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Aggressive Drivetrain Shake??

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FSJunkie View Drop Down
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Post Options Post Options   Thanks (0) Thanks(0)   Quote FSJunkie Quote  Post ReplyReply Direct Link To This Post Posted: May/23/2020 at 2:59am
I've generally found the torque tube driveline develops some noisy vibration at around 3200 RPM shaft speed no matter what u-joint it has. I say that because I have cars with both single and double cardan joints in them and they both do it, though at different road speeds because of different axle ratios.

It sucks because it usually happens at the speed you want to go while on the interstate.

Supporting an engine's weight by the end of it's crankshaft is not usually a good idea. Crankshafts are not as strong as you think and it's not great for the main bearings either.

Still, I find it hard to believe that raising the engine would damage the driveshaft u-joint. It's more likely it damaged one of the several transmission mounts and torque links in there or the rubber doughnut seal that holds the torque tube to the transmission tail housing. In any case...some piece of 50 year old rubber that just got flexed more than it ever had before.

Edited by FSJunkie - May/23/2020 at 3:01am
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2020 at 7:41am
If you're having a noticeable vibration something is wrong. You might get JUST enough vibration to notice, but nothing that should be a real bother, typical old car road noise (it's not a new/modern car!!). Usually it's the rubber mounts on the ends of the transmission crossmember are worn out, and/or motor mounts. The rear axle pushes on both. There are two thrust arms, one each side of the crossmember, but they have very thin hard bushings in them. I don't think there are replacements for those available, but they don't seem to be a problem. The center bearing (on both V-8 and six -- CV and single joint, respectively, also single joint 56-61 V-8) could be worn. It rides in a rubber bushing made onto the bearing. They usually don't wear out easily, but if there had been a transmission seal leak for a long time the oil can deteriorate the rubber around the bearing. I've not seen one of those cause an issue yet, but haven't had one in a long time either. Time could be a factor...

So if the CV joint isn't "floppy" at the center and seems to be holding itself straight, check the rubber end mounts on the crossmember and the motor mounts -- replace if they look old. The end mounts should hold the crossmember about centered in the channel on the ends. The channel keeps the crossmember from falling out or lifting too high if a rubber mount fails. If the crossmember is sitting on the channel it has obviously failed. Should be at least a 1/4" gap between lower edge of crossmember and the flat of the channel at the bottom -- channel extends a little over an inch from the inside of the sill ("frame" rail).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote 73Gremlin401 Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2020 at 8:44am
Have you checked the bell housing to engine block bolts? Seems ridiculous, but this happened to me, and I couldn't believe it either. And sure enough they were loose just enough to throw things out of alignment - or back in - based on engine load. If everything else is checking out - why not check this.

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Post Options Post Options   Thanks (0) Thanks(0)   Quote rsrguy3 Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2020 at 11:58am
In the very first sentence in the first post, you mentioned that you lifted the drivetrain by the balancer. How old is the HB? Is it possible you compromised the elastic during the lift? My knee-jerk reaction to vibrational issues is generally drive line, however in this case it sounds like you didn't touch the driveline. You did state you lifted by the balancer. This should be a point of closer inspection. You may now be needing a HB overhaul. 
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: May/24/2020 at 2:32pm
let it idle and watch the balancer on the crank

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Post Options Post Options   Thanks (0) Thanks(0)   Quote jzurek Quote  Post ReplyReply Direct Link To This Post Posted: May/26/2020 at 11:06am
Thanks for all of the advice guys!
The harmonic balancer is in good shape (I jacked it up with a piece of wood underneath to try to prevent damage). The engine idle smoothly and revs up as it should in neutral.

I decided to to try with the easiest parts first, torques down all the transmission mounts, and they look to be ok shape, but due for replacement. They're not really sagging but are all cracked up. 
The bellhousing bolts were a little bit loose, though. Torqued the ones that I could get to from underneath to 40-45 lb-ft per the manual, but could not get to the top ones. I'll order an angled wrench and try to get to those this weekend.

Took it out for a drive after going through all of this, and it did do the shake, but only intially for about 3-4 seconds, and then it never came back, seems a little bit better, might be that the bellhousing is not torqued down completely.
The weirdest part about this whole thing is that it only seems to do it after it has been sitting, and this last time that I drove it, the shake never came back even under heavy acceleration. I will tighten up the rest of the bellhousing bolts and get back with what I find.

Thanks again!
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Heavy 488 Quote  Post ReplyReply Direct Link To This Post Posted: May/26/2020 at 1:38pm
transmission shudder when it's cold?
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Post Options Post Options   Thanks (0) Thanks(0)   Quote farna Quote  Post ReplyReply Direct Link To This Post Posted: May/27/2020 at 6:36am
Driving will loosen things up inside, especially if it sat for a while. ATF has lots of detergent in it to keep the trans cleaned out, it may be working. Wouldn't hurt to change the fluid and clean the screen (no replaceable filter, just a really fine mesh stainless steel screen.. clean with solvent, brake cleaner, etc.).
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jzurek Quote  Post ReplyReply Direct Link To This Post Posted: May/28/2020 at 4:15pm
Possibly...I did change the fluid in the pan and adjust bands but did not drain the torque converter.
I'll drain the converter and refill the transmission

Trying out everything I can before I have to drop the torque tube, haha
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Post Options Post Options   Thanks (0) Thanks(0)   Quote jzurek Quote  Post ReplyReply Direct Link To This Post Posted: Jun/01/2020 at 9:35pm
Is there any way to drain the torque converter on the M8/M10 transmission? I took off the access panel and all I see is the front of the flywheel, but no obvious drain bolt even after rotating the engine. 
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