379 Peterbilt vibrations while under a pull & letting off throttle

500-700k.

 

I run a 1991 379 and replace anything that gives me a hint that it might fail. I was inspecting the radiator on the fan side when I saw that the harmonic balancer looked as though the outer ring was moving forward on the inner section. That meant the outer ring was moving towards the fan blades but had not made contact yet. Got her back home and into the shop. The decision was made to go with a aftermarket type that had great reviews. What a difference it made. There were things going on that I did not feel or detect in the old one's failure until it was replaced. The new one is larger and changed everything about the running characteristics of the old 3406-B engine. Not saying this is your problem. Sounds like drive line problems to me. All because things were replaced does not mean that they're not defective or installed improperly. Good luck, keep us informed. Inquiring minds want to know.

 

Felt a bad vibration on my 93 years ago. Stopped raised hood, reached in with one hand and pulled outer ring the rest of the way off. That was close.... To the OP you might check your ride height. That will cause what you are describing, exactly. Btdt.

 

My truck is doing the same thing, 2003 Pete 379. Can’t figure it out, driveshaft is tight. Adjusted my ride height to factory specs.... did you ever figure it out

 

My 03 pete doing same after I changed all the spindles did you figure yours out

 

I had my drive line rebuilt and balanced solved the vibration issue for me.