99 F150 XLT 4x4 SCab Short Box
5.4 Triton w/ 4R70W & BW4406 TC, 3.55 NON LS Rear
150,000 miles

Problem:
When I let off the gas while going about 60 mph, I get a heavy "thud". It repeats that thud when I lightly get back on the gas pedal. It happens at mostly highway speeds. It's slowly getting worse. I've tried several fixes, but to no avail.

Fixes Tried:
Changed tranny fluid and filter. {NO CHANGE}

Pulled driveshaft, inspected slip yoke (no obvious problems). greased the crap out of it with synthetic grease and reinstalled it. {SLIGHT CHANGE, BUT ONLY TEMPORARY}

Checked U-joints. Installed new Front U-joint. Rebalanced Drive Shaft. Lubed the Slip Yoke with Moly Lithium Grease. {NO CHANGE}

It's shifting good. It's running great. I just feel slack in the drive line somewhere. I cant find it. I need help.

My company mechanic thinks that it may be the "Rear Output Shaft Bushing" in the Tail Housing of the Transfer Case. (I drained and refilled TC Fluid. it was pretty and clean like brand new) This bushing is brass I think, and it goes around the slip yoke inside the TC tail housing.

I think it could possibly be the Transmission Mount. How can I tell if that bad short of removing it?

Thanks guys.

 

How's the rear u-joint? That's the one that usually goes first.

Pull the front driveshaft and see if that makes a difference. You gotta nail down where it's coming from a little better.

 

I thought that the front drive shaft didn't spin unless it's in 4wd. Am I wrong?

I had the drive shaft balanced because my company mech (he's seems fairly competant at working on cranes and rebuilding diesels, but this shook my confidence) pulled it and removed the front U-joint and played around with the slip yoke. While getting the ujoint out, he broke the cast iron damper ring off the slip yoke. Then it started vibrating at about 60mph. WHen I had my wife take it to the only place in frikken Birmingham, AL that balances drive shafts, they said the front U-joint was shot. They replaced it for $15 and balanced the shaft for $40. When I got it back, it still kinda vibrates, but not as bad, and seems to only do it in deceleration between 70 and 60mph. I assumed that they didn't replace the rear U-joint cause it didn't need it. I havent seen any slack in it.

I feel like the clunk is coming from between the transfer case and the rear tires. When I'm in 4wd, the sound is gone. It only happens in 2wd.
Tons of fresh grease in the slip yoke that slides into the transfer case seems to help, but not totally eliminate the noise for about 10 miles.
It's starting to clunk now when I take off from a dead stop. Not every time, but fairly often. When I say dead stop, I mean a red light, where I never take it out of drive.

 

My guess would be check inside the rear end. Whether its the gears themselves or a bearing, it would make sense that it would go away when in 4wd, because the front wheels are providing force as well so the rear end isn't getting all the torque. Hope that makes sense

 

Well it sounds like it could be in the diff w/the fact that it is between the tcase and rear wheels that is the only place you haven't checked. To start I would jack up the rear end and spine the tires and see if their is any play in them w/shacking them down and spining them.

 

If you didn't use a teflon grease at the slip yoke, you did a lot of work for nothing. It MUST have teflon grease or you'll get the thump like you describe. you'll need to clean up all of the mating parts and regrease with the right grease.

 

Well, I believe I found my culprit. I removed the rear diff cover, jacked up the rear end, and started spinning the tires and the drive shaft by hand. I didn't notice the problem till I started turning the ring gear with a pry bar. Every time I torque the ring gear with the pry bar, I see a gap appear between the "Pinion Shaft" and the "Differential Case". It is accompanied by the elusive popping noise I've been hearing. Any change in direction of the spin or the amount of torque applied, I get my Pop sound and the gap appears.

For reference, if you have a haynes manual for 97 - 03 ford pickups, go to chapter 8 -16and look at picture 16.4a and 16.4b.

Does this sound plausible? Is the pinion shaft supposed to be tight?

 

Yup sounds about right.

 

my pinion nut would loosen constantly...i finally loctited it and torqued it down super tight...that was 30K ago...it was being caused by a bad rear u-joint...

 

you need to reshim the diff over to the right to tighten up the clearance issue you are talking about

 

How do I reshim it? I'm new to rear diff issues. I got a company mech, but i'm waiting for him to get the time to look at it.

If it needs reshiming, what would cause that?