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Discussion Starter · #1 ·
Lately my 1988 ford Bronco, with a 5.8L engine and c6 auto transmission, has been making a loud thunking noise while driving. I can feel each thunk underfoot in the driver and passenger seat. This usually occurs while coasting but occasionally also while accelerating. I do not have much experience with transmissions or u joints on drive shafts but those are the places I would venture to guess the problem is. Any advice on where to start diagnosing the problem would be appreciated.
 

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Discussion Starter · #3 ·
Is it a single noise or is it a repetitive sound?
It is usually a repetitive noise (as in when the sound occurs it will thunk continuously as i coast or sometimes as I accelerate or maintain speed) sometimes louder than other times. Occasionally shifting into neutral then back into drive while make it stop for a while.
 

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Check your motor mounts. Open the hood, foot on brake, press accelerator slowly and see if engine rises up. Then listen to hear if you hear the same "clunk" when you suddenly take your foot off the accelerator. (KEEP YOUR FOOT FIRMLY ON THE BRAKE!) If the engine moves a lot, the mounts are shot. Next you'll have to crawl under and with the engine off, pry the engine with a decent prybar and look to see if you can see any cracks in the mount.
While you're under there, look at the transmission mount, and the radius arm bushings, all of those can cause the clunk you hear.
 

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Discussion Starter · #5 ·
Check your motor mounts. Open the hood, foot on brake, press accelerator slowly and see if engine rises up. Then listen to hear if you hear the same "clunk" when you suddenly take your foot off the accelerator. (KEEP YOUR FOOT FIRMLY ON THE BRAKE!) If the engine moves a lot, the mounts are shot. Next you'll have to crawl under and with the engine off, pry the engine with a decent prybar and look to see if you can see any cracks in the mount.
While you're under there, look at the transmission mount, and the radius arm bushings, all of those can cause the clunk you hear.
Should I test that while in neutral?
 

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No, you test that in drive, so you load the motor, which will cause it to torque, putting a load on the motor mounts. That's why the admonishment to keep your foot on the brake.
 

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Discussion Starter · #7 ·
No, you test that in drive, so you load the motor, which will cause it to torque, putting a load on the motor mounts. That's why the admonishment to keep your foot on the brake.
I see. I will test and get back to you. I forgot to include earlier. Out of curiosity I loaded the back of bronco with 800lb of concrete and the rattle did not happen. Could have just been a coincidence but it is rare it does not rattle atleast once or twice between point a and b.
 

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The Tennessee Warden
96 XL, built 357W, E4OD, BW1356, 4.56 gears
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I see. I will test and get back to you. I forgot to include earlier. Out of curiosity I loaded the back of bronco with 800lb of concrete and the rattle did not happen. Could have just been a coincidence but it is rare it does not rattle atleast once or twice between point a and b.
Hmm - could be shot leaf spring bushings.
 

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Discussion Starter · #9 ·
No, you test that in drive, so you load the motor, which will cause it to torque, putting a load on the motor mounts. That's why the admonishment to keep your foot on the brake.
I tested out the engine mount theory today. It did not make any rattling while foot on brake and in drive. I revved up to 2500 and let off quickly and nothing happened out of the ordinary. Any other ideas or things I could try?
 

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Does it thunk when braking as well? I had a issue where I would get a thunk when accelerating, and a thunk when coming to a stop. I found the issue to be my slip yoke, though I never had it thunk when simply driving down the road repeatedly.
Does not mention it in the thread, but you need a 12mm 12-point socket to remove the drive shaft bolts. I picked one up at the depot of homes.

 

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Discussion Starter · #11 ·
Does it thunk when braking as well? I had a issue where I would get a thunk when accelerating, and a thunk when coming to a stop. I found the issue to be my slip yoke, though I never had it thunk when simply driving down the road repeatedly.
Does not mention it in the thread, but you need a 12mm 12-point socket to remove the drive shaft bolts. I picked one up at the depot of homes.

It thunks when braking, coasting, or even sometimes when I am accelerating it has started to do it more when I am accelerating.
I think I might try what was in the thread you attached just to be safe.
 

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I’m going to go out on a limb here... it thinks when braking, thunks when accelerating, and thunks when coasting.... rear wheel bearings or bad u-joints... check for play in your rear drive shaft by pushing and pulling on it... jack up your rear axle (get both tires off the ground) then try to lift the tire up by hand... any movement and the wheel bearings are toast... try spinning the tires while listening... no noise should be heard... while spinning the tires also feel for excessive resistance.
 

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Discussion Starter · #13 ·
I’m going to go out on a limb here... it thinks when braking, thunks when accelerating, and thunks when coasting.... rear wheel bearings or bad u-joints... check for play in your rear drive shaft by pushing and pulling on it... jack up your rear axle (get both tires off the ground) then try to lift the tire up by hand... any movement and the wheel bearings are toast... try spinning the tires while listening... no noise should be heard... while spinning the tires also feel for excessive resistance.
I will test both ways and get back to you. I’ve been toying with the idea of changing the ujoints but was put off by the expensive kits offered online. Have you done a unjoint change before and if so how does one go about finding the right ones and where is good to buy them from?
 

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I will test both ways and get back to you. I’ve been toying with the idea of changing the ujoints but was put off by the expensive kits offered online. Have you done a unjoint change before and if so how does one go about finding the right ones and where is good to buy them from?
I have done them in the past, but with the price of a new drive shaft shaft, I don’t find it worth my time. By the time you buy all the u-joints and the the repair kits, you are almost at the price of a drive shaft from a reputable company.
 

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Discussion Starter · #15 ·
I have done them in the past, but with the price of a new drive shaft shaft, I don’t find it worth my time. By the time you buy all the u-joints and the the repair kits, you are almost at the price of a drive shaft from a reputable company.
Good to know.
 

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The Tennessee Warden
96 XL, built 357W, E4OD, BW1356, 4.56 gears
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It cost me $100 and about two hours in the garage to rebuild my driveshaft with this kit from JBG.

I paid 150 for a new driveshaft from NAPA... but then again I also have a business account... so not sure what I would have paid as a walk in customer...
 

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The Tennessee Warden
96 XL, built 357W, E4OD, BW1356, 4.56 gears
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I paid 150 for a new driveshaft from NAPA... but then again I also have a business account... so not sure what I would have paid as a walk in customer...
This is what I find on a quick search.
 

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If it is not a Bronco, it's just not worth driving.....
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More food for thought......

When I got my Bronco 6 years ago it had a noticeable clunk or shift in the driveline that I could feel when accelerating from a stop. I did not notice it at any other time. To me it felt like the rear axle was loose and when I put torque on the drive line the axle housing would start moving forward in relation to the leaf spring. It felt like it moved about an inch or so. I searched under the truck far and wide for the cause of this sensation. I had no luck in finding it.

Fast forward...

Three years ago when I had my transmission rebuilt the transmission shop told me my bearings coming in and out of my transfer case were gone. They said the pinion gears were slapping the bearing races where they entered or exited the transfer case housing. They told me I needed to have the transfer case rebuilt.
I kinda suspected they were trying to make more money off of me but the shop had a real good reputation so I gave the go ahead for the transfer case rebuild and I am glad I did. The sensation I was feeling coming off of a stop totally disappeared. What I was feeling was the pinion gears shifting in the transfer case bearing races when I would apply torque to the drive line. They were just bouncing around in the housing.

This could be a similar situation for you as well. I don't know of a way to test the output shafts for the transfer case without separating the transfer case from the transmission, but maybe one of the gurus on the forum may be able to shed a little light on the testing process of the transfer case bearings....

To rebuild my transfer case only cost me $900. I have a lot more confidence in my totally new driveline now. :)
 

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Discussion Starter · #20 ·
More food for thought......

When I got my Bronco 6 years ago it had a noticeable clunk or shift in the driveline that I could feel when accelerating from a stop. I did not notice it at any other time. To me it felt like the rear axle was loose and when I put torque on the drive line the axle housing would start moving forward in relation to the leaf spring. It felt like it moved about an inch or so. I searched under the truck far and wide for the cause of this sensation. I had no luck in finding it.

Fast forward...

Three years ago when I had my transmission rebuilt the transmission shop told me my bearings coming in and out of my transfer case were gone. They said the pinion gears were slapping the bearing races where they entered or exited the transfer case housing. They told me I needed to have the transfer case rebuilt.
I kinda suspected they were trying to make more money off of me but the shop had a real good reputation so I gave the go ahead for the transfer case rebuild and I am glad I did. The sensation I was feeling coming off of a stop totally disappeared. What I was feeling was the pinion gears shifting in the transfer case bearing races when I would apply torque to the drive line. They were just bouncing around in the housing.

This could be a similar situation for you as well. I don't know of a way to test the output shafts for the transfer case without separating the transfer case from the transmission, but maybe one of the gurus on the forum may be able to shed a little light on the testing process of the transfer case bearings....

To rebuild my transfer case only cost me $900. I have a lot more confidence in my totally new driveline now. :)
I think I may try some cheaper options first but I will most certainly keep that in mind. I truly hope my issue is not that bad.
 
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