Theory of operation

Dual Range Hydra-Matic

Last used in the 1955 Oldsmobile

Oldsmobile was given the task of developing transmissions for the other upper car lines of General Motors. Chevrolet had their cast iron Power Glides and Aluminum Turbo Glides and Buick the Dyna-Flow for their automatic transmissions. I was just getting into the automatics around 1965 when the Aluminum Power Glide had been out a couple years. I was taught the Jet-a-way transmission at Dunwoody Institute in Minneapolis in 1963 but never worked on them until many years later. The Jet-a-way was first used by Oldsmobile in 1956 and I had a 1957 Olds with one in it. Earlier, I had a 1955 Oldsmobile with the Dual Range and never gave a thought as to how it worked until recently.

The forgotten Dual Range

It was at least forgotten on my part as my lifes work kept me moving onto the newer transmissions as they were introduced and immediately ended up in my repair stall with problems. After a life time of rebuilding thousands of various types, one day I reflected back as to what type of units were they. Then it occurred to me that there was this Dual Range of which I know nothing about. Between buying a service manual from 1955 and the internet, I'm very impressed with the engineering and success of that unit. I now refer to it as the ultimate four speed shifting machine.

About the drawings below.

I used to teach automatic transmissions in Phoenix in the mid seventies. I found out that it was best to learn about a subject and then put it into my own words and drawings for teaching.


 The Dual Range had been perfected for twenty years before I owned one. Lately, I have talked with the Oldsmobile car collectors that have this trany, and their statement is they still work flawlessly. Of course some will wear out or need rebuilding in time, but that is expected.

The first drawing is simply a layout of the parts that are enclosed in a cast iron case. The transmission doesn't use a Torque Convertor. Torque Convertors have a third member inbetween the two turbins called a stator. This stator redirects the oil flow between the turbins for greater efficency. The Hydra-Matic as G.M. called it, used a fluid coupling to act as the "clutch" between the engine and transmission.

Planetary Gears

Automatic transmissions use planetary gears that are helical cut (teeth cut at an angle). A gear set is in three units. A sun gear is in the middle with three or four gears in mesh called planet pinions. The planet pinions are mounted in a planet carrier and rotate on axles pressed into the carrier. The planet pinions are surrounded in a ring gear or internal gear that looks like a drum with teeth cut inside of it. Planetary gears are always in mesh with each other so their id no grinding of gears possible like a manual transmission where gears slide in and out of mesh with each other.

Reduction or direct drive

Reduction is obtained by driving one member, holding another member to get reduction in the third member. Direct drive in accomplished when two members are driven then the input and output are the same gear ratio. In this transmission, holding a gear member is accomplished by bands in the two front planetary sets or a cone clutch in the reverse gear set. Direct drive is by applying a multiple disk clutch to drive a second planetary member.


Gears with external teeth in mesh with each other, rotate in opposite directions. A gear with external teeth in mesh with an internal tooth gear, they both turn in the same direction.

 When the engine is running, the flywheel turns the fluid coupling which turns the front pump to keep the coupling full, supply the valve body at regulation oil pressure and also provide a lubrication system for the various shafts and gears.

  I thought it to be very odd how the fluid coupling worked. The coupling housing is kept full of oil with the front or rear pump which makes sense. The engine turns the outer shell ( coupling housing) just like a Torque Converter on other cars, ok? The coupling housing turns a shaft that operates the front pump like torque converters do. Here is what's odd. The shaft that drives the pump, continues back and drives the input ring gear of the front planetary. The sun gear is held by a band so the planet pinions and carrier are turning clock wise and connected to a shaft that goes forward and drives the drive coupling at reduction. The driven coupling now wants to turn but won't as long as your holding the brake. The oil from the drive coupling against the driven coupling is just churning and creating turning resistance to the engine. This is why you feel the engine slow down when you pull the shift lever from neutral to drive.

When you release the brake, the driven coupling begins to turn, it turns a shaft that's it's connected to and drives the rear planetary sun gear. The rear band is applied that holds the rear unit ring gear. The sun gear drives the planet pinions counter clock wise that make them walk around the held ring rear clock wise. The pinion gears are connected to the planet carrier and they turn as a unit. The carrier is splinned to the output shaft and car creeps forward in low gear with a combined reduction of 3.82 : 1 At this point, the transmission valve body, Governor, throttle position valve is prepared and waiting for what you'll do next. Transmission oil control circuits are way to complex for this simple narrative at this point.

 For second gear, the fluid coupling is going to change from being in reduction to direct drive. That means the band that hold the front sun gear has to let go and the clutch that drives the sun gear must be applied. This shift could occur at five miles an hour. Low gear is really low. This is why an Oldsmobile can really light up the tires at full throttle in low gear with that 324 cubic inch Rocket V/8 up front.
 Third gear is complicated. The front unit goes into reduction and the rear unit goes into direct drive. That means, the front band now applies and the clutch must release. The rear band releases and the rear clutch applies. The transmission valve body works with governor pressure, throttle position pressure, different mainline pressures, shift valve changes and other factors under all speeds and loads without lock-up or engine run-a-ways.
 Can you imagine accelerating a Super 88 at wide open throttle up to 75 miles an hour and then feel the trany up shift into forth gear and keep climbing the speedometer? That front band has to let go as the front clutch applies and all you feel is a firm up shift that presses you into the back of the seat. The sun gear has to go from standing still to a few thousand revolutions a minute in a split second. That's what I call engineering and workmanship by our Grandfathers. Oldsmobile had ten body plants, 8 assembly plants, one transmission and engine plant in those days and every car was expected to be a performer. GM dropped the Olds a couple years ago, but in my memory it was some automobile.
 The Dual Range had tremendous torque for backing up a car as two planetary were in reduction with a 4.3 : 1 ratio. I can imagine some old fart taking a wrong turn leading down a steep grade ending at a dead end with no way to turn around. He did this with a mumbo Air Stream trailer in tow. He could put the Rocket 98 in reverse and have all the power in the world and torque to go with it and pull it off.

 The old transmission designs like this Dual Range all had the problem of timing the adding and dumping clutches when up shifting and down shifting. If a band needed to apply for an up shift, a clutch had to release elsewhere to make the shift happen. If the band came on before the clutch released, the trany would bind up for at least a split second. If the band applied to slow after the clutch released, you had engine run-a-way (slipping). Considerable engineering went into valve body design to prevent any of the glitches. Then a fella by the name of "Simson" changed all that.

Simson gear flow

The Torque Flite was the first to use it. Two planetary units with a common sun gear was all it took to get three speeds and reverse. The sun gear was either held or driven to make this happen. The beauty of this design was it added clutches to up shift and dumped clutches to down shift. Every transmission for the last 45 years uses this power flow now. The draw backs are, only three speed forward and reverse has a second gear ratio, not a real low like the Dual Range.