc mm3 driving down the road, it'll do it at 25mm3-35mm3. Definitely intriguing