Bad drivetrain noises when I let off gas.

[hitechredneck]

Just installed a 14Bolt FF in my 90 K5. The axle came from an 86' CUCV chevy truck. I have taken the rear axle, th400, and np208 tc out and had them all rebuilt. The th400 has a transgo 400-S manual shift kit installed, the 208 is rebuilt (stock), and the rear axle has 4.56 gears with detroit locker. There doesn't seem to be a problem when I am driving the vehicle, but as soon as the "load" is taken off the rear shaft, it sounds like my shaft is dragging. There is nominal vibration, but ALOT of noise. I called Eaton about the problem and they said the there is no moving parts in the locker assembly if the vehicle is moving forward (while giving it gas or not). They said that the problem is most likely coming from the T.C. I had a new shaft made (because my old shaft was to short - less than half of the yoke was in the T.C.) The only thing that I can think of is that the rear bushing and seal in the 208 is worn and letting the shaft vibrate when there is no load. Anyone have any similiar problems or ideas?

 

[Chevy305]

is it a clucky/ gear chattery noise?

 

[obijuan]

what are your ujoint angles like? also when i let off the gas it sounds like my truck is gonna explode. i have a stretched chain as well as whack joint angles.

 

[Goose]

x2 what are your u-joint angles like? Are they phased properly? Did you get your new driveshaft balanced?

 

[kgblazerfive]

Does your pinion has alot of free play side to side? well the yoke move alot before it turns the ring gear?

 

[WJACKSON11X]

Put it in neutral, with the parking brake on or chock the tires. Grab the rear driveshaft and see if you have any play near the transfer case or the pinion, could be one of the bearings in either.

 

[Chevy305]

Put it in neutral, with the parking brake on or chock the tires. Grab the rear driveshaft and see if you have any play near the transfer case or the pinion, could be one of the bearings in either.

I tried that, and I can't feel any play. Also the splines don't seem to have any play in them. However, there is some rotational slack inside the trans/tcase if i spin the output.

 

[hitechredneck]

Fluid levels.

Check your fluid levels. My TC was approx. 1 1/2 - 2 qts. low. Rear seal is bad and POSSIBLY the rear bushing of the TC. The driveshaft I had made was balanced, and new u-joints installed. The rear diff was about a qt. low as well.
I called Eaton and they told me that I have a Detroit Locker installed rather than a gov. locker (Difference in the spider assembly and springs.) They said that the rear being so noisy is normal, but when I let off the gas, there shouldn't be any moving parts in the rear to cause the noises. (Locker is engaged). They said for me to check the diff "backlash" of the ring and pinion. It seems to be OK.
All I know is that I ran the Blazer for over a year with too short of a driveshaft (Less than 1/3 of the yoke was in the TCase.) This undoubtedly ruined the seal and possibly the bushing in the case. I hope so, I don't want to have rebuild the rear end. All I know is that it sounded like my rear end was coming out, but it sounded as though it was coming from the TCase.

 

[hitechredneck]

NO vibrations.

Also, there are no vibrations that I can tell from the drivetrain while I'm driving. Just that terrible sound that happens when I let off the throttle. I stuck with the slip yoke design for the driveshaft. The angles were "eye-balled", (I know.... not a good idea.) I have the TCase lowered and I figured I could adjust the angles of the diff by using shims and/or by raising the TCase. I don't think it has anything to do with the u-joints being out of phase because I would be able to tell by driving it under a load. (I think)

 

[Mudbug1979]

me too

Just did, shackle flip, 4" 14bff, detroit, 4.56's 37's, slip yoke, new joints, shimmed. Cruising fine, let off gas and POW, Vibration like crazy, like the pinion bearing has gone out.

Check, Low BLACK fluid, maybe a quart, not dropped transmission, so I'm thinking the driveshaft might not have enough in the tailshaft to cut the vibration down. Sounds like the Xcase is coming apart.

 

[JNygard]

Your pinion is most likely diving down when you let off the gas, binding u-joints. Mine did that after I went to 6" lift and 14bolt. I did the ultimate fix bye changing the drive shaft to a Double-Cardan joint and pointing the pinion just slightly under the tailshaft. Took care of all the vibrations when letting off the throttle.

 

[isaac]

had the same issues after i lifted my blazer sounded like the driveshaft was going to buzz right off the truck. after a shim and cv shaft no vibe or crazy sounds from the drivetrain.

 

[MTMike]

Check your driveline angles. I bought my truck with a 12" lift and it would make sounds like that all the time. Once I lowered the truck with stock 56's and a shackle flip the noise went away.