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Avalanche chassis harmonic vibration
My 2005 Chevy Avalanche has developed over the last 2-3 months a harmonic vibration in the chassis. It mostly occurs around 50-55mph and perhaps 2000 rpm. The transmission can be in Drive or in 3rd, doesn't matter. The vehicle has 271K miles on it, runs great...

It seems to be initiated by pavement ripples or by the transmission just starting to be loaded.

I have checked all suspension components and in fact replaced all shocks. The shocks reduced how often it happens, and the severity.
But the vibration can still be initiated and felt strongly when it occurs.

I have checked the drive shaft and U joints. All within tolerance.

We've thought about motor mounts and transmission mounts, no action taken there.

Any suggestions on where to look for a solution are appreciated
Re: Avalanche chassis harmonic vibration
there are so many things that can cause a vibration, but without actually driving and feeling the problem, I wouldn't know where to start looking.

do you feel it in the steering wheel or do you feel it in the seat? where does it feel like it's coming from? What does the vibration feel like? do you just feel it of does it have a noise also? Is it steadily there or does it happens sometimes and sometimes not? When did you first feel it?

Wheel balance happens to have a shake at 50-55 mph. Front wheel balance problem will shake the steering wheel. Rear wheel balance will be felt in the floor or seat. Wheel balance will shake at a certain speed (mph) so if you have it in overdrive and take it out of overdrive, your engine and trans may be at a different rpm but the vibration will still happen at the same mph. If you go faster the shake will go away (most of the time)

there is a vibration that could happen from inside the trans. It will feel like driving over rumble strip in the road but the road is nice and flat. It will happen sometimes but not all the time. Different from wheel balance because wheel balance will be every time you hit the certain speed.

bent or out of balance drive line could cause a vibration but they usually start at 55-60mph but get worse if you go faster.

Wheel bearings could cause a vibration but but they usually make noise too. The noise normally starts at 30-35 and gets louder the faster you go.

You say the vibration can be initiated by pavement ripples. That would make me look more closely at suspension components. If it is felt in the steering wheel, I would look at steering linkage, gearbox, intermediate shaft. If it's felt in the rear I would look at rear control arms and the track bar.

You should have s shop drive and feel the vibration. It's something that should be actually felt. Otherwise everything is just a guess.
Re: Avalanche chassis harmonic vibration
I feel it in the chassis, seat of the pants.
Wheel balance was the first check. Fine.
Drive shaft and u joints next. Jacked up at rear diff and ran it fwd and rev. It rotated smoothly and didn't show excessive play, so joints seem good.
Suspension components were then checked, and I went ahead and put in new shocks since they had 200k on them.
Only minor benefit. Vibration / shudder still apparent, still randomly occurring but always around 50-55mph and around 2k rpm. Never happens in other conditions. Only happens in 3rd or OD, never 1st or 2nd.
Now starting to think its the Torque Converter Clutch or the trans EPS.
Having the trans checked today.
There is also a thought that a Ujoint might be seized rather than loose, and that our earlier test might not have caught that. Requires removing the drive shaft to test.
Re: Avalanche chassis harmonic vibration
if it always happens around 50-55 and 2k rpm the it isn't going to be 3rd or overdrive because they would have different rpm's. overdrive should drop a couple hundred rpm's. Or does it happen at 50-55mph when you have it in either 3rd or overdrive? which would mean that the vibration is mileage based and not rpms.

check the drive line to see if you have any weights that have come off. They balance the drive lines by adding weights that are tacked on. See if you see and spots that look like a weight has come off.
Re: Avalanche chassis harmonic vibration
The driveshaft balance hasn't been checked at speed of the noticed vibration. When we did that check, nothing in the way of bare spot / missing welded weights was noticed but I'm not sure we were looking so much at that as just whether we saw any out of round.
The trans has now been evaluated and it's okay. Just needed a little tlc.
The mech looking at the trans noticed a vibration in the body just sitting in the seat at idle. He threw an analyzer on the OBD plug and it's showing random misfires on cyl 4,5,6. He thinks that could be the issue, might need coils. But until the vibrations become more consistent, it's kinda chasing a tail, spending money so far.
Re: Avalanche chassis harmonic vibration
Also re the drive shaft, the vibrations would be happening very consistently if a balance weight was off
Re: Avalanche chassis harmonic vibration
yes, if it was a drive shaft imbalance it would be more consistent than not, probably. how often does it happen on your vehicle?

If you feel the vibration, can you make it keep the vibration going for a long time or when it happens it happens and then just goes away? Can you prolong the condition? If when it happens, it just happens for a short while and then it's gone then I wouldn't think it's a balance thing but more an occurrence problem. without feeling it myself, I would tell you to test drive it with a person from a tranny shop and have them feel it.
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