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77 F150 Clunk

Hi Everyone!
I have been fixing up my 77 F150 since I got it in January and it’s really starting to look nice! I love this truck, but I have noticed a clunk sound when I put it in reverse and also a same if not similar sound when I’m driving and slow down. It makes a clunk sound when it shifts to a lower gear, (automatic). Now, I was thinking it was a U joint at some point and crawled under there and started to grab them and see if there was any play in any of the spots but found no play. I checked it a few times, thinking well it could be just the point the U joint stopped and that would explain no play but after a few times of crawling under I still never found any.

Any ideas?

Thanks!

If the U-joints have no play then it’s probably the differential. Just remove the driveshaft and rock the differential pinion back and forth. If it clunks…it needs repair.

I’ve seen it on a different vehicle where the brake caliper bracket mounting hole had worn into an elongated hole and would slide around when going from forward to reverse. You’d hear this weird ‘clunk’ when the mounting hardware slid into the other end of that hole. Your truck no doubt has drums but a similar thing could conceivably happen to the backing plate, etc.
It would be an easy thing to check and someplace to start.

Does the truck have a 2 piece drive shaft? If so the slip joint at the hanger bearing or the hanger bearing can make the ‘clunk.’

Sometimes a worn U-joint can’t be detected by just grabbing the driveshaft and trying to move it. Try raising both rear tires off the ground and place the transmission in neutral. Rotate one of the rear tires back and forth while observing the U-joints. If there’s a worn U-joint this method will reveal it.

Tester

Have you checked your motor mounts?
Did you check for axial play in the driveshaft?

In addition to the motor mounts, there is a transmission mount that should be checked. (at least I think there is on your vehicle) If this is worn or the rubber is nearly gone from it, the transmission can bob up and down when it shifts or you go from forward to reverse. The transmission may be knocking against the frame if this mount is bad.

Thanks everyone for the thoughts! I haven’t bothered to get it fixed, because now I have an even more serious problem with it. I’ll be starting a new post about that shortly. After looking at all the ideas and getting back under the truck again, I think it is the defferential. I even see it leaking a little.

Thanks for all the help.

One more thought; I’ve read where these driveshafts can bottom out on the transmission tailshaft under certain conditions. Get the truck safely elevated and try wiggling the driveshaft both laterally (sideways) and axially (along its axis).