Mopar taxis, 1935-2003

The reliable 3.5 V6

1997 Plymouth Pronto
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The odd M6 and M4 four-speed semi-automatic transmissions

Thanks to Argent, Bill Watson,
Sam Chase, and S. Berliner.

The M6, sold as Presto-Matic, Fluidmatic, Tip-Toe Shift, Gyro-Matic, and Gyro-Torque, was a two-speed manual transmission with an electric 1:1 ratio unit (loosely called an overdrive unit) attached. It was used on Dodge (1948-1953), DeSoto (1946-1953) and Chrysler (1946-1953) cars, coupled to either Fluid Drive (fluid coupling) or Fluid-Torque Drive (torque converter).

Gyromatic

The driver would select either the low range or the high range — the two manual forward gear sets — and “shift” within either range by lifting his foot off the accelerator pedal within a set speed range. Then the “overdrive” (really, direct drive) unit would kick in. Thus, there were four speeds: Low, Low Overdrive, High, and High Overdrive.

Most people would start out in High range and, between 15-20 mph, lift their foot off the accelerator; the hydraulic unit would then kick in for the cruising gear. The overdrive kicked back out at any speed under 11 mph, and when the car was brought to a stop.

For better low-speed acceleration, you could start in the low range and first gear, at 8 mph release the gas and wait for the clunk as it shifted, then go up to 25 and shift into the high gear.

To use all four speeds, you could start in low range, and, while shifting to high range, floor the gas, which caused the transmission to kick down to the low gear, bringing third gear into play. This could provide a performance boost.

The clutch was necessary any time the gear lever was moved between Low/High, or Reverse. A fluid coupling was attached to the flywheel, and a conventional clutch was mounted in tandem.

Later models had shift quadrants to make the configuration appear as automatic as possible. On the 1954 Dodge, the quadrant had four positions in this order: R L Nu [Neutral] Dr, with the same H pattern as a conventional 3-speed, minus 1st gear ("L" was in the 2nd gear position, "Dr" in the 3rd gear slot.)

Bill Watson noted “The shifting of gears within low or high range was not done automatically as you had to lift your foot off the accelerator and wait for the ‘clunk’ when the higher gear engaged. Chrysler described it as a ‘click,’ but in my 1949 DeSoto, it was a ‘clunk’! The downshift occurred automatically when the car slowed down.”

Fluid Drive referred to the fluid coupling between the engine and the clutch. The Fluid-Torque Drive (torque converter) was optional starting in the 1951 model year; it had a shift quadrant, but as it was coupled to the M-6, it still had a clutch.

Mike Sealey noted that the M-6 was marketed under the name Prestomatic at Chrysler, Tip-Toe Shift at DeSoto, and Gyro-Matic, Fluid-matic, Fluidtorque, and Gyrotorque at Dodge. On second-series 1949 through 1952 Dodge Meadowbrooks and Coronets, if it says Fluid Drive below the model name, that originally meant the standard transmission with the fluid flywheel action, while if it says Gyro-Matic, that emblem belongs on a car with the M-6.

Sam Berliner III added, “My 1941 Chryslers all said FLUID DRIVE on the trunk light cluster, even my Windsor with a 3-speed manual and the fluid coupling, as did my 1950s — even my 1949 C-46 Highlander 8 does — bracketing the back-up light.”

The M-6 transmissions often confuse modern drivers, who believe them to be 3-speed sticks that refuse to go into first. Besides the clutch pedal, later versions can be told apart from the later PowerFlite by their strange “D-N-R-L” range of gears - the position of reverse making a strong argument for keeping the clutch pedal!

Bob J. wrote:

Regarding the M-6 transmission in the 1946, 1947, and 1948 Chrysler and DeSotos: I have disassembled and reassembled one using the 1948 DeSoto Shop Manual. It has two gear ratios in low range in the column shift pattern and two gear ratios in the high range of the column shift pattern. It is very difficult to use all four gear ratios in a normal start.

Starting in low range, first gear ratio, and then allowing the automatic shift into the second gear ratio also positions the internal gear ratio into the fourth gear ratio. In effect, the transmission uses only first, second, and fourth gear ratios. Third gear is skipped because the transmission has already made its hydraulic shift into fourth gear ratio. The only way to pick up third gear ratio is to depress the accelerator pedal to the floor, through the detent, causing the transmission to automatically do its hydraulic shift back into third gear ratio.

All this is exactly as the shop manual details the operation and that is exactly as the parts work in the transmission.

Two other points: The foot operated clutch is strictly mechanical and not hydraulic. The fluid coupling is just that - a fluid coupling and does not multiply torque therefore is not a torque converter.

Hemi Andersen wrote:

Cars that had the name 'Fluid Drive' on them had only a fluid coupling, no special transmission. That was mainly the Dodge. Dodge had Gyro-Matic which was fluid coupling with a M-6 transmission. Gyro-Torque was the self contained torque converter with the M-6 transmission. The distinction between Fluid and Torque told you which hydraulic coupling was used. Chrysler and DeSoto used the M-6 across the board as I recall.

The M-6, like the cast iron Powerflite and Torqueflite transmission, were bolt-on to the bellhousing transmissions with various hydraulic couplings ahead of them. In those days the coupling was not part of the transmission. The couplings were directly bolted to the crankshaft, so when you removed the transmission, the coupling was left attached to the engine.

M4 (Vacamatic / Simplimatic)

The M4 was a pre-war 4-speed, vaccuum-hydraulic unit used on Chryslers and DeSotos in 1941 and 1942. Chrysler called it the Vacamatic and DeSoto the Simplimatic, in the U.S. Chrysler of Canada sold the unit as Simplimatic on both DeSoto and Chrysler cars. It was not available on the Dodge. The Vacamatic / M4, as one might expect, was operated by engine vacuum. Like the M6, it had a low and high range, and a low and high gear, providing four speeds.

Other Chrysler, Dodge, Plymouth, DeSoto, and Jeep transmissions.


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Mopar taxis, 1935-2003 The reliable 3.5 V6 1997 Plymouth Pronto