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Technical discussions specific to 1964-1967, 1968-1970, and 1971-1973 Classic Mustang. Discuss all tech related to in-line six cylinder and V8 powered Vintage Mustangs here.

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Unread 12-07-2010   #1 (permalink)
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Default Clunk sound when I accelerate

Hello my fellow Mustang enthusiasts. I have owned my fairly stock 66 convertible with a 289 for about 2 years. Ever since I have owned the car I have noticed that the car makes like a "clunk" sound when I accelerate after coasting. So if I am driving on the road and let off on the gas or brake and then accelerate again it makes a "clunk" sound when it engages. I am wondering if this is more of a rear end issue or maybe just the U joints. Your advice is always appreciated.
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Unread 12-07-2010   #2 (permalink)
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more then likely it's excessive backlash in the ring and pinion.
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Unread 12-07-2010   #3 (permalink)
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I had that problem with my 67 mustang and thought the same thing that you did. I went in and had the U joints replaced and they said the problem should stop but it still made the noise. So i started looking under the car and saw that my newly put in power brakes hard lines where in the way of the back U joints and it hit every time the drive shaft would spin.
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Unread 12-07-2010   #4 (permalink)
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I would put my money on bad u-joints. IMO, excessive backlash in the ring and pinion would result in a howling noise as the gear teeth are not meshing properly.

You might also check the transmission mount on the cross member. I have seen bad ones (cracked) create the same "clunking" noise.
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Unread 12-07-2010   #5 (permalink)
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Quote:
Originally Posted by rj8806 View Post
I would put my money on bad u-joints. IMO, excessive backlash in the ring and pinion would result in a howling noise as the gear teeth are not meshing properly.

You might also check the transmission mount on the cross member. I have seen bad ones (cracked) create the same "clunking" noise.
+1

Check both of these items first which are prone to failure with a high mileage vintage vehicle. Luckily they are both pretty easy and relatively inexpensive items to replace...

Might want to take a look at the motor mounts and suspension components too...
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Unread 12-07-2010   #6 (permalink)
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I guess I should expand on why I think its a bad u-joint or possibly the tranny mount.....

When your foot is on the gas, you have "load" going through the drivetrain (which includes the transmission, driveshaft and rear end). When you let off the gas and/or brake the "load" disappears and the drivetrain is basically free wheeling/spinning. Your driveline angle also changes in this condition. When you get back on the gas, the driveline angle changes again, load is transmitted through the drivetrain and a bad u-joint will make a clunking noise during this transition.

A bad/cracked transmission mount acts much like what I described above in that it will move up/down with the "load" being applied and taken away.

Look for signs of grease leaking from around the u-joints caps. Check to make sure there is no play in the u-joints. In other words, grab the driveshaft near the u-joints (both front and then rear) and make sure it does not move up/down or side/side. If that checks out, I would remove the driveshaft from the rear end pinion yoke and make sure the rear u-joint is not binding and moves freely but smoothly back and forth. You can lift up and down on the rear of the driveshaft to check the front u-joints, this way, you don't have to remove it from the transmission tail housing and risk dumping fluid all over the ground. If you decide to remove the whole driveshaft, I found that a 20oz plastic bottle of soda (empty of course, your choice of flavor ) makes a nice tail housing plug. Simply cut the bottle in half, stuff a few rags into the bottom half, slide a hose clamp over it and then slide the bottle over the tail housing and tighten up the clamp to hold it in place. This will catch the fluid. If you prefer, skip the rags in the bottle part and that will let you know how much fluid you actually lost during this and you will then know how much to replace once you have finished.

As for the mount, that should be obvious....make sure it is not cracked and dry-rotted. If it looks good, try putting a floor jack under the tail housing and GENTLY lift with the jack. The mount should not have any up/down play in it at all.

I'm not always right, but this is where I would start looking.....
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Unread 12-08-2010   #7 (permalink)
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Thanks for the responses. I guess I will start by inspecting the U Joints and then move on to the ring and pinion of the U joints seem fine.

Richard, my car has a T5 conversion in it and one of the slave cylinder mounts on the tranny broke off. I have to drill a new whole in the case and monkey rigged a new bracket that has been holding so far. I was wondering if I could weld on a new bracket to the case or do I need to basically get a new tranny ?

Thanks
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Unread 12-08-2010   #8 (permalink)
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Quote:
Originally Posted by Topdown66 View Post
Thanks for the responses. I guess I will start by inspecting the U Joints and then move on to the ring and pinion of the U joints seem fine.

Richard, my car has a T5 conversion in it and one of the slave cylinder mounts on the tranny broke off. I have to drill a new whole in the case and monkey rigged a new bracket that has been holding so far. I was wondering if I could weld on a new bracket to the case or do I need to basically get a new tranny ?

Thanks

Are you running an internal or external slave cylinder? We deal with internal ones that just slide right over the bearing retainer and there is no drilling or anything required. I'm a little confused as to what you are describing...can you post or e-mail me a picture of what you are seeing?

Being that the case is aluminum, I think welding would be next to impossible but I don't claim to be a welder...:nogrinner
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Unread 12-08-2010   #9 (permalink)
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i cant say anything that hasnt already been said, it seems to be your u joints, i have the same problem but not the time to fix it.
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Unread 12-10-2010   #10 (permalink)
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Quote:
Originally Posted by rj8806 View Post
Are you running an internal or external slave cylinder? We deal with internal ones that just slide right over the bearing retainer and there is no drilling or anything required. I'm a little confused as to what you are describing...can you post or e-mail me a picture of what you are seeing?

Being that the case is aluminum, I think welding would be next to impossible but I don't claim to be a welder...:nogrinner
Richard,

Sorry...I should have been more specific. I have a CNC hydraulic clutch set-up. The slave bolts onto the transmission housing but I snapped of the mounting bracket on the tranny case. I made a new hole in the case and rigged up a new mount that is working but is no the best set-up. Ya...I don't know about welding the piece that snapped back on either. That might be difficult. What about just getting a new case ?
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Unread 02-18-2011   #11 (permalink)
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I had this same problem in my '73. When I would left off the gas and idle down the road and then step on the gas it would clunk. also, when at a stop in drive, I put it in reverse it would clunk and then again when I put it in drive.

I took it to the shop and they had no idea. I got the response of just excessive play in the system. I thought that was BS.

So, I was going down the road a couple of days later and I started hearing a crushed grinding glass sound. I went straight to autozone and changed both u-joints in the parking lot.

The needle bearings were nearly, as in 75%, gone!!! I was amazed it was working at all.

Since the change, I've had no more clunks.

My advice, change both of them. It's easy and cheaper than one breaking and you ruining your drive shaft.
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Unread 02-18-2011   #12 (permalink)
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For your broken slave mount, I assume you are talking about the aluminum bellhousing where the bracket bolts to? If that is the case, either you could have a welder fix it, or you could get a new bellhousing. They can be purchased for as little as $35 and can cost as much as you want to spend. A welder might charge around $50 +/- to fix yours. If you have a picture that you could post, I might be able to give a more specific answer.
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