Hello again,
On our 08 F 450 King Ranch toward the end of our last trip towing you could feel the rear end grabbing while taking tight turns? It feels almost like its a locker and its not wanting to turn. The truck has 88,000 miles on it also. So what could it be,
Thanks
Nick

 

Rear axle requires a service, or has been serviced and the proper amount of friction modifier was not added. You have some sticky clutches in the LS unit that are chattering. Add the ford friction modifier and make sure the level in the diff is correct. Add fluid or service accordingly and your chatter will smooth right out.

 

How do I know how much modifier to add?

 

Usually a bottle is suitable for a complete sevice. If you have not changed the fluid in the axle, it may be time, and if you follow the service requirements you will use a bottle of friction modifier.
Otherwise, if you are adding fluid, add an ounce or so and drive the vehicle. May take a couple of tries to get it right but by adding small portions, you will find that the chatter will go away. Unfortunately there is not a magic answer, because all clutches reat differently, so you add as much as it takes to solve the problem. Real scientific huh? Lol.

 

The reason you add a little "friction modifiyer "at a time is,, there is a fine line between clutchs chattering and clutchs slipping all the time,,,to little FM,,clutchs chatter,,,to much FM and clutchs just slip,,and you have no LS rear diff,,,I recall hearing add 2 oz of FM at a time until chatter stops,,,thats all.

 

Where can I get FM and should I change the rear gear oil is it time?

 

1234567890

 

I think Ford says it is not required until it is submerged or leaks but I would recommend doing it. Most of the new gear lubes have the modifier in it so you may or may not need to add additional. You can get the Ford stuff at the dealer, and the 4 oz. container at my dealership in MN was $5. The way to do it is to change it then drive it. If it chatters add an ounce drive again, etc. etc. until it works. I am doing mine this weekend because my '01 had the diff cover rust thru and it is leaking. I am doing the change with AMSOIL sever gear 75W140 and it has some FM in it already but I did get the extra from the dealer just in case.

 

WOAH! You have an '08 with that many miles?!?! Sorry, just surprised.