Bang, what is that?

[86in]

My 99 Z71 has a "thud" sound from underneath somewhere whenever I start out from a stop. It feels like somesthing slid and hit my tailgate. What the heck is it? Thanks for the info in advance.
Mike

[Z15]

Could be the slip yoke on the drive shaft into transmission. They can bindup and make a banging noise from launch. You can try to stop it by applying some good synthetic grease (Amsoil) tot he yoke teeth. Remove the drive shaft and apply grease coating, you may have to clean the yoke of corosion or replace it. GM cameout with a nickle plated yoke to solve this problem years ago.

[sector95]

Yup...what Z15 said. I believe GM calls it a "Stop clunk noise" in their TSB files. I have a 98 Sierra with the single driveshaft and when I bought the truck (used) it would do exactly what you are experiencing. Solution is to remove the driveshaft, clean the slip yoke and grease it.....I use Bel-Ray waterproof grease because it is thicker than plain chassis grease and it's what I have on hand....seems to work like a charm; no more stop, thunk sounds. You'll need to do this maybe every third or fourth oil change. I actually found in my owners manual where this is a mileage-based service procedure.

Mike
98Sierra

[86in]

Thanks for the help, I am no "mechanic", is this a difficult task? Do I need any special tools? I appreciate the help.

[maxwedge]

Drive shaft has to come out to do this of course, some fluid will be lost from the trans also.

[sector95]

No special tools are required and it is not that hard to do. Before you begin, go out and look at your truck. Look at the rear u-joint to see how it is held to the axle input yoke; you'll see two u-shaped straps held with four bolts (two each per strap) retaining two of the bearing cups of the u-joint (lotta "U"'s here). Look at the tail end of the tranny and see the slip yoke and how it fits into the tranny. There is a lip seal at the back of the tranny sealing around the slip yoke.

You will have to raise the back end a bit to get under the truck comfortably and allow you to rotate the driveshaft to remove all the bolts. You will need a floor jack; you can raise the back end by placing the jack under the differential housing....*Do Not* use the tire jack under the differential. Wal-Mart, Pep Boys, AutoZone, etc. usually have a floor jack and jack stand kit for some easy money....these would be some good tools to have on hand anyway. When raising the rear end, *MAKE SURE YOU CHOCK THE FRONT WEELS ON THE FRONT AND BACK SIDES OF AT LEAST ONE TIRE*. Jacking the rear wheels off the ground will negate both the parking brake and the parking gear and the truck can roll (floor jacks have wheels on them) until it is supported by the jack stands. Using a pair of jack stands place each one just to the inside the spring plates on the axle (where the leaf springs are held to the axle). Before you get under your truck, *MAKE SURE IT IS SECURELY SUPPORTED* by giving it a good wiggle back and forth; it should not move.

I think the four bolts that hold the two rear u-joint clamps to the input shaft are 13mm....maybe 15mm (which is a common metric size GM likes to use). Before you start unbolting, you *must* match-mark the drive shaft to the axle input shaft so the two will go back together in the same reference to each other....this is to insure the driveline rotational balance is maintained....some "White Out" typewriter correction fluid works great for this. You will also need a plastic or rubber mallet to break loose the u-joint from the axle input yoke. With the truck on the jack stands you'll need to take it out of "Park" and release the emergency brake so you can rotate the the driveshaft to position it so you can unbolt/rebolt the U-clamps. Once you have a u-clamp in the 6 o'clock (bottom) position you'll need to reset the e-brake (or put it back into "Park") to keep the driveshaft from turning while you unblot/rebolt the clamps. With both U-clamps removed, firmly hold the driveshaft and try to push it forward into the tranny to release it from the axle input saddles...this is where the mallet comes into play as it will no doubt need some "persuasion" to come free. Once the drive shaft is free from the rear axle, simply lower it a bit at the back and slide it backwards; the splined slip yoke will come out of the back of the tranny and usually a little tranny fluid will drip once the slip yoke is completely free.

Once you have the driveshaft out from under the truck wrap some masking tape around the exposed bearing caps of the rear u-joint....this will keep them from falling off and allowing all the needle bearings inside to fall out. Get some carb cleaner and a toothbrush and clean the old grease and mung from inside the slip yoke (be careful not to wash away your match mark with any dripping carb cleaner). I use Bel-Ray waterproof grease but any thick grease should do (GM has a part number for the correct grease but I've not had any problems with what I use). Use your finger and a small brush to thoroughly work the grease into the splines of the yoke...be generous and completely cover the inside of the slip yoke. Put a thin coat on the outside of the slip yoke where the lip seal rubs (it'll be a shiny area).

With the prepped driveshaft, get underneath and orient your matchmarks and then carefully slide the slip yoke into the tranny; be careful not to booger up the lip seal. At the back end, check to make sure your match marks line up; if not, unslip the yoke and realign the splines so that your match marks line up. Remove the masking tape from the u-joint and push the bearing caps into the axle input saddles. Replace the U-clamps and snug down the bolts (don't get carried away with tightening them too much...and you'll need to rotate the driveshaft to get the wrench on the other set of u-clamp bolts).

Make sure everything is secure and re-engage the e-brake and put it into "Park". Lower the vehicle and enjoy clunk-free driving.

Hope this helps you out...it sounds like more than it really is and once you do it you'l find out it is a relatively simple procedure. Good luck and let us know how it goes....

Mike
98Sierra

[mad1517]

Thanks for taking the time to explain in this detail.Tom