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Subject: Torque Converter -- Bad Vibrations?

Original Message
Name: Bob Johnson
Date: February 23, 1998 at 07:30:58
Subject: Torque Converter -- Bad Vibrations?
Can a bad torque converter cause vibrations in a car when it is standing still, i.e. in park and reving the engine through its operating range?

Here's why I ask: My 72 Chevelle (350 V8, THM350 transmission) has developed a seat of the pants vibration at two places, between 1150 and 1250 rpm and between 1750 and 1850 rpm. Anywhere else in the rpm range from idle through 4000rpm there is no vibration, only at those two specific ranges. Cruisin' down the highway at 70 mph plus the car is smooth as a baby's butt -- no vibration.

The vibration is there when driving but also when running the engine in park. I say that means an engine problem since the vibration is present in park. But my transmission guy says it could be a bad torque converter or flex plate.

I've had my engine scoped and completely checked for missing in those two rpm ranges and there doesn't seem to be an internal engine problem. So maybe the guy is right. But, I still don't understand how a bad torque converter could cause vibrations in a car that is in park and standing still... the torque converter's got no hydraulic pressure on it at this point, right???

I really don't want to spend a bunch of cash chasing this vibration into the transmission unless that is really a likely cause. PS) The engine and transmission were both rebuilt about 65,000 miles ago.

Anyone's experience and advice on this one is very welcome.

Thanks for your help.


Response Number 1
Name: fred ont canada
Date: February 23, 1998 at 10:02:21
Subject: Torque Converter -- Bad Vibrations?
Bob what you are discibing is a mechanical vibration has nothing to do with pressure. At this point in park you have a rotating mass that is out of balance.Pull the flywheel cover and run the engine check for run out in the converter.If you use a timming lite hooked up to 1 of the rear plugs you can strobe the flywheel converter will it is running.
Use caution and stands BE CAREFUL AND SAFE...FRED

Response Number 2
Name: Leo
Date: February 23, 1998 at 10:19:25
Subject: Torque Converter -- Bad Vibrations?
Your transmission guy is partially correct. He didnít go far enough in his explanation of the cause of vibration. The convertor or flex plate could have lost the balancing weights that are sometimes welded on to balance them. The flex plate could also have cracked at the crankshaft hub.
Any of these three items could cause your vibration. You can drop the flywheel cover and inspect for missing weights on either convertor or flex plate. The cracked hub is a little harder, usually experience with cracked flex plates makes it easier to diagnose that. In the years of working on these vehicles I have replaced all three at one time or another, all for a vibration. Sometimes with a cracked flex plate it is possible to hear a light crack or pop when coming just off idle, not all the time though. Your have to have a rather quite engine and exhaust to hear this and with a muscle care I donít think you will.
Your can inspect for lost weights but you may have to slide the transmission back to find if you have a cracked flex plate.
Good luck in finding the problem, I hope that this is of some help. Leo

Response Number 3
Name: Bob Johnson
Date: February 23, 1998 at 13:01:33
Subject: Torque Converter -- Bad Vibrations?

What am I looking for when I illuminate the flywheel and torque converter with my timing light in the manner that you describe?

How do I check for run-out on the converter? I'm guessing that you mean check to see if it is out of round... is that right or not? If so, how can I tell by strobing it with my timing light?

Thanks very much for your help Fred and Leo!


Response Number 4
Name: Gene McGill
Date: February 23, 1998 at 16:37:05
Subject: Torque Converter -- Bad Vibrations?
I'm not sure what a timing light would do either, as it would create a "stop motion" effect, and I'm not sure what that would tell you. Have you checked everything on the front of the engine, as in the harmonic balancer, pulleys, and fan? Personally, I think I would pop on a new harmonic balancer just to eliminate is as a possibilty prior to seperating the engine and transmission. On anything that spins, make sure all of the bolts are installed and are the same as each other, including the pulley on the balancer and the tourque convertor bolts.

Response Number 5
Name: Ray
Date: February 23, 1998 at 19:09:57
Subject: Torque Converter -- Bad Vibrations?

Just a thought - are your engine-to-tranny bolts all present and tight? I had a vibration problem like this many years ago, and found some of the bellhousing bolts loose; one missing.

When you're at idle, there is no torque on the fasteners so they can 'act' loose and move around, causing your vibration problem at idle.

But when going down the highway, the engine is putting a torque on the tranny, so whatever bolts are holding the two together, are loaded up. Therefore no vibrations on the road.

The reason that you may have more than one rpm at which vibrations occur is what we call multiple resonances. (Dont ask, just take my word on this.)

Anyway, ask yourself why GM put in 6 large bolts in this location if there was not a large force involved.

Good luck

Response Number 6
Name: fred ont canada
Date: February 24, 1998 at 07:37:36
Subject: Torque Converter -- Bad Vibrations?
Bob with the engine running look for wobble back to front on flex plate and converter this could mean the neck was not welded in staight or is cracked.
As far as strobing it you can see it in operation and check for any thing abnormal.There is more that one use for a timming lite "IE" checking fuel flow in the carb stick it in the throat of the carb and hit the throttle or check for leak down in the rear 2 barrels.Can be used in a number of different ways...FRED

Response Number 7
Name: Ron
Date: February 24, 1998 at 18:58:20
Subject: Torque Converter -- Bad Vibrations?
Gene has a good point, for this or any other problem: identify all the possible little things first before taking things all apart. Look at the car as a whole system (rad to tailpipes) and eliminate one thing at a time. Sometimes a couple of simultaneous problems can make professionals unsure of what to do.... Good luck

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