1950 DeSoto semi automatic fluid drive (M6) questions

Indian 101 · June 25, 2011

Hello!

I apologize for my bad English. Nonetheless, I hope someone can help me with my questions. I have taken over a 1950 Desoto custom (S14) with an M6 semi-automatic drive transmission (tiptoe). My father started a restoration of the car 30 years ago, but it was never completed. With the help of a friend of my father, I want to try to get the car on the road again. The questions that I hope someone can help me with is:

It is fitted with a non original carburetor (no kick down switch and anti-stall control ..) and wiring harness from the control unit for the gearbox is in poor condition and was not connected to any devices in the engine room when we began work on the car. Is it possible to run the car without disconnecting the wiring from the solenoid and governor to carburetor? Would it be possible to get the gearbox to function without the kick-down function, but that it still shifts between 1. and 2. in the power range and 3. and 4. in driving range? How will the wiring diagram then look like? So far I have noticed and removed all the wiring and then it's maybe 2. and 4. I get if I try to drive the car?

I have trouble getting purchased described motor oil 10W. Are there other more easily available oils that can be used in the transmission? See the description of a hydraulic ISO32. If I find a hydraulic fluid that is referred to ISO32, is it OK to use this?

I am very grateful for any advice and tips. It would be really fun if the car after 40 years out of the road, could again roll on bumpy and winding Norwegian roads

Rusty_OToole · June 25, 2011

A good oil for transmission and fluid drive unit is TDH tractor oil, ISO32 grade. TDH stands for (transmission, differential and hydraulic).

This oil is available where auto parts and farm supplies are sold.

The transmission will function without the electric controls but will not go into high gear. You will have only 1 and 3 gears.

There are 2 switches on the carburetor. One is energized when the throttle is completely closed. It controls the shift to high gear. The other switch is energized when the throttle is completely open. It controls the kick-down. The kick-down causes the transmission to shift down from high to low for passing or climbing steep hills.

It would be possible to make the transmission shift with manual switches. This would do for testing purposes but would not be practical for everyday driving.

Your best option would be to buy a new or rebuilt carburetor. This would cost about $250. You might be able to find one on Ebay cheaper but there is no guarantee it would not need to be rebuilt.

If you go to the Chrysler and Dodge sections of this site, and do a search for Fluid Drive you will find a lot of information. About 2 years ago there were 2 or 3 long threads on the operation and servicing of this transmission that are very informative.

This website reproduces some original Chrysler service technical literature, very valuable to understanding the Fluid Drive system.

Master Technician Service Conference - Chrysler's Training for Mechanics

You will find a 1950 DeSoto works very well on winding and bumpy roads as long as you are not in a hurry. They have enough room for a family of Vikings wearing horned helmets, lots of road clearance and they have a suspension that soaks up bumps.

Edited June 25, 2011 by Rusty_OToole (see edit history)

Indian 101 · June 26, 2011

Many thanks for the answers to a Viking:) with his wife and two children who hope it will be possible to get the DeSoto back on the road.

I do not know why the carburetor has been replaced and how this has worked, but they have switched to an adjustable carburetor, perhaps to save expensive Norwegian petrol. I had hoped it was possible to use the car without the kick-down function, but with only 1. and 3. gears it becomes difficult. I thought maybe it was 2. and 4. gear I got, since the wiring harness is removed and there is no current to the solenoid?

If there are no practical ways to get the transmission to shift to high gear without the original carburetor, I think I must try to get a new carburetor and a new wiring harness (and hope that there is no fault in the transmission that is why the carburetor is replaced and wiring disconnected). Do you have tips for places where I might buy a new carburetor and wiring?

The link to the website with some original Chrysler service technical literature is very useful!

Rusty_OToole · June 27, 2011

The wiring is very simple, you can make the wiring using a wiring diagram as a guide.

If you replace the wiring, and add 1 manual switch you can make the transmission go into high gear. The switch you need, is the one that operates at closed throttle. When you are driving above 14MPH lift off the gas pedal and actuate the switch and the transmission should shift into high.

What happened to the original carburetor? Is there any chance your father took it off and kept it in the garage? If you can find it, it is a simple carburetor to rebuild and adjust.

The transmission is very simple, rugged and trouble free. Most problems are simply due to low oil level, or faulty wiring. The electrical contacts in the governor may need cleaning once every 150,000 miles or 50 years. They are soft silver, a shot of electronic contact cleaner spray will clean off any oil, then draw a strip of white paper or a business card between them. This is only necessary if the governor fails to work.

9 chances out of 10 if you return the wiring to factory configuration the transmission will work.

Here is a typical wiring diagram. This should be correct for your car. Some models use a relay but I don't think yours does.

There should be a square box on the air cleaner support. It is about 50cm square. It contains a resistor and a circuit breaker. If it is missing I can look up the spec of the resistor and circuit breaker. If it is not in the manuals already linked.

On reviewing the wiring diagram it appears both switches on the carburetor are wired together. In that case only one manual switch would be necessary to make the transmission work.

46661d1264108238-48-fluid-drive-trans-wont-upshift-gyro-tranmission.pdf

Edited June 27, 2011 by Rusty_OToole (see edit history)

June 27, 2011

Indian-I just finished going through 48 and 49 Chrysler cars with M6 fluid drive transmissions. Neither would downshift at all.It is possible to make an electrical downshift mechanism which you manually trigger with a switch at the steering wheel. This switch would replace the pushbutton mechanism on the carburetor which is actuated when you floorboard the gas pedal.The carburetor has 2 devices which are wired together which ground the electrical system to the transmission when you floor the throttle pedal. One is the pushbutton mechanism tied to the throttle lever at the carburetor and the other is an electromagnetic mechanism on the carburetor which causes extra gas to be squirted by the carburetor accelerator pump.

Chrysler dealers sold an accessory pushbutton mechanism which mounted in a replacement knob on the end of the shift lever. Pressing on this button would cause the transmission to downshift any time you pushed the pushbutton. A Chrysler parts vendor sent me one of these accessory pushbuttons but I didn't install as I got the carburetor pushbutton to work properly when you floorboard the accelerator pedal by adjusting the throttle linkage.

I did temporarily install some extra wiring so i could ground the downshift mechanaism when the carburetor pushbutton did not work and was able to do the downshift with a pushbutton..

I also was able to have a wiring harness manufacturer make me a new wiring harness to connect the various parts of the transmission, carburetor, and the switchbox on the LH inner fender using the original harness as a pattern.

You need to have a proper wiring harness even if you don't hook anything up at the carburetor as the coil has to be grounded when you trigger the downshift.

Contact me if you need more info on a fixup.

Martin Lum

marty@oldercar.com

1948-49 Chrysler

Indian 101 · June 30, 2011

Thanks!

I have now set the car down on the wheels and tested it a little out of the garage (the car has not been out of the garage for 30 years, so this too was a milestone). I have only tried power range and reverse, because the driving range will require a better road, and more mounting of the car (I'm sitting on an old milk crate in a car without doors, etc.). When the vehicle is in neutral, I hear a sort of "rattle "noise from the transmission. If I pull gear lever to me and pushing it slightly in the direction of the reverse, the noise disappears completely. What could be causing this sound, and how can this possibly be tried repaired?

The non-original carburetor (Tillotson) that are in the car is the same as was in the car when my father bought it 30 years ago. However, I have some parts from a similar car in boxes ... I see that there is a disassembled carburetor of the type Carter E7L4, but unfortunately I doubt that it is complete. Is this the original type carburetor of my car?

If I'm using extra wiring and switch(es) to connect a wiring harness that allows up- and downshift with the non-original carburetor, I would find out if the transmission is ok and have at least a temporary solution? Unfortunately I think I need a very detailed description of how this should be done, when current not is my greatest strength ... I can then continue working on other parts of the car while I'm looking for an original carburetor or rebuild the one I have in parts.

I have found a supplier in the U.S.,that according to the website can deliver new wiring-harness to this transmission in the DeSoto. Is this also a good starting point for a connection without the original carburetor, or can I as well try to use parts of my old wiring?

There is a square box on the air cleaner support that hangs on the old wiring harness. I'm not sure if this could be the resistor and circuit breaker??

Rusty_OToole · June 30, 2011

The Carter E7L4 is the correct model for your car. It is called a Carter Ball & Ball or B&B type carburetor. It was made for many years in different versions for different cars. If you examine it carefully you should see the 2 electrical switches.

The best thing for your car would be to rebuild this carburetor or buy a replacement. Easiest would be to buy a rebuilt from the US.

Use the old wiring if it is in good condition. Usually the wires in the engine compartment wear out first. The heat dries out the rubber insulation until it cracks and falls off. You can repair any bad spots with heat shrink tubing or electrical tape, or replace the bad wires.

The square box on the air cleaner support is for the transmission wiring. It contains the resistor and circuit breaker. If you clean it off you should see that the terminals are marked.

At the top should be Ign and Int. Ign stands for Ignition, this terminal connects to the coils positive terminal. Int stands for interrupter, it connects to the carburetor and to the interrupter switch on the transmission.

Below those should be Bat and Sol. Bat connects to the coils negative terminal. Sol connects to the Solenoid on the transmission.

Refer to the diagram above for more details.

If you wish to connect a manual switch you will have to run a long wire from the passenger compartment into the engine compartment and connect it to the carburetor switch wire. 1 wire to the switch wire, the other wire from the switch to ground.

This will allow you to control the high to low shift manually. Just press the button when you wish to shift.

I am told the transmission will shift up without the switch but will not shift down. You will have to experiment and find this out.

If you know a mechanic or electrician, see if you can get some help. The transmission system is unusual and not easy to understand but if you just put everything back the way the factory made it, it should work grand for you.

I would also strongly recommend that you buy a repair manual. They turn up on Ebay regularly. The factory made enough for every dealership and there are still a lot of them around. Usual price from $2O for a reprint to $5O for an original. Well worth it for the time and money it will save.

The rattle noise may be from the shift mechanism or some other part under the car. One of us should have a look. You are closer than I am.

Edited June 30, 2011 by Rusty_OToole (see edit history)

c49er · July 8, 2011

The M6 requires the wiring and electrical controls to be able to control upshift and downshift at the engineered/designed road speeds.

The M5 and M6 transmissions will always upshift into high range (as long as no mechanical gear damage) without any electrical controls hooked up. They will upshift imediately once pump pressure gets to 30PSI or more. Then the trans will downshift back to low range only when oil presure drops below 5 lbs.

I bench test these transmissions after repairs without any electrical hooked up on the transmission stand. Just spin the input shaft and it will shift into high range. Simple. For those who are just beginning to drive/own a M5/6 MoPar they need to go buy the shop manual and and learn how to operate and fix these cars with this 2 range transmission. The old timer mechanics who fixed them are all just about gone now.

Bob

Indian 101 · January 19, 2014

Thanks for all the advice! After installing a new wiring harness and a rebuilt original carburetor the transmission still not functioned, but after running the car for a while it began gradually to function properly. I still dont now what the rattle noise is, but apart from this the semi automatic fluid drive now works excellent .

Rusty_OToole · January 19, 2014

Congratulations on getting your car working after 30 years. The rattle may indicate something worn or loose in the transmission shifting linkages and rods. Or in the clutch pedal mechanism. The clutch is supposed to have a spring to return the pedal, if this is missing the pedal can rattle.

I hope with the new carburetor, you should burn about 11 liters per 100Km at a steady 90Km/hr. This is not bad for a large car of this age and of course, you can burn the cheapest low octane gas.

Wilf Sedanet · April 6, 2014

Seeing as this topic is quite recent and being in the same ballpark: Are there still internal parts for the M6? My M6 does not shift normally: 1. if I lift off the gas it just gnashes teeth, I have to disengage the clutch, wait a few seconds until the speed of the gear sprockets matches the rest and then engage the clutch again. 2. If I downshift by going WOT it may just shoot into neutral thereby almost disintegrating the engine by high revs, so I never downshift that way. 3. When driving up steep slopes the tranny will try to downshift, thereby not engaging, thereby the car will roll backwards uncontrollably. Very scary when people are driving behind you. Seeing as this is some sort of synchronisation problem and the oil looked more like sludgy metallic paint when I refreshed it, it is surely a mechanical problem. I also live across the pond so it's expensive and hard to get a spare tranny from the USA. I would consider it if it was safe, though, but as I'm not able to see and inspect it in person that's a big risk.

c49er · April 6, 2014

Gold colored oil is a sign of extremely worn syncro rings.

You most likely will need a new Mopar input shaft, a blocker ring and direct speed blocker ring sleeve to fix the into high range lock up problem.

The oil pump pressure also needs to be checked, needs to read at 40lbs or more.

You need a factory shop manual and or a good mechanic who understands these M5/M6 transmissions.

Bob

Edited April 6, 2014 by c49er (see edit history)

Wilf Sedanet · April 6, 2014

The oil was silver colored metallic, on the drain plug there was a big sludge of silver colored fine flakes. I think there's not a lot of knowledge in our country but I do know a great mechanic. I only have the standard workshop manuals (1949 Chrysler and 1950 DeSoto - I owned a Chrysler too), not special M6 manuals. I know there is a lot of knowledge to be found on the imperial car club website.

Are these internal parts hard to come by? Or is it maybe more cost-effective to find a swap tranny?

c49er · April 7, 2014

I'd think it is easier to swap a known good trans into your car. In 1950 there are two different M6 transmissions-an early and a late. The early type will have an external e-brake band on the back of the trans while the late M6 trans will have two internal brake shoes hidden inside the rear trans brake drum.

They are not a direct swap over as the rear extension housings are different and the e-brake cables are completely different too.

Edited April 7, 2014 by c49er (see edit history)

Wilf Sedanet · April 7, 2014

Thanks for this extended information, much appreciated! I recognize the late M6 from when I drained the oil. So I need to find a late model tranny. That's gonna be a hard assignment; a few years back there were a bunch on the Dutch market but then I did not need them. Thanks!

1950 DeSoto semi automatic fluid drive (M6) questions

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