Dodge :: Clunk Or Heavy Sounding Thump Under Very Light Acceleration At 2 - 3 Upshift

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 accelerating 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" ....as 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 onerous 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.