Vibration over 70 please help!

[SamJam05]

so basically, only happens under load and at high speed? goes away when coasting at high speed?
what happens if you manually shift to the next lower gear (4th to 3rd) at high speed before you hear it? does it start noise?
what happens if you are in a lower gear at high speed making noise and then shift up (3rd to 4th)? does the noise go away?

What type of information with this provide just curious? I’m relatively new to working on cars (last 5 years) and self taught. But I’ve never downshifted to feel/hear the vibration, but I can tell you it’s worse in second under heavy load and then if you bump 3rd it goes away until the rpm reaches a similar point. But it’s definitely still related to speed more than rpm.

 

[SamJam05]

Hard to say just by description, but as a young budding mechanic ... I broke more than I fixed, until I realized that's a bad business model and got my self into classes, and hung out with the older folks that knew way more than me.

Low to medium pitch, gaining pitch under load and speed is typically pinion to ring mesh. It might sound great on a lift but there is no load of the car weight on the bearings and etc. It is possible it is fine as is, but changes once vehicle is on its full curb weight and moving forward shows it could be bearings or other issues in the setup.

You did thorough work on diagnostics of the noise so balance of the peripherals is probably not it.

Things changed were TQ so is possibility easy to rule out but will have to pull trans to swap out.

Main two things I see is differential noise, or internal in trans.

To me and based on experience, it sounds like a differential setup issue, yet could also be slop in the trans. The internal total run-out is controlled by selective nylon washers and not a lot today understand how to setup an older TH350 or TH2004R/700R4. Engine RPM at 3,000 can easily have internal components at 20,000 RPM or more. This can produce the issue you are having.

Then also have to ask... Did you try rotate tires? Did tread style change? Tire with slipped belt?

I did rotate tires and they’re the same as before the rear gear change. I do appreciate the help a lot. I’ve gone to a few local mechanics about this and have changed everything they felt was the culprit that’s why I’m here. Started with “pinion angle is probably off” to the driveshaft is out of round, or the ujoints are bad. I’ve had a sneaking suspicion the entire time it was either rear gear or trans related, but I’ve checked the rear gear 3 times now. I’m slowly getting through my “broke more than a fixed” phase because I’m definitely still in that! Love that analogy. Again, I much appreciate the help.

 

[SamJam05]

Just rereading though this again..

You say you can "feel" this and increases as speed increases...

It could still be differential, but more inclined to think transmission.

Mainly you can "feel" it and that seems to point to excess slop in a rotating item. TH350 have noted Drum problems, and I have rebuilt a few where the matting surface of the Forward Drum was completely worn away.

If you have ruled out the other things, I would pull that trans and pull apart.

I was unaware the internals could cause it to vibrate as well. Other thing I forgot to mention, if you turn the tires and listen to the transmission in neutral you can hear a click underneath the trans pan. Never heard it before, is that common?

 

[86LK]

so basically, only happens under load and at high speed? goes away when coasting at high speed?
what happens if you manually shift to the next lower gear (4th to 3rd) at high speed before you hear it? does it start noise?
what happens if you are in a lower gear at high speed making noise and then shift up (3rd to 4th)? does the noise go away?

What type of information with this provide just curious? I’m relatively new to working on cars (last 5 years) and self taught. But I’ve never downshifted to feel/hear the vibration, but I can tell you it’s worse in second under heavy load and then if you bump 3rd it goes away until the rpm reaches a similar point. But it’s definitely still related to speed more than rpm.

if the problem only happens at a certain speed and it goes away, or happens, in a certain gear, then you know the issue is not just speed but also RPM related, whether it is engine or transmission. every clue that you can pick up of when, and when it does NOT happen, is a clue to getting it resolved. if it happens at a certain speed whether under load or not , that can help eliminate some potential causes.

 

[SamJam05]

if the problem only happens at a certain speed and it goes away, or happens, in a certain gear, then you know the issue is not just speed but also RPM related, whether it is engine or transmission. every clue that you can pick up of when, and when it does NOT happen, is a clue to getting it resolved. if it happens at a certain speed whether under load or not , that can help eliminate some potential causes.

It does begin to vibrate when I downshift from 3rd to 2nd around 50 mph

 

[64nailhead]

If the vibration is only present under load as you described, then it’s in the driveline FER SURE. If it were a tire, front end component, axle, driveshaft, etc, then the vibration would be consistent regardless of load.

Since the only thing that is different from what you’ve described, is the the differential/ring and pinion. My guess is the backlash is too tight. A cheap, and relatively easy test is to increase the backlash by .002 - .004”. Pull the axles, then pull out the differential/ring gear and move .003-.006” of shim from the left side and put it into the right.

Gear setup is like rod and main bearings clearance - a little too loose and no one will know, but too tight and everyone (and you) will know. I run the backlash at the top of the spec or .001-.002” too much. The gears will be fine a little, repeat a little, too loose. Considering I’ve been doing this in the 800-1000hp range, I’m sure you’ll be fine. If you feel that this adjustment is over your head, but you can do basic repairs, then you’re wrong. The only tool needed is a budget caliper. The only parts needed are a rear shim kit (less than $50) and some silicone.

 

[Rt Jam]

Eliminate the most you can. Not with new parts but with removing from equation.

If you have access to a drive on dyno. You can recreate the rpm and speed. This removes front wheels from the equation.

Run it on jack stands with the suspension loaded. Repeat with wheels removed and wheel nuts holding the drums. Repeat with no wheels and no drums but not touch brake pedal. Does the slip yoke have good engagement into the transmission?

When the car is up in the air, check tread runout. A balanced tire does not mean it's round.

 

[popeye1978]

If it were a tire, front end component, axle, driveshaft, etc, then the vibration would be consistent regardless of load.

I disagree with that statement due to my own experience -- if the Road Force Balance of a wheel/tire combination is off, vibrations will rise as speed rises.

 

[64nailhead]

I disagree with that statement due to my own experience -- if the Road Force Balance of a wheel/tire combination is off, vibrations will rise as speed rises.

Yes, but it will be consistent with speed and independent of load. The OP is saying that when he lifts the vibration is gone/dissipates or is much less. A tire won’t do that.

 

[Built6spdMCSS]

Yup this is driveline