I Need Fluid Drive Advice . . .

[grey beard]

Not sure of the proper term - may be Hydrive - 1950 Dodge Coronet that upshifts with a "click" when you let up on the gas - this unit slips on a pull.

Is it not a conventional 3-speed with a convertor next to the clutch, or is it the forerunner of an earlier automartic? I'm looking for any advice and am open to all suggestions and local wisdom.

Thanks

[48mirage]

Gyromatic

[PatS....]

Some great info from the Imperial club:

http://www.imperialclub.com/Repair/Lit/Master/012A/index.htm

http://www.imperialclub.com/Repair/Lit/Master/012B/index.htm

http://www.imperialclub.com/Repair/Lit/Master/024/index.htm

Despite all the different names, yours is the M6 transmission. Pretty simple and bulletproof once you understand it.

[De Soto Frank]

Greybeard,

Sounds like your Dodge has Gyromatic Fluid Drive, which was the division's sales monniker for fluid coupling with the M-6 "Hydraulically-operated" transmission.

First let me emphatically state what this is NOT:

It is NOT like any "conventional" automatic transmission found on US cars and trucks built since about 1955. There are no bands or clutch-packs, and the fluid-coupling is NOT a torque-convertor, even though it looks quite similar.

(Many younger mechanics try to approach pre-1955 MoPar semi-autos trying to relate it to "modern" (post-1955) true automatic trannies, which is really just heading down the wrong path. Went there myself with a '48 Chrysler when I was in my 20's... )

Here's what it IS:

1) Fluid Coupling - the conventional "Dry-clutch" flywheel is replaced by a Fluid Coupling, a sealed unit with two turbine blade assemblies: one driven by the engine, the second attached to the output shaft. The coupling is filled with a special oil (MoPar Super Fluid Drive Fluid originally; now you can use "Universal Tractor Hydraulic Fluid ISO 32"). It is the oil that transmits the power from the engine to the tranmission.

If the unit were 100% efficient, it would transmit power at a 1 to 1 ratio; there is NO "torque multiplication" achieved by this unit. Due to frictional losses, and other gremlins of physics, some power is lost as it goes through the fluid coupling, being dissipated in the form of heat.

The output shaft of the fluid coupling has a "driving flange" attached to it, to which is bolted a conventional Borg & Beck clutch.

This is the original "Fluid Drive", appearing on Chrysler, De Soto, and Dodge in 1939, and had a 3-speed or 3-speed+OD tranny behind it, manual shift.

Fluid Coupling & manual tranny was available as an option through 1954 on the six-cylinder MoPars.

(Remember this is the first and most basic form of "Fluid Drive"... but wait, there's more! )

2) Transmission: In 1941, responding to GM's Hydramatic Drive (first true "modern" automatic tranny), several car makers began cooking-up semi-automatic trannies to try to compete. In 1941 & '42, Chrysler Corp. offered the M-4 Semi-Automatic transmission (on Chrysler, Imperial, and De Soto), a four-speed manual (sliding-gear) tranny, that was split into two operating ranges (Lo and Hi, 2-speeds in each), plus reverse, and had a vacuum servo and control system (governor, relay, kickdown switch, ignition interrupter switch), that would cause the transmission to "automatically" upshift when the driver would lift his foot at the appropriate time. ( Think of it as up-shifting a conventional stick-shift, except the car operates the clutch & gear-change; the driver only lifts his foot off the gas and "waits for the clunk".)

These trannies had a feature that allowed them to "free-wheel" when in the lower speed of each range, eliminating the need to de-clutch when upshifting.

This is the basic operating premise for all MoPar semi-autos, 1941 through '54.

But it's still a sliding-gear tranny that really belongs to the Manual-transmission clan.

3) After WW-II, Chrysler Corp re-designed the semi-auto a bit, replacing the vacuum servo with an internal hydraulic piston & oil-pump to provide the force for up-shifting. This version is known as the M-5 "Hydraulically-operated tranmission". (Still a 4-speed semi-auto). The internal gear ratios were revised somewhat.

The M-5 was available only on Chrysler, Imperial, and De Soto through '48. Dodge owners could only get a 3-speed manual, with or w/o Gyrol Fluid Drive(fluid coupling).

4) 1949: The semi-auto is revised again: this time, the control circuit is revised, with a new solenoid & governor, and eliminating the relay. The rear end of the tranny case is revised, and legnthened about 4 inches. This latest version is called the M-6 Hydraulically-operated transmission, and sees no further changes through its final season (1954).

The M-6 is available on Dodge now (Gyromatic), De Soto (Tip-Toe Shift), and Chrysler/Imperial (Prestomatic). These units all still use the original 1939-design fluid coupling. Regardless of the marque brand name for it, the semi-auto is just referred to as "Fluid Drive" in terms of badges on the dash or script in the rear bumper.

5) 1951: Chrysler introduces its Fire-Power Hemi V-8, and realizes that the 1939 1-to-1 fluid coupling is overwhelmed by the power of the new engine, and that road performance is suffering; still very smooooth, but oh-so-slow...

So, Chrysler engineers develop a true torque-convertor to replace the fluid coupling; it still has the conventional dry-clutch and is mated to the M-6 semi-auto. This package is called "Fluid-Torque Drive", and greatly improves get-away and hill-climbing. But, you still have to take your foot off the gas and "wait for the clunk", while the tranny upshifts.

Fluid Torque is available only in Chryslers for '51, then in De Soto for '52, and Dodge for '53 ( as the junior marques got V-8s). Fluid Torque ( and the other "Fluid Drive" variants) remained available through '54.

In 1954, Chrysler (finally!) offers a true, modern automatic tranny: the Powerflite, a very stout 2-speed unit, avilable in all marques except Plymouth (Plymouth gets P-flite in '55.)

So, that's a long, detailed answer...

Bascially, a true "modern" automatic transmission has:

1) Torque Convertor

2) no sliding gears; reduction & reverse is achieved by bands, disc clutches and planetary gear-sets.

3) Will up-shift under load.

4) Requires no further participation from the driver beyond selection of a driving range.

If you can obtain a reprint of the origninal factory shop manual for your Dodge ( Roberts MoPar, Bernbaum, to name two sources), or find a MoToR's or Chilton's shop manual from 1960 or earlier, they will have good info on servicing the semi-auto.

There is some "slippage" inherent in the non-Fluid-Torque MoPars, but if it seems excessive ( car comes slows to a stop on a hill with motor screaming), check the oil level in the Fluid coupling...

Hope all of this is of help...

Good luck !

De Soto Frank

(BTW: that original 1939-style fluid coupling was used in MoPar engined airport tugs and forklifts well into the 1960's & '70s...)

[PatS....]

Here's the naked system:

Shows the fluid coupling (replacing the flywheel), then the regular clutch, then the M6 transmission.

[bob westphal]

I don't think anyone has really adressed Greybeard's concern. He is wondering about the slipping! There is a clutch in this system that can slip while going up hills or under heavy power when worn out.

[James_Douglas]

Dave,

Check the fluid coupling. Take up the mat on the passenger side floor board and take the access plate out. There will be a big plug in the bell housing about 2" in diameter. Remove the plug. Have someone turn the engine over with a wrench on the crankshaft (take the plugs out it will turn easier) and watch the hole for the fill plug in the fluid coupling.

Take the coupling plug out and see if the thing is full. You must check it with the hole in the bell housing at about 2 o'clock as that is the correct fill hight.

If it is full then the clutch is slipping. If it is not full, fill it with tractor fluid as Frank said and try it again. If it is the clutch, then you need a clutch.

Best, James

[De Soto Frank]

If you suspect the clutch itself is slipping, there may be some shuddering and the smell of hot clutch facings ( like hot brake linings).

If the motor just seems to rev-up, it's probably low oil in the coupling.

James cited the way to check on that...

I would think clutch failures were a lot less frequent with Fluid-coupled cars...

De Soto Frank

[Merle Coggins]

De Soto Frank,

That was an interesting history on the Fluid Drive and Semi-Automatic transmissions. However, you can also add in the fact that Dodge trucks began offering Fluid Drive with a 3 or 4 speed transmission in 1950. Then in 1953 they added the M6 and called it the Truck-O-Matic.

Just thought I'd add that in, coming from the truck side.

Merle

[De Soto Frank]

Thanks Merle;

I've haven't had the pleasure of owning a vintage Dodge truck, so that is a void in my knowledge base !

I wonder how many trucks were optioned with the Fluid Drive or Truck-O-Matic ?

My family have been Chevy truck people since "forever", mostly the '47 -'54 Advance Design series...

I have a '54 3/4 ton that was my grandad's, and last fall I acquired a chassis & running gear from a '54 Chevy 3/4 ton that was a factory Hydramatic truck

(owner got railroaded into rodding); I've only seen one other AD Chevy that had the Hydro...

I wouldn't mind having a '39 or '40 Dodge pick-up...

De Soto Frank

[Merle Coggins]

I don't know how prevalent they were in the trucks back then. I know that my '50 3/4 ton truck has Fluid Drive with a 4 speed. I can't wait to get it completed enough to try it out on the road.

Merle