Unknown Harmonic Vibration??
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
Likes: 0
From: Vancouver Wa
Unknown Harmonic Vibration??
My 81 with 235k on it with original motor has a harmonic vibration that I can’t figure out at about 1,500 to 1,700 rpm’s and then it smooths out after that until about 3,000 rpm’s. I replaced the old harmonic balancer thinking that this might be the cause but that was not it. Any idea as to what could create a vibration like this? The vibration is the same whether accelerating, coasting or running the idle up in park so I don’t think it is internal in the engine because it doesn’t change under a load, would like to take care of as I prefer a smooth ride. Could this be due to a cracked flex plate? Has anybody experienced this of motor.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
Likes: 0
From: Vancouver Wa
Thanks, seems like a harmonic balance problem as I have had a harmonic balancer go bad before, just replaced HB and that wasn't it and vibration is unaffected by load. So there are balancing weights on the Flex Plate and the Torque Converter?
Racer
Joined: Feb 2009
Posts: 343
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From: San Diego CA
They are tack welded on. If you see a tack where there is no weight, you'll see where one is missing.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
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From: Vancouver Wa
Can you think of anything else that could create a harmonic type vibration between 1,500 and 1,700? Could it be internal? Seems unaffected by load and car runs fine with good oil pressure.
Safety Car
Joined: Aug 2000
Posts: 3,711
Likes: 43
From: San Antonio TX
Take all the belts off the engine and try it. Obviously you can't drive it in this condition, but you can run it up briefly in your driveway. All you're doing here is trying to rule out your accessory drive system. You'll also want to inspect your engine mounts and your exhaust system.
All engines have a harmonic range and some people are more sensitive to it than others. You could be trying to solve a non-issue. You don't state if this is a recent problem so it's hard to say what needs fixing.
All engines have a harmonic range and some people are more sensitive to it than others. You could be trying to solve a non-issue. You don't state if this is a recent problem so it's hard to say what needs fixing.
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Drifting
Joined: May 2005
Posts: 1,629
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From: Somers CT
I chased a somewhat similar vibration for 2 years until I just discovered the A/C belt was vibrating at just the right rpm. There's a pretty long unsupported section that get whipping pretty good at just the right rpm's. I'd try the suggestions about belts before anything else.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
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From: Vancouver Wa
Thanks for all of the input. I ran the motor up in the garage without the belts attached when I did the harmonic balancer replacement and the vibration was still there. It is a healthy enough vibration to where I don't think that it is exhaust or motor mounts and the vibration is there whether moving or not, has to be somewhere between the harmonic balancer and the input shaft of Auto Tranny. Given that fact that is not affected by load I feel that it might be cracked flex plate or missing weights, but am looking for input as I hate to take the car apart only to find that "Oh that was not it", we have all be there before.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
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From: Vancouver Wa
Should probably mention again that vibration is prominent between 1,500 and 1,700 and then smooths back out until about 3,000. Vibration seems a little too prominent to be a natural harmonic vibration.
Team Owner
Joined: Jan 2006
Posts: 37,637
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From: Crossville TN
Remove the inspection cover from the bottom of the bell housing. You can check the condition of the flexplate and mounting bolts from there. Just have someone else 'bump' the motor over to index it a bit at a time. (remove the dist. main feed wire, if you want to prevent it from starting) I'm still betting on a cracked flex plate...but it could be a loose bolt, too.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
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From: Vancouver Wa
Thanks 7T1 Vette, so I should be able to see the bolts with the torque convertor on? Do I have to remove the starter to remove the inspection cover? Can't remember as it has been a while since I have had to put a new starter in.
Thread Starter
Instructor
Joined: Jan 2003
Posts: 143
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From: Vancouver Wa
Thanks c31980, drive shaft u-joint is good as I just pulled out and serviced rear diff (that was fun) and vibration is the same whether parked and not moving, accelerating or coasting down a hill in drive. Once I hit around 1,500 vibration is like that of a Harmonic Balancer that is a little out of whack. Will be checking out Flex Plate / Torque convertor.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
Remove the four little bolts on the inspection cover (7/16" wrench/socket) and you can work it around to get it out without removing the starter. Just get some light down there to check it all out. See if the attachment bolts are loose or if there are any cracks in the plate.