Hey guys I'm new here but I have a 77 F250 i could use some guidance on.

Here's the story, I was driving my truck the other day and while under normal driving conditions on a straight stretch of road the truck just abruptly bucked up like it hit something. I stopped and got out of the truck expecting to find something broke and fluids draining everywhere but nothing at all looked out of place. i continued down the road and it did it again but not as hard of a jolt and now every time i take my foot off the gas there is a grinding/vibration that is very noticeable. If i'm on the gas there is no vibration, only when i let off and let the truck coast.

I'm at a loss of where to start looking to find out what's going on. Everything still works fine in or out of 4 wheel drive.

I'm assuming the problem is in the Transfer Case but before we start down that road is there anything else that should be considered?

Thank you for any imput!

 

 

I agree, something in the drive-line.

 

Or the rear differential.
But a u-joint is an easy to inspect first culprit. Usually they don't make the whole truck buck when going out, but they certainly can if given half a chance.
Any maintenance done to the truck by you at all? Had it a long time, or just a short time? Know what the PO did, if anything?

Since it's a four-wheel drive truck (is it part-time or full-time?), you can remove the rear driveshaft and drive only on the front to see if it still has the problem. And since you're taking it off to do that test, you're going to be looking for any problems with it anyway. Something like this should be visible unless it's in the differential.

Is the truck sporting a suspension lift?
How many miles on the truck?
Big tires?

Just a few things to think about.
But fix it soon...

Paul

 

The exposed portions of the drive line are easy to check for looseness and should be done first. Then try to isolate the front drive line from the rear. No one without ESP can tell you what it is without more information. But it does sound like a drive line shortfall. It should be resolved before it becomes a major problem. If it were mine, I wouldn't drive it before it was fixed.

 

Thanks guys for the replies.....I have owned the Truck for about 3 yrs, the original owner was a mechanic who worked for Ford in Michigan and he took good care of it. I bought it off of Craigslist and drove it home to Tennessee. It has a total of 6 inches of lift and was running 39" tires but I have put 37's on it for now. It is part-time 4wd with manual hubs.

My first thought was to look at the u-joints and they appear to be ok,
As i stated in the original post that it only does it when coasting, If i'm in the gas there is no problem but once I take my foot off the gas pedal it immediately starts the noise and you can tell it's something in the driveline by the way it relates to the speed of the truck.

 

It's absolutely possible for it to be u-joint related then. There may be by-the-book diagnoses of driveline issues, but in my experience there are so many different clues that a driveshaft or shafts can give you that you can't rely on previous experience to always pick a u-joint as the culprit in every case. So you simply have to mistrust u-joints (especially double-cardan style!) to the point that you simply assume that any funny noise or feeling you have "might be" a u-joint.

And FYI, this new info would have been pretty nice to have right off the bat in the first post. A 6" suspension lift and a truck running 39" tires for even a short period of it's life is subject to many failure points that might not be thought of on a stock truck. And that info is a huge part of the diagnosis trail we all have to go down.

Another thing to know, is what kind of shafts you have. I can think of at least three different designs used on 1/2 ton trucks, plus another two or three iterations of that on the heavier duty trucks. Not to mention what might have been put on by a PO to compensate for the lift.
Do you have a single long rear shaft? Double shafts with a pillow-block carrier bearing in the middle? Single u-joints at each end? Double-cardan (CV) head at one end?
Got pics?

Let us know some more. Sounds easily like it could be driveshaft related, or transfer case, or rear end still. You'd be simply amazed at how a problem in a differential can transmit itself in the form of noise all the way up the shaft, and through two gear boxes into the bell housing!

Paul

 

Oh, and do you know for sure it's originally a part-time transfer case (NP205)? Or could it be an NP203 that's been converted to part-time?
Not that it's a guarantee, but if the t-case shift lever says Hi, Hi-Lock, Low, Low-Lock, then it's a good chance it's been converted.

Reason I ask is that converted 203's often have long-term trouble looming down the road for the next owner. Chains stretch normally, but add some conversion kits and things get loose inside quick. Add 39" tires and odd shaft angles, and everything just accelerates more!

Paul

 

43Hfarmall,
I'll share what symptoms my 1977 F-150 4x4 from double cardon U-joint was making. I could hear my truck making a low but audible squealing noise when I would start off in either Reverse, first or second gear. By the time I shifted into third, I could no longer hear it probably because of wind and exhaust noise. This past January 2018 I took my truck on a ~ 65 round trip miles. When I got home, I got underneath and looked for any visual signs of leaking and then put my hands close by the rear drive shafts U-joints. I felt noticeable heat from the front U-joint which is a double cardon style. It's the U-joint directly behind the rear of my NP 205 transfer case. Upon closer inspection with a good flashlight and wearing my reading glasses, I noticed that particular U-Joint had visual signs of rust colored dried leaks on the sides of the U-Joint caps. I then jacked the truck up and placed it on four jack stands and rotated the drive shafts, front and back. Front U-joints no problem, as expected since it doesn't get torque nearly as much as the rear's. When I was rotating the rear drive shaft I could see some ever so slight slop and irregularity. Also while underneath I noticed new GO 90 leaking from the front side of the transfer case shift rails seals. This past weekend I changed the U-Joints and got the transfer case removed. Spent a couple of hours cleaning the transfer case, first using a putty knife, then wire wheel in a drill and finally gunk to wash it off. In the interim since mid-January I research parts, U-Joint, needle zerk fitting and seals for the NP 205 transfer case. Got everything from High Torque out of Billings, Montana dealt W/Roy whom I can't say enough good things about.
I suspected I had one final leak, the transfer case, all others I've eliminated since acquiring the truck in June 2016. Overall, this repair has taken longer than should of, because I had to work up the mental attitude before tearing into it, I get a headache working on a creeper for extended periods. In short, recommend getting your truck lifted on stands and rotating the drive line to try and better detect where the noise (grinding is originating from). I hope it's U-joints and not your differential or another drive line component. Another thought Thinking some more to myself, you might try and remove the differential filler plug and lower a magnet into the housing in so much as possible to see if you get any metal. If your truck is a LWB, it might be the drive shaft's mid-carrier bearing. Had a friend's mid-carrier bearing go out and the symptoms were similar to your description. Whatever you discover, please share W/us, we can benefit from some more first hand experience. Thanks, good luck and most importantly, enjoy your labor.

 

Just a quick thought on u-joints.It is VERY common to not be able to feel any play or movement out of a installed/in place u-joint. Even with pry bars,there is ,many times ,NO perceivable play or movement in the joint. The tell-tale signs tho', are looking very carefully around each cup for a rusty dust slightly emerging out of the culprit cup. If nothing to notice or feel then just the SYMPTOMS of vibration,clicking intermittingly or all the time, will tell you to take the u-joints apart. It is then a slight surprise to find a u-joint with a cup with no needles/bearings at all,the u-joint stem 1/2 worn out or a bone dry,rusty cup&needles.That is where ,MANY times it will make you kinda' scratch your head and wonder why it showed no play/movement when checking.

 

Definitely! By the time the rusty grunge is creeping out, the joint has been gone for at least awhile.
Kind of on that same theory, I have some modern cars with unit-bearings, or "hub bearings" for wheel bearings. No more adjustments or maintenance. They're just good, or they're not.

But when I bought the car, three of the four were completely shot and the PO didn't know it. When driving it had the distinct whaaa whaaa whaaa sound of snow tires with some wear on them. Know that sound? I've heard it many times over the years.
But the tires looked really good and almost new, so I didn't want to just throw them away immediately. On a hunch then I changed the bearing in the corner that made the most noise. Got rid of 80% of the noise in one fell swoop!
Looking at it when I removed it though, it was leaking grease. And though it had no perceptible play, you could tell it was shot when you rotated it by hand.
Did the second one and got rid of another bit of the noise. And it had some leakage as well as excess play.
The third one got rid of the last of the noise, but had zero external symptoms. No mung, splooge or muck. And zero excess play.
Replaced the fourth one just because of the other three!

Just goes to show, like you were saying, you can't always tell by excessive movement.
Oh, and now I have another car that's just starting to make that whaaa whaaa sound, along with a little more "road noise" that I'm guessing is wheel bearings.
Guess I know what I'll be doing with some of that "spare time" I have on my hands!

Paul

 

Hey guys......well due to family/children priorities my truck has sat for a while and i'm just now getting back to the original problem. We have diagnosed the problem and the rear end is gone out of it. The mechanic i'm working with said it was broke when i bought it as parts were missing that could only have been removed from taking the diff cover plate off. He told me there was a pin holding things together and that's what finally broke and resulted in the problem i'm having.

He gave me 2 options...he can rebuild the rear end that is in the truck now (most expensive option) or he can get a newer rear end out of a 2000 model Super-Duty with 4:10 gears for 1/2 the price of the rebuild.

I am not a mechanic at all and am learning this stuff as I go along so any advice is appreciated!

Thanks!

 

That's an option that many have done for the same reason. Or some just because the new stuff is real beef and they might be replacing a lesser axle with it, or simply wanted the disc brakes from it.

But the big downside seems to me to be the wheel bolt pattern. The wheels are 8 on 165mm (or some such metric number, maybe 180?) whereas our old ones are 8 on 6.5 bolt circles.
I don't know if there are easy and cheap kits that allow you to run the old pattern, or if you re-drill the axle shafts for the old pattern, or just get new wheels for the rear only. Saving the money gives you options, but most of them add up to getting close to what you would have spent to rebuild the old one.

I'm sure others have run down this hole so have more insight as to whether it was worth it or not. But basically it's a good axle. It just has a different wheel bolt pattern.

Paul

 

Thanks for the feedback, i do appreciate it!

Is there any market for the factory rear end I'm replacing or just send it on to the scrap yard?

 

If you're not in a hurry to get rid of it someone will buy it eventually. Toss it on Craigslist and see what bites