I have a '78 Ford w/ a 9" posi-rear. When I make right hand turns I get this grinding type noise from the rear. It only happens when I am turning to the right, straight ahead and to the left are a-ok. Any suggestions? Thanks, Rick

 

How many miles are on the axle, and what type of "posi" is in it, i.e. limited slip, Detroit Locker? If high miles, it could be a side gear or a bearing. My 9" ate through the bearing support at 230,000 miles, never made a sound. It just stopped pulling.

Nathan

 

Hmmm, not sure about the miles. The gentlemen who previously owned the truck had changed the rearend. Supposedly, he used a heavy duty axle with 4.11's in it and then swapped in a 2.75 combo. I am not real knowledgable about the types of rearends but I can tell you that with both wheels off the ground they both rotate in unison - would this mean a Detroit Locker?

 

Lift one tire off of the ground. If you can turn that tire, but its really hard, its a limited slip. If you cant turn the raised wheel, it could be a Locker. At any rate, it sounds like rebuild time, or you could purchase a complete "drop out" for your 9" and install it yourself. Just make sure you check the splines on the axle shafts for wear.

Nathan

 

Try driving the truck straight ahead along the side (not up and down) of a hill or incline to put some side load on the bearings. With the right side of the vehicle higher than the left, this should be similar to making a right turn except you are going straight. If you hear the grinding noise, then there is a bearing problem. The noise may actually be comming from the left brake drum which is being allowed to be pushed sideways into the backing plate and/or brake shoes due to the sloppy bearing.

If you do not hear the noise then try this alternative experiment. Jack up the left tire just enough to clear the ground. Chain your rear bumper to a tree and slowly try to drive forward. If you have a working limited-slip, then the tire in the air will spin helplessly. The tire on the ground should cause the truck to tug on the chain a little, but it probably will not spin unless the ground is of poor traction (wet grass, loose gravel) or you have a locker. If that is the case, then this experiment will not work.

If you hear the grinding noise with the tire slowly spinning in the air, then there is a problem with your limited-slip differential, probably the clutch packs. You might try changing the oil. Most limited-slips require special gear oil that have friction additives in it. It may be possible that the guy who installed the limited-slip did not know this and just added regular 90 weight. Cheap fix if that's all it needs.

While you are still chained to the tree, swap sides and spin the opposite tire in the air. Does the truck still tug on the chain about the same amount as before? This will tell you wether one side of the differential is weaker than the other for some reason.