Hey guys, I've got a 98 F150, and I'm new to Ford 4 wheel drive systems. I know that the front diff engages when you put it in 4 wheel drive, but my question is, when you're not in 4 wheel, should the front drive shaft spin freely when parked, by this I mean can you spin the front drive shaft by hand? The reason I'm asking is because I think my front differential is locked in all the time. I've looked at the vacuum diaphragm and it's in the locked in position all the time, and when I move it, seems like nothing happens and sometimes it goes back to the locked in position on it's own when it's not in 4 wheel. I hope someone can explain this system to me a lil better. Thanks for any help.

 

with your truck on the ground your front drive shaft will not spin free, at least mine doesnt, when you put it in 4 wheel it also locks in something in the transfer case too that puts the whole thing in 4 wheel

 

is there a way to test the diaphragm to see if it's working properly, or do you think it's working properly according to what i've explained?

 

mine turns free

 

Mine and every other 97-03 F-150 I've been under (about 4-5) all turn freely when in 2wd. Sounds like your vacuum diaphragm isn't working properly.

 

toys are for kids!

 

^^^ What?

 

I know this thread is very old but another member had recently brought it up as a reference to a certain function that he wasn't sure about and the info here isn't clear on it...

So just to clear this up for others that actually use the search function

Yes, your drive shaft should spin freely by hand in 2H with your wheels on the ground if everything is functioning properly...

If it doesn't

Check the vacuum actuator on the front diff, it is most likely stuck in the locked position... Also check for vacuum leaks, it is possible that the pink line could be cracked or severed or melted to the exhaust manifold... Check the function of the vacuum solenoids on the firewall behind the battery, they should click when switching from 4wd to 2wd and vice versa...

 

Also check fuse #23 top right of the fuse panel, red 10A fuse I believe