I have a 96 f-150 4wd automatic. About 50 percent of the time when i take off it thuds. I checked the U joints, back differential, back brakes to see if they were sticking, and trans fluid. Any suggestions as to what could be casuing this. it does thud occassionally going into reverse. It also shifts pretty smoothly. Only once in a while will it shift a little rough. Thank you

 

Do you happen to know what transmission you have? I know there may not be any codes for the thud but it wouldnt hurt to see if the computer is seeing something to help narrow down the possiblities. If the U-joints, differential, etc. parts all check out then it leaves the transmission to be the suspect by shifting hard into gear. How many miles are on the drivetrain currently?

 

How did you check the u-joints and rear differential?

 

it has 124,000 miles on it. I checked the u joints by putting it in nuetral and turning the driveshft back and forth. I also checked the pinion bearing by pushing up to see if there was play. I took the differential cover plate off flushed teh fluid. I checked for metal shavings there wasnt any and i put pry bar under the ring gear and tryed to see if there was a worn carrier bearing or anything. That was about all ive done so far

 

welcome to fte.

the best way to check ujoints is to remove driveshaft.

sometimes they can feel tight when on but are hiding a problem.

 

it's usually too much backlash that makes the thud or your springs are loading and unloading

 

I was thinking something in the suspension too....clunk and thud are two very different sounds.
To narrow it down without seeing the truck is pretty much impossible.
A thud on takeoff helps a little....
I would check rear springs as suggested...check spring hangers, and work your way forward. If you can narrow down where the sound is coming from it might help, but that can be very difficult when you're behind the wheel. Will it happen no matter how fast you take off???
More details......

 

I don't know if your driveshaft slip joint is the same design as my 1990, but mine was thudding real good as I took off from a stop.
I searched old threads and found a remedy. Seems when the slip joint is very worn, and there is enough slop in it to allow the splines to bind when I apply the brakes. This forces the splines in the clockwise direction as the truck stops. When the truck starts up the splines become unstuck, and rotate counterclockwise until they make contact again, and give a good thud. Replacing the slip joint and the splined end of the drive shaft is one option, but dropping the shaft and sticking a finger full of grease in the slipjoint is a temporary fix. On my truck it lasts a couple thousand miles. I did this a few times, and then pulled the slip joint and drilled and tapped it for a grease zert. A two of pumps of grease every couple of months controls the thudding pretty well. After doing this for 1.5 years, I dropped the shaft during an oil change, and cleaned out all the old grease. Gave it a new load of grease and it has been ok for several months now.
You can test for this by coasting to a stop and not applying the brakes at all. If the thud does not occur when you take off, the slip joint may be the problem.
Good Luck Frank

 

if it i take off very easily it will not do it. Thanks for everyones input on this I appreciate it. I'll check some of the things you guys mentioned. I'll also take the driveshaft off to check my u joints as well.

 

You'll figure it out....just gotta keep narrowing it down.
All the best!