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Manual swap vibrations 91 Cummins

Old 04-16-2019, 08:14 PM
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Manual swap vibrations 91 Cummins

I just finished converting my 91 automatic to a getrag 5 speed and I've noticed more vibration at idle its enough that the mirrors are vibrating it goes away as soon as I start driving wondering how much is normal and if i should be worried. i did replace the trans mounts so I'm thinking that's probably the difference other than that the truck runs and drives amazing!
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Old 04-17-2019, 06:44 AM
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The aftermarket Anchor transmission mounts are much harder and transmit much more vibration into the frame and cab.
I don't like them myself, and re-use old OEM ones that are still in decent shape.

The new mounts could likely be the source of you new vibe..

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Old 04-17-2019, 10:12 AM
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That makes sense i figured if it was something out of balance it would get worse at higher rpms. Thanks!
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Old 04-17-2019, 10:27 AM
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Just to rule them out, you might also consider the harmonic balancer, and since a transmission swap was involved, the flywheel......Ben
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Old 04-17-2019, 10:32 AM
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If it was either of those wouldn't it be worse at higher rpms?
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Old 04-17-2019, 01:49 PM
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The straight six is inherently balanced but there are some vibrations induced by adding on accessories such as the alternator, A/C, hydraulic pump and the such. For a vibration at idle I would be looking closely at the engine mounts and the transmission mounts. A stiffer then stock mount can and does lend it self to having minor vibrations but will be fine and long lasting. With a manual transmission I have seen and felt more vibrations at idle then with a automatic as the automatic has the ability with the fluid coupling to absorb the vibrations a manual transmission is a direct connection. One thing to test is to SET THE PARKING BRAKE and then place your manual transmission in neutral at idle and see if the vibration is still present. If the vibration is still present it might be coming from the engines harmonic vibrations. I suspect it is a combination of the harmonic balancer and the stiffer mounts. I would not really worry about it unless it really bugs you.
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Old 04-17-2019, 03:52 PM
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Originally Posted by Mikeske View Post
SET THE PARKING BRAKE and then place your manual transmission in neutral at idle and see if the vibration is still present.
Using this method, and if possible having a helper to hold the foot brake down, you can also use a prybar from under the truck to take some of the weight off each mount and see if the vibration changes......Ben
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