Short Story - I have a drivetrain vibration at a very specific torque output(2000rpms and 10psi boost). What could it be?
Long Story:
I have a vibration(moderate) on my 86 diesel that occurs at a VERY narrow rpm band (2000-2200 rpms) that feels like, if I was forced to guess, it is coming from the carrier bearing for the rear driveshaft. It doesn't happen all the time, and isn't real bad. Well, after my connecting rod broke last spring, I just assumed it was the rod/engine and I was wrong as to the location of the vibration.
However, after the rebuild, it still happens. As part of the rebuild, I installed a boost gauge though and it tells the tale: While it doesn't happen all the time at 2000-2200 rpms, it ALWAYS happens at 2000-2200 AND 10 PSI boost. In other words, I can cruise the town at 35 mph in 4th gear (which is about 2000 rpms) without so much as a noise, but as soon as I mash on the pedal and raise boost to 10psi I get the vibration until 2200 rpm. It is most pronounced in 4th and 5th gear. It is barely noticeable at all in 1st through 3rd, but only because I think you pass any particular rpm band so quickly in those gears though.
In other words, this vibration seems to be very specific to a certain transmission torque output. Does anyone know/think they know what this is, or have a troubleshooting tip? I assume I should rule out driveshaft balance since torque seems to be the determining factor and not rpm.
During the rebuild I did try to address this in case it wasn't the engine. I have replaced trans and diff fluids and found nothing out of the ordinary, greased sliding shafts and universal joints, replaced fasteners at the universal joints, and replaced the transmission mount. I also inspected the carrier bearing and universal joints and they all seem fine. I can't for the life of me narrow this down to any one thing past the transmission since I am not sure what kinds of logic/troubleshooting techniques to use for this problem.
By the way, stuff in the front (loose change, etc) and the stick shaft does not vibrate any worse when this happens, just stuff in the back so I am pretty sure it is rear of the transmission.
NCDiesel
