confusing vibration

[beastman]

i havent been on here i a long time, but ive ran into an issue i cant figure out. i have a nasty vibration under 2000 rpm, mostly in first gear, a little in 2nd, and in 4th gear at 70 theres a humming sound. i have the a833 od transmission, rebuilt under 1000 miles ago, and ive put in a centerforce duel friction diaphram clutch. then i installed a eaton helical gear lsd, and with 295's on the back, i grenaded the slip yoke, 7260 series. so to fix that, i bought a chromolly spicer 1350 series slip yoke and a new driveshaft made with 1350 both ends. the 489 case with 3.23 had the 7290 yoke, so i installed a coonversion u joint. still had the vibration, and twisted the rear strapps off the yoke and destroyed the u joint. so i went and had to get the driveshaft re tubed and balanced, and this time had the 489 case rebuilt with brand new 3.91 gears, bearings, and a billet 1350 yoke. i thought i had to have fixed it by now, but upon todays test drive still a nasty vibration and humm. i put the rear on jackstands and ran through the gears looking for a vibration, but of course with no load there wasnt one. so im stumped. all i can think of is maybe i bent the tailshaft in the transmission, but would like some input first before i go drop a couple more grand making a bullet proof tranny

[712edf]

Is the vibration at the same engine RPM in each gear?

I have seen engine imbalance cause transmission/driveline vibration & failure.

Bucky

[beastman]

Yes, typically between 1500-2200 rpm. There's no vibration at higher rpm, and I haven't changed anything on the engine except the clutch. Though that rpm in 3rd gear doesn't seem present, note in 4th, just a noticeable hum at higher speeds

[gww]

i havent been on here i a long time, but ive ran into an issue i cant figure out. i have a nasty vibration under 2000 rpm, mostly in first gear, a little in 2nd, and in 4th gear at 70 theres a humming sound. i have the a833 od transmission, rebuilt under 1000 miles ago, and ive put in a centerforce duel friction diaphram clutch. then i installed a eaton helical gear lsd, and with 295's on the back, i grenaded the slip yoke, 7260 series. so to fix that, i bought a chromolly spicer 1350 series slip yoke and a new driveshaft made with 1350 both ends. the 489 case with 3.23 had the 7290 yoke, so i installed a coonversion u joint. still had the vibration, and twisted the rear strapps off the yoke and destroyed the u joint. so i went and had to get the driveshaft re tubed and balanced, and this time had the 489 case rebuilt with brand new 3.91 gears, bearings, and a billet 1350 yoke. i thought i had to have fixed it by now, but upon todays test drive still a nasty vibration and humm. i put the rear on jackstands and ran through the gears looking for a vibration, but of course with no load there wasnt one. so im stumped. all i can think of is maybe i bent the tailshaft in the transmission, but would like some input first before i go drop a couple more grand making a bullet proof tranny

morn n, where do you feel this nasty vibration, steering wheel ,seat,? sounds like you ve covered alot of changes. your thought of trans, tail shaft being bent, does the rear trans. seal leak,when on blocks watching it is it out of round? while on blocks did you spin rear wheels for run out? poss. twisted/bent axle? could this be a harmonic vib. from a bad belt/ harmonic balancer ? just thoughts. hope not to add to your prob. . gary

[Roxyflash]

Had that tranny in a ramvan did you replace the tail shaft bushing?I was fighting the same vibration also.In my opinion those trannys are weak.I under stand the need for the over drive but the 323 with a non over drive would be better then a 391 with the overdrive tranny,

[Roxyflash]

Another thing was the rear mount if its the one that has the mount that uses a single bolt tru it they have a tendency to warble out and vibrate metal to metal,When there in over drive the stress on the tranny makes them howl i had mine in and out of the ram vam van three times replaced all kinds of parts finally just learned to live with it i had it to where it had the howl in 4th gear only the van was beater 318 with 4 on the floor was a odd ball short wheel base was a blast to drive till the ac quit the back windows in the doors didnt open was a sweat box so i junked it .

[beastman]

The vibration is felt through the whole truck, steering wheel, seat, the whole truck shakes. It's nothing suspension wise, it's only über load between 1500-2200rpm, mostly in first gear, less in 2nd. Everything in this truck is new as of 10000 miles ago when I finished restoring it. I'll double check the tail shaft, the tranny was also rebuilt less than 1000 miles ago, i know stumped

[712edf]

If the vibration occurs at the same RPM regardless of gear, then its most likely an engine or input shaft vibration.
How are motor mounts? Are bellhousing to block bolts tight? Flexplate bolts? Wrong balancer or TC?

When did this start, has it always done it, or just after tranny/clutch work?

Bucky

[beastman]

It's new as of a month ago. It started when I got a clutch that didn't slip and a good working lsd, I started breaking stuff. I broke the pinion in the 489 case, but it was also the original 40 year old pinion. I'm gonna double check all the engine and tranny mounts, but all that stuff was new per finishing the restoration. I'm gonna double check bell housing bolts and clutch bolts, even though I used lock tight and torqued to spec. I hope it's something simple as a tranny mount, but I doubt it. I'll probably start tearing into the tranny to see if any things broken. I doubt it's the motor since I haven't changed anything besides the clutch, and it's been smooth from the get go

[413wagon]

I had a truck years ago that had been pieced together. Same issue, vibration at certain rpms. regardless of road speed. I suspected engine imbalance. I bought the high priced fluid damper for the front..very little change. I obtained sticky weights for wheels and stuck them on the flywheel (which was accessible quickly) I used about 2 ounces. The vibration rpm range changed considerably. I took the whole recipricating assembly to have it balanced (crank,rods, pistons, balancer, flywheel) We found the truck had been assembled with the wrong flywheel, externally balanced, had a cast weight in it. The guy could not even attempt to balance it with the wrong flywheel (too far out of balance0 I found the correct flywheel and still had the recipricating assembly balanced. I'm not saying that is your problem, but if it is engine rpm related..think about the front pulley/balancer and the flywheel/clutch assembly. loose bolts in those parts will cause vibrations as well as incorrect parts