[StuartT]

On my 2001 F250 SD LB 7.3 with auto, I am beginning to notice a bit of a slam when I start moving from a stop. It feels like the transmission is slipping for a second and then grabbing low gear. Is this a problem anyone else has noticed? Could there be another cause? There is now 101,000 miles on the truck, the tranny was flushed 15000 miles ago with Mercon V.

 

[brotherxb]

Check you slip joint on your drive shaft may need greasing. You have to cut the metal clamp on the rubber boot and use a tie wrap to put it back. Also mark the position of the driveshaft at the rearend yoke and put it back the same way or it will be out of balance.

 

[StuartT]

You have to pull the slip joint apart to lub it then? I take it there is no zerk?

 

[klhansen]

You have to pull the slip joint apart to lub it then? I take it there is no zerk?

Correct. If there was a zerk, then it would get filled with grease and hydraulically lock (not able to compress).

 

[StuartT]

Okay, thanks.

 

[StuartT]

Hey, brotherxb, you nailed it. Lubed the driveshaft and the jerk is gone. Thanks much!

 

[Machiner1]

I have the same problem , couldnt figure it out. I just replace the rear pinion seal and had the driveshaft down. Wish I would have read this earlier. I will definitly try greasing the slip joint, but isnt that a splined fit, there shouldnt be any room for torsional play grease or no grease right? It just moves in and out for u joint service/ or am I looking at it wrong. anyway if a little grease fixes this terribly agrivating clunk, awesome.

 

[Oneof6]

I have the same problem , couldnt figure it out. I just replace the rear pinion seal and had the driveshaft down. Wish I would have read this earlier. I will definitly try greasing the slip joint, but isnt that a splined fit, there shouldnt be any room for torsional play grease or no grease right? It just moves in and out for u joint service/ or am I looking at it wrong. anyway if a little grease fixes this terribly agrivating clunk, awesome.

This is the way it works. When the unit launches it produces rotational torque at the wheels. That rotation causes the height of the unit to change which moves the slip splines on axis to compensate for the change in angle between the fixed point of the front shaft half and the moving point of the rear shaft half. When the slip spline is not lubricated properly it jumps on axis instead of slipping and that produces what some call a thump, slap or banging. You can reduce the chances of this by doing a couple of things. One in any case lube it with a PTFE grease, Ford makes some for it but I use Slick 50. The second is that some make a 1/4" spacer and put it between the truck frame a the front shaft half carrier bearing assembly. That reduces the angle and the likelihood it will thump. I never did that to mine, just lube it with a good PTFE grease. Regular chassis grease will work just not for as long as you might like it to.

 

[Machiner1]

that makes sense. My boot is gone around my joint so Im sure theres very little or no grease. that may be why mine is so bad.