My problem is when the truck is in 4H or 4L. Manual hubs locked in the truck will pop out of 4WD at a low speed. With the truck raised and locked in, one wheel turns one way while the other wheel turns the other way. That tells me the hubs are locked in. So as I begin to drive there's this loud clunking noise from underneath and as I continue to drive it clunks again. And if I'm in mud it'll the 4x4 doesn't work. But if I put the truck in reverse and then back into forward gear it'll grab like it's locked in but it will soon clunk again and the 4x4 doesn't work. Anyone have any suggestions on what this might be?

 

Forgot to mention that truck is a 1975 F250 Hiboy 4x4 w/390 4 speed. NP205 T-Case

 

What do you mean by pops out of 4wd, meaning the shifter literally shifts out or something else? Does this happen when the hubs are locked and your in 2H? Sounds like a problem with the transfer case, axle seems fine because it would do it in 2wd too if it was the axle and the hubs locked.

 

Could be a broken ring or pinion gear, broken gear in the t-case or just popping out of gear in the t-case. I would get it on a sand hill or somewhere you can easily spin it out and have a friend look under it while you put it in 4x4 and try to go. Find out if the front driveshaft is turning or if it stops when the clunk is heard. If it stops then it's a t-case problem. If the shaft keeps turning and the front tires are not then it's a front diff/axle/hub problem and we can isolate it further from there.

If we decide that the fronf axle is the problem (the front shaft is turning) then you need to look and see if either front axle shaft is turning at the steering knuckle (axle u-joint). I hope those have open knuckle front ends but I think they do. If the axles are both stopped and the driveline is turning then the diff's broke. If either axle shaft is turning then the diff's probably ok since only one axle should turn since you have an open diff (as evidenced by the two tires turning opposite directions when you had it jacked up). If the axle is turning and the tire is not then you probably have a bad hub. This would be the best news as the hubs are relatively cheap and easy.

If the front driveline is not turning when in 4x4 (after the clunk) then there's a problem with the t-case. Hopefully it's just popping out of gear but since it does it in 4lo and 4hi then that's unlikely.

If this doesn't make any sense I'll try to clarify some. Just ask.

 

Hello Rando75:

Did you figure out what the problem is? My truck does the same thing. 81' f-150 302, NP208

Thanks,
Keith

 

I have had 2 NP-205 and both of mind would and will have loud clunk and disengage the the back wheels in low range. the front wheels will still be pulling. If push it hard into low range this does not happen. Is this your problem or did you find it.

 

My truck was making a clunking noise when in 4 wheel drive also. I investigated and found the right locking hub (Warn) was broken and allowing that wheel to disengage from the axel. I relpaced both hubs and the clunking is now gone.


81' f-150, 302 c6