Ok, I have done a search on the 80-86 forums, and I only found one post that was close to my problem.

I have an 86 f-150 4wd xlt. My problem is a clunk. I haven't done that much work on it yet research wise. The symptoms are a loud clunk under three conditions, that all seem to come from the sam spot (it sounds like my rear diff). The three conditions are

1) when I first put the thing into reverse, it takes a second, then you can feel the transmission engage and there is a loud clunk

2) if I let off the gas suddenly when driving (only at moderate speeds. I have never gotten up past 65). I 'think' i only hear it in second gear, but I am not sure

3) if I push the gas too quickly when accelerating from a stop.

I got the truck used, and as far as I know there has never been any sort of work/maintenance done on anything in the rear drive/suspension.

So, if you have any suggestions as to what I should look for to start checking around, I would really appreciate it.

I had thought maybe I am low on diffy fluid, or that something (like a band?) was loose on my tranny. I am just not too sure where to start digging around.

Thanks for the help everyone.

 

Sounds like too much backlash in the ring gear and pinion.

 

too much backlash? So, how is that fixed? Is that one I need to take to somebody, or is there adjustments/tweaking I can do in my driveway?

 

Backlash can be adjusted but you have to go inside the rear to do it. Unless you have experience with rear ends let someone else you trust do it. You need to check the bearings to make sure they are good. Whether you can get by with just adjusting the ring gear closer to the pinion of have to replace the crush spacer and shims on the pinion depends on what the pattern looks like on the ring gear and pinion.

Several other things to check before deciding if it is the rear end - Motor mounts, transmission mounts and u-joints. Especially the transmission mount. A broken tranny mount will allow the transmission to bounce up and down causing a bang.

 

Check the u-joints first. Cheaper fix. More likelly the cause.
Unless the rearend has been severely abused, it is not likely the rearend needs adjusting.

 

I agree with checking the u-joints along with the mounts. It is always better to check the easy things first. As for the backlass in the rear, high mileage wear will also cause excessive backlash. It is fairly easy to check also although not cheap to fix. Place trans in neutral and turn drive shaft back and forth to see how much play there is. Can be done while checking the u-joints and trans mount.

 

its slop in your rearend.my 81 is the same.common on trucks with miles.the 9 inch seemed to be bad for this.

 

From what you've described, I agree that you should check the u-joints first. It's the most common problem in a truck that old, and if nobody did any maintenance, it's a sure fire bet that they are original.

 

I ahd the clunk in my '84. A new U joint fixed it,

ChipperB

 

Replace the u joints is easy, just remove the driveshaft, and remove the clips at each end of the u-joint(s) get two one for the front an one for the rear, If this doesn't fix the clunk then its possible that the rearend needs some parts, but usually unless its been out in the woods and lots of road wear, it'll be the u-joints on the driveshaft. Some u-joints come already sealed and doesn't require lube in them, whale others has a fitting that does require lube, its your choice to determine which to buy.....

 

ok, I checked, and the ujoints were replaced about 2 years ago (talked to the previous owner). Also, when I got underneath and checked them, they are all solid. I pushed up-down and back forth and nothing.

I wanted to change out the diffy fluid and filter (it's been 126K miles since anyone touched that) but the thing is sealed. I am going to try putting it in neutral and check that.

My tranny mount looks a little strange. anyone know a good place for me to look to find a pic of what it should look like?

 

Most likely the noise is just normal wear. By the time you add up the slop you get in the tranny, transfer case, u-joints, slip joints, cv joints in some cases and rear differential the more miles you have on the rig, the more wear and the more movement. Add em all up and it can be a significant thump when you put it into gear, from reverse to forward or anytime you go from torque on the drivetrain to no torque and back again.

 

Filter on the differential???
No such thing.

Just get a suction gun to suck the old oil out.
Buy 90W gear oil and a pump to install the new lube. Synthetic costs a bit more but not a bad idea.

Mechanic informed me that gear lube has anti-foaming agents. Prolonged service life causes these agents to breakdown. Causes air to become entrained in the oil when the vehicle is moving, reducing the lubricating properties of the gear lube. I believe the average service time for gear oil is about 25K miles.