I am going to assume its an Allison unit, I been driving it lately alot when it downshifts it is harsh and bang when coasting to a stop, Its hard to tell but i think it happens when over 3rd gear, its not all the time either. I visually looked for leaks , noticed a rod going to trans is that a tv rod?
Question #1, how is the oil level?
Probably, and may need adjusting. In my experience the Allisons were always harsh shifting, especially when empty, YMMV.
Typically Allisons have a passage in the valve body where there is a relief body that allows oil pressure to pop-off or bleed a little right before a gear change. Otherwise you have the full pressure going from one set of clutches to another; causing a bang. Oil is the first place to look, then the filter. Check not only oil level, also the color and smell. It should be very red like a cherry extract for a drink and not clouded. It should not smell burnt either.
that was my first thought ! LOL- the dipstick is missing and has a cap. I was trying to locally track down something with same length!. YMMV? Its firm on 2-3 ,3-4 upshift They unit appears to have newer filters, no rust or residue on them. Seems like someone cared for the truck, hoping to swing by a junk yard see if i can find a 3208 with an auto close in year. What is the unit called? (AT545 from MT643) Does anyone have a pdf for overhaul or adjustment procedure? A book from atsg or atra?
YMMV = Your Mileage May Vary That's Don Lanway's famous catch phrase.
Tag missing from the side? Is it aluminum or iron case? In 75 I might have thought it would have a MT60 or HT70 (?) but those were before my time. Does it have 6 speeds or 4? Or 5? Pictures?
Lots of the 3208 in Fords had aluminum cased, 6 speed, 653s in them around here. Especially in the heavy duty models. YMMV
The 6 speeds were the MT60 style transmissions. But they were not 6 progressive speeds, 1st and 2nd were like granny low manually selected and normal driving was in 3-6. But I thought they were iron cased? Maybe that was the HT70. The MT 6xx and HT 7xx series were all aluminum. They were 4 speeds and could have an additional granny low bolted on for 5 speeds but again, not progressive.
The MT 60 was an iron case unit. Had a late 60s ford tandem with a 477 gasser and the MT60 as a water truck, and a mid 70s (74?) International Loadstar with one too. The shifters had a 1-2,3-4, 5-6 range marking.
All the 60s and 70s former state owned single axle snow plow trucks had the aluminum cased Allisons. You are correct on shifting as I forgot the lower gears were not progressive but were granny gears that never self shifted. The road salt ate thru the cases in the tail section over time and they turned into never ending leakers. I still got one in a early 80s Ford that is waiting for scrap price to go up.