Okay, I'm checking this out for my Dad who doesn't do computers. He has a 77 F-150 4x4 with an automatic trans. He was pulling a trailer up a hill the other day and suddenly lost power to the rear wheels.

Upon further examination, he found he could lock the hubs & get home with the front axle propelling him. You can jack the rear wheels up and turn the yoke by hand and it will turn the tires, however, when you set it back on the ground, the pinion will turn without turning the wheels (with some resistance). You can turn each tire and it will turn the yoke (it does have some kind of a limited slip differential).

He took the yoke off and the splines are okay, could it be the ring & pinion slipping (worn out), spyder gears ? We just would like to have an idea before opening the whole thing up.

 

My bet is either an axle side gear or the axle splines have stripped.
Pull both axles out and see if there is any damage to the splines.

 

My SWAG, broken axle.

 

Before pulling the axles, remove the driveshaft and with the truck on the ground and using something like a pipewrench , try and turn the yoke and whatch the pinion nut. You may have a stripped yoke and it's the easiest fix possible for your problem. I had 2 fail , the good part is the pinion shaft is harder than the yoke and the splines may be ok and you'll just need another yoke. You can do this with the driveshaft on but it's harder to see if the nut turns with the driveshaft or not. If this isn't it, your going in deep. Good luck.

 

My guess is a stripped axle spline where it goes into the side gear. Just a guess though. Good luck.

 

Could be a stripped axle(common on these trucks) or a broken case where the limited slip clutches go(common on these also.)

 

And with a Ford 9" rear you need to pull the axles first before the diff comes out. That would be a fairly easy fix. And finding a used axle for a 9" would be pretty cheap and easy in my neck of the woods anyways.

 

In other words (based on all of the above), the axle needs to be inspected in more detail.

 

If it has limited slip it might have split the case around the bolt holes. Only way to know for sure is pull it apart and have a look.

 

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Thank you all very much for your help... you are correct, it will have to be taken apart to tell for sure.

 

Did you drop the shaft and try turning the yoke? If that is the issue it won't need to come aprt. Only the punion nut needs to come off. You have to drop the driveshaft regardless so mike as well check the yoke first. May save you alot of work.


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Axles look good & everything else 'appears' okay, no chunks of metal or shavings in oil... thoughts anyone?

 

How does it feel when you turn the gears by hand? and roughness , binding or skipping when you rotate the gear? It's hard to see the side gears but if the splines look good and gear teeth look good they should be fine. How did the axle splines look? You did check the yoke first right? If the yoke stripped the gears will all turn fine, feel fine and splines will be fine. You will not be able to make it slip by hand either.

 

The splines all look good, yoke, axles & differential. With the wheels on the ground you could turn the yoke with a bare hand. it had normal backlash too. There is no perceptible noise by hand or when driving the truck. Is possibly the gears or clutches in the limited slip unit that are offering no resistance?