Hello!

I have my '76 F100, 4speed manual, 360, 4WD (with no front driveline) here at work today and I had an interesting trip in this morning.

There are a whole plethora of issues going on right now so I'll only include those symptoms that seem related to this.

I have a knock/rattle happening at certain times and conditions. I can't nail down where the source seems to be. It feels and sounds like it is either coming right under the center of the seat or from up near the right front wheel. It is easy to replicate. It is fine on flat ground or going downhill. No problems when I accelerate at normal pace.

Problems start up whenever I hit the slightest incline. I hear a knock/rattle as though I'm in way too high of a gear. I also lose power pretty quickly. As soon as I take my foot off the gas the knock/rattle stops. I do hear it slightly in 3rd but 4th gear is where it is really bad. This morning I was cruising in just fine until I hit a low-grade hill that goes on for almost a mile. I was not 50 yards up the incline when I felt it starting. The more gas I gave it the worse the knock/rattle. When I let off the gas it immediately went away. I'd hit the gas and it was back. It didn't take long until my momentum was scrubbed. I shifted down to 3rd and it went away but I was red lining the RPMS, back into 4th and it was knocking/rattling like holy hell. I ended up just moving right, dropping to 3rd, and putzing up the hill at about 30 MPH. As soon as the road leveled out I got speed up, back into 4th, a bit of knock but speed climbed, and the knock went away.

Off and on I have heard a "metallic "clang!" from the trans/driveshaft area when releasing the clutch in 1st gear.

From what little I know about this and the research I have managed to do today it sounds like the U Joint is out of grease or just plain bad. I haven't done any work to the rear driveline in the year I've had the truck but I did just have the motor out and did some shifting of the trans which may have affected the U-joint.

I don't have any issues with the wheels as far as I can tell. I replaced the front bearings and ball joints a year ago.

I am a bit concerned as I drove in today because I have to go help a friend move after work. I live in Seattle and he is in the hilliest part of town there is. Yikes.

Any recommendations for diagnosing?

 

I'd start with a visual inspection of the motor mounts and u-joints. See if there are any loose/missing bolts, broken mounts, etc. Sometimes it's apparent and other times you'll have to pry on things with a bar to see if they're loose. Look at the u-joints and see if they're sloppy or if you see any metal filings, excessive grease leaking out or wear on the yokes.

Sometimes it helps to have someone in the truck applying power while you're looking underneath and under the hood while going from forward to reverse while the e-brake is applied to see if there is anything moving around excessively. Needless to say be very careful doing this and keep yourself out of harm's way so you aren't getting hurt!

 

Have you done any rear spring work lately?