Fluid Drive

by Richard Langworth

 from The Complete History of Chrysler Corporation 1924 - 1985

Imperial Home Page -> Repair -> Transmission -> Fluid Drive

Fluid Drive was one of Chrysler's many splendored engineering feats, probably the most popular option (when it wasn't standard equipment) on Mopars of the Forties.  It was usually sold along with Chrysler's hydraulic transmission of that time.  The impetus for Fluid Drive was simple enough: eliminated shifting.  The actual concept was a bit more complicated.

Fluid Drive discarded the conventional flywheel in favor of a fluid-coupling torque converter that performed all the same functions except providing a contact surface for the clutch plate (a separate clutch was mounted behind the coupling).  All other flywheel services were provided: storing of energy, smoothing of power impulses, and carrying the ring gear that meshed with the starter pinion.  The coupling was a cylindrical drum, filled to 4/5ths capacity with low-viscosity mineral oil.  It contained a casing, or "driver," with radial vanes on its inner surface.  This faced a driven "runner" having another set of vanes running freely inside a cover, which was welded to the opposing drum to make a solid unit.  the oil allowed the casing to be sealed for life, since it provided essentially permanent lubrication.  The coupling was bolted to the crankshaft as a flywheel would be, and the fluid was retained by a leakproof seal around the rotating central shaft.  The filler hole was designated to avoid overfilling.

When the engine was started, the fluid coupling revolved as a flywheel would, but the vanes attached to the driver casing also rotated, thus throwing the oil outward in whirlpool fashion.  The oil circulated across a quarter-inch gap between the driver and runner and onto the vanes of the runner.  The runner then turned through the action of the moving oil, though always a bit slower than the driver.  This transfer provide a "cushioning" effect that accounted for the smooth flow of power for which Fluid Drive was known.  there was no metal-to-metal contact.  The cushioning effect prevented the engine from stalling when the car was stopped with a gear engaged, allowing the gearlever to remain in position without depressing the clutch pedal.

There were two Fluid Drive gearlever positions.  "Low" controlled first and second gear and was used only for extra pulling power.  "High" connected to third and fourth gears, and was used for all normal driving.  "Low" was located where second would be on a conventional column shift; "high" was in its usual place.  To start off, you normally shifted into High and stepped on the accelerator.  At 14 mph you released the pedal slightly,  waited for an audible "clunk" and continued on in fourth or High.  To stop, you braked as you would with a modern automatic transmission.  To start again, you just stepped on the accelerator.  The system eliminated about 95 percent of all gear shifting.

Ted West provided one of the most entertaining driver's impressions of Fluid Drive in a 1968 issue of Road & Track magazine.  West was certainly the first journalist to suggest that this semi-automatic transmission could be used like a manual four-speed: "To shift from high range of low gear to low range of high gear (second to third) it is necessary to de-clutch, change the shift lever position to high, reengage the clutch, press the accelerator to the floor to activate the electrical kick-down switch, and with luck you will continue forward.  See how simple and relaxing?  To reach high-range speed, just let off the gas again...you can run through the gears or simply leave it in high, a la Dynaflow (another legend of the Old West), let the torque converter do its stuff.  This latter method results in very "dignified" acceleration and is recommended only for people with several gasoline credit cards.

Fluid Drive must have been a boon to neophyte drivers in its day, but the factory had a strong warning about it: "The fluid used in the coupling must be of the correct chemical analysis and viscosity.  This fluid is obtainable ONLY through the Chrysler Parts Corporation, and no other should be used under any circumstances."  A small price to pay for so many virtues?

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