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Uncomfortable Annoying Clunk Noise

 Can anyone help?  I have the mystery question of the century.  I have a '74 Dodge B-100 van, 318 V-8, with a 3-speed A727 TorqueFlite tranmission (which was totally rebuilt about 20,000 miles ago).  Overall, this old baby that we call "Blue Thunder" is in almost pristine condition.  It starts well, runs great, and over the years has been highly reliable.  It's been in the family for almost 38 years and is special to all of us.  I've maintained it extremely well from day one.  

 Here is the big problem.....under very light acceleration at the 2-3 upshift, it makes a loud-sounding and aggressive-feeling "clunk" or heavy sounding "thump".  Under hard acceleration, or accererating on an upgrade (hill), it does not make this sound at all and I do not feel or hear it at all.  There are no other accompanying symptoms such as whines, roars or vibrations.  I recently took it to a reputable (in business for 35 years) transmission  shop recently with the above problem description.  He checked the fluid and adjusted the kickdown band.  This same shop had serviced the transmission a year earlier. He recommended that I let him him take it to a general auto repair shop down down the street for a diagnosis on the "loud clunk" he did not think it was a transmission problem. 

  I agreed for him to do so since, the shop "down the street" is also known for quality repair work.  The written problem description was sent aong with the van.  In a week or so, the auto repair shop called and said all they could find was a little wear in the spider gears in the rear differential.  I gave them permission, since the differential was already open. to replace anything that looked like it needed to be replaced.  It took a while to get the parts, but they replaced the spider gears, both side bearing cups, and 1 of the differential bearing adjusters.  Of course the work included new gear oil and gasket for the differential cover.  It was an enorous bill, but I didn't complain, thinking that the terrible "clunk" would be gone.  I picked up the van and immediately noticed no difference at all.  The loud "clunk" was still there under the exact same conditions.  

 I took it back and asked the owner to ride with me where upon I easily recreated the "clunk" by accelerating very lightly.  He said, "Oh, that's in the transmission, because it's only makes the sound when it shifts from 2nd to 3rd gear".  I called the transmission shop back and they want nothing to do with it....saying that a noise like that one is practically impossible to pinpoint.  He said even if the transmission were rebuilt, he cannot guarantee the noise would not be there.  Both shops say the U-joints, yokes, engine mounts, and transmission mount all look OK.  So, I'm heart broken, disappointed, sad, and don't know where to turn next.  I was hoping that someone on the forum might have some thoughts as to what the problem could be based on work experience or whatever.

This is unreadable. It makes my eyes hurt.

Ever heard of paragraphs?

That’s better. I think you’re going to have to keep looking until you find a mechanic or a transmission shop that is willing to help you.

I have to ask, how much are you willing to spend on this '74 Van?

If the transmission was rebuilt 20K miles ago it should be perfect. Was there a warranty on the rebuilt transmission?

Can you describe where the clunk is coming from and do you feel it as well as hear it? Assuming the transmission, drive shaft and differential are all OK I would reconsider the engine and transmission mounts. If the clunk is coming from the back you might have the mechanic check the leaf springs and shackles.

For me, the van is historically special. I’ve had it since early 1974. Richard Nixon was President then, and now, it has an “antique auto” tag. Yes, the tranny was rebuilt 20K miles ago (in “years” however, that was 1989). Shortly after the rebuild, the van sat for several years and was only driven on occassion. “Sitting” and being inactive for several years was probably not good for it. After I retired, I decided to restore it. All has gone well with the restoration until this “clunk” issue.

Both the transmission shop and the mechanic say the u-joints, yokes, transmission mounts and engine mounts are OK. I don’t know about the springs and shackles, no one has mentioned it, but the sound only happens at the 2-3 shift only, and when “lightly” accelerating. Yes, I can hear it and feel it. The sound is a “clunk” or sharp loud “thud”. Concurrent with the sound, I feel a quick stiff “jolt”. If I manually keep the transmission in 2nd and don’t let it shift to 3rd, I never…ever…hear the clunk or feel any jolt at all…under any condition.

Even though all other functions of the transmission are perfect, the mechanic thinks its in the 2-3 shift apparatus. The transmission shop won’t touch it with a 10 foot pole. Quite frankly, I don’t think he (the transmission guy) knows much about the older Mopar TorqueFlites that are fully hydraulic, but won’t admit it.

Insofar as where its originating, I first thought it was in the differential. That’s where the “sound” seems to be coming from. The mechanic however says the “rapid-overly-quick” 2-3 shift is transmitting the “jolt” through the driveline and frame, so I feel and hear much of it to the rear and behind the driver position. The mechanic drove the van 3 or 4 times, and says that aside from the clunk, the vehicle is in perfect condition…in fact better than many of the 2003s and 2004s that he sees in his business.

I think Mcparadise is right. I’m going to have to look for an older transmission fellow, perhaps someone who used to work at a Dodge or Chrysler dealership. Its obvious that the expertise I need doesn’t exist at the typical transmission shop. I anticipate that no matter who does the work, they are going to want to rebuid the whole thing or replace it with a re-manufactured one. Its the way of the world! Nobody wants or knows how to troubleshoot and repair like 40 years ago. They just want to “replace”.

The TorqueFlite A727 is still one of the most durable automatic transmissions to ever come out of Detroit. It’s used in most of the Mopar muscle car restorations such as the Dodge Chargers, Plymouth Road Runner/GTX, and all the Mopar light trucks from about 1965 to 1980.

Thank you both for your responses and comments.

This is one for Transman. He’s around once in a while if you ask or run it down to Georgia for him to take a look.

You might look for slack in the slip joint behind the hanger bearing.