Truck "Jumps" When Shifting into Reverse

[Subzero]

On my 1968 F100 Ranger with the Cruise-o- Matic trans sometimes the truck with make a knock noise and "jump" when I put it into reverse. Other times it will shift right in very smooth. When driving the trans shifts relatively smooth into second and third gear for a 40+ year old trans with no rebuilding done.. If you put it in drive 1 or 2 while at a stop it will do the same as the reverse but not nearly as bad. I was told it could be the universal joint. What do you all think could be causing this?

[papabug71]

Sounds like a classic example of U joint slop to me. Does it make a "PING" sound when you feel it jump?

Crawl under the truck ( with the truck in park). grap ahold of the drive shaft. Rotate it side to side & look at the U joints. Look for slack or play in the joints.

Most likely it's just time for new U joints. No biggie. They will wear out over time.

[Donnie]

.........or could your idle speed be a little too fast..Just remember these units will break the rev band if the engine is running too fast & rev is engaged....MAJOR cause of rev band breakage in this trans.==don't put the truck in REV with the engine on fast idle...
We always told the owner to back in the driveway when coming home from work if you must jump in & go in the AM with out a proper warm up to open the choke..........just a word of caution to ALL COM drivers.......probably is the U joints though...........Donnie

[Subzero]

I park mine out by the curb so I never use reverse till later so I don't think its runningb too high, but that is a good tip. Is the U joint something that will "fail" eventually and cause the truck to not be able to drive? Does this cause damage to other things?

[papabug71]

Yes. When a U joint goes out the truck will be dead in the water so to speak. But, U joints usally last a very long time. It's not like you have to replace them every 10k miles. Chances are the U joints in your truck are factory equipment.

If you lose one going down the road it can cause serious damage to the drive line components. It's best to replace them at the first sign of trouble.

Last time I blew a U joint, I was moving 40mph down the road & it bent the drive shaft, split the transfer case & ruined the yoke on the rear end. Not to mention the drive shaft put a huge dent in the floor of the vehicle (91 Jeep Cherokee) when it was banging around under there. I can only imagine what would have happened if I had been going 70mph down the highway.

[Subzero]

With mine doing what it does when putting it into reverse how long will the u joint hold out?

[Whateverman]

While it could be your u-joints Its likely more that its the combined lash in your drivetrain -a little slop here,a little slop there-especially if you have a 2pc driveshaft,mines been making the same kinda noises as yours does for as long as i've had it (over 20yrs)...it went away when i swapped a 4spd in for awhile....

[Donnie]

You may also want to check your engine mounts...loose or collapsed mount will cause this feel, too. .....donnie

[papabug71]

Good info from Whateverman & Donnie.

It's hard to to tell exactly what it could be without being there in person to hear/see what it doing. Check the U joints first. Look for slop. You're gonna have to be the judge as to how soon to replace them (if that's the problem). Just remember if you find one with alot of play, or one that has thrown a cap, it's cheap insurance to replace the defective unit.

[hazelnut]

The biggest reason that u-joints go bad is because they never get greased. A little bit of grease every 10k will make them last a long time.

[Subzero]

Ok thanks for the replies. I will be sure to look at the u joints soon and check them out.

[Subzero]

The U joints don't "look bad" and when I had the exhaust gasket replaced the mechanic said that its not the U joints that make this happen. He just said possibly the combination of all the drive line slack could do this. Could the trans fluid be low?

[Donnie]

Low fluid, not likely, but I would suggest a band adjustment if it has never been done...You WILL break the band if it is banging into rev> Donnie