IanM

Howdy all,

I've done a few searches on this and I'm fairly sure its the u joints on my drive shaft, but I wanted to run it past you guys first...I'm still a bit of a newbie when it comes to problem solving.

From standstill, my truck will jerk twice when accelerating. Once at about 1000 RPM, second time at 1700 RPM. It will also jerk if I'm accelerating or cruising at about 50-60mph and I take my foot off the go pedal.

Could it be a stuck drum brake on accel. and injection pump on decel? or gear slop in the rear diff?

Thanks in advance!
Ian

gorms

U joints can do that, as can a center support bearing (if equipped with a 2 piece driveshaft) if they are bad. Best way to check is crawl under, grab the driveshaft and try to move it back and forth. If you have any play in any joint, replace it (spicer original replacements are the best IMO) and if you have a center support bearing, there should be a little play (movement rather) in the rubber isolator, no metal to metal sound should be heard. Even give the tranny a good shake, make sure no mounts are deteriorated, same goes with your leaf spring bushings.

Could also be shifting problems, assuming you have an auto, like starting out in 2nd gear then down shifting after take off. Does it do it all the time? If it does, try manually selecting 1 from start, then 2 when it would normally shift, then "D" with o/d off, then turn on O/D around 50 and see how it does. If that clears it up, tranny flush or at least fluid/filter change and band adjustment would be a start. Good luck.


PS other than the jerking, how's the new truck?

IanM

Hi gorms,
Its a 1 piece drive shaft. I just crawled under and tried moving the drive shaft and couldn't get it to move anywhere.
Tranny is rock solid. I pushed up on it several times and it didn't have any play at the mounts from what I could see.
The tranny was also freshly rebuilt when I bought it. Literally the night before I took the truck home a few weeks ago. The problem existed previous to the tranny rebuild as well, but is getting worse now.

Asides from the jerking, the truck is awesome! 12v is muuuuch easier to work on than my previous 2 cummins and a million times better than my gasser. I still have a small list of repairs that need to be done, but its still a great truck!

gorms

Oh yeah forgot they put a new trans in it. It being a new truck, I wouldn't overlook anything, as there's a whole bunch of moving parts that can cause a jerk. Check rear drums/shoes/hardware, front calipers, front control arms, ball joints, rear end....etc etc. Spend a solid day checking everything out and get to know that new truck, check "hard" joints (ball joints/leaf bushings) with a prybar. Might find something else ready to go while you're in there. Good luck!

Just a shot in the dark...my CAD was getting sticky a few weeks ago (loose vacuum connector), when shifting in/out of 4wd I'd get a pretty good jerk in the truck several seconds after shifting, sometimes not even close to when I shifted, on slick roads too. Make sure all t-case linkage is correct is what I'm saying, don't think that'd be your problem but speaking from experience.

IanM

Awesome. Thanks gorms. It'll have to wait until I'm done killing the dowel pin though...hopefully nothing falls off!!

The strange part about the jerking is that it's consistently at 1000rpm and 1700rpm right from a stop. If I'm already in motion and lower the rpms and speed, it doesn't happen. I'm leaning towards rear drums, but who knows?! Maybe I'll just throw in new ujoints anyways.

IanM

Driving home from work, I manually shifted from 1st as you suggested...the jerking didn't happen!! Would this be Tranny bands? It hasn't been broken in yet, and is still shifting rough when put into drive or reverse from park.

I was also thinking the deceleration jerking might be from throttle linkage...sudden withdrawal from the go pedal may be starving the motor of fuel?? Just a theory but not sure how to check it. Thanks again!

gorms

When I first did my tranns over it did shift a bit too firm for about 500 miles, then started to smooth out. How many miles on rebuild? If it gets any worse or doesn't go away after a few hundred miles I'd take it back to who did the trans for any adjustments needed, mistakes happen sometimes. Keep an eye on the fluid, make sure it stays nice and clean. Fluid should be changed soon (IMO) to get out all break in material. Good luck and keep us posted!