The Hydra-Matic dates back to 1938 or 39, depending on which historian is talking. The first models shifted automatically, but a manual clutch was used for starting and stopping. In 1940, a fluid coupling (not a torque converter) was used, which allowed the engine to idle in gear. Oldsmobiles and Cadillacs got most of the Hydramatics, with a very few installed in Buicks.
Along comes WW II, and a lot of the 2 1/2 ton trucks used the transmission, and at least one medium-sized tank was powered by a Cadillac flathead V-8 with a Hydramatic transmission. After the war, GM sold the Hydramatic to other manufacturers, such as Nash and Lincoln. By the mid-1950's Chevy and GMC trucks got a beefed-up version of the trans, with slightly higher hydraulic line pressures for more positive shifts. The bigger GMC's used Pontiac V-8's with hydros, up to and including a few road tractors. I drove one of them in the mid-1960's.
The Dual Range Hydra-Matic is a 4-speed without a torque converter, that does not multiply torque for starting from a dead stop, so it needs a very low-ratio 1st. gear. It makes a 1-2 shift at 3 or 4 MPH in some cases, depending on throttle opening and driveshaft speed. Shifts were VERY positive when the throttle pressure was adjusted properly, causing some owners to refer to them as "Jerk-O-Matics". Lowering the throttle pressure resulted in soft, slippy shifts and rapid wear on the clutches and bands. The transmission was used in Chevy and GMC pickups and some larger trucks into the late 1950's or early 1960's, much longer than their use in cars. The upgraded Hydra-Matic that was introduced in cars in 1956, the Controlled Coupling model, was never used in trucks. It lasted into the mid-60's, 1964, I believe, and got replaced by the Turbo 400.
Your truck is not terribly scarce or valuable, but it is somewhat rare because of the relatively few, percentage-wise, that were sold with the automatic transmission option. It's worth keeping in original condition if possible, because there are several unique f-eatures of the automatic transmission trucks.
Jerry
Last edited by cletis; 09/20/2010 6:34 AM. Reason: tricked automatic link.
