Hello all, I have been able to answer most of my own questions by just reading through old threads or following all the random links. But i cant seem to find a definitive answer on my newest quandary. The Vehicle in question is a 1951 Desoto Deluxe, flathead i6 with an M6 semi auto fluid drive. I have rewired all transmission leads, rebuilt the solenoids and checked and rechecked the fluid levels. It up shifts and downshifts properly, however i believe the fluid coupling itself is bad. This car acts just like a standard transmission, i have to use the clutch when i come to a stop, and i have to stab the clutch when i let off the throttle for the up shift. My question then, Am i correct in assuming the Fluid Coupling itself is not functioning? If so do i have any options other than pulling the trans and sending off the coupling to be rebuilt?
Once it is in "drive" you should not have to use the clutch again, either at a stop light or on hills. It should function like a modern auto. It should also accelerate from a stop just like a modern automatic by just releasing the brake and accelerating. Initial starts in drive and when reversing require the clutch. As I somewhat vaguely remember, you can start from the lower gears but I think you may have to use the clutch to get from 1st, to 2nd, to drive in this "manual" mode. Don't quote me on that - someone else can say more about that. I learned to drive on a 53 Desoto Firedome with that trans and I do remember running it up thru the gears with the clutch. The manual mode was very useful if you wanted to stay in a particular gear for poor weather or road conditions, as that mode would prevent the trany from automatically shifting into high gear if you didn't want it to. Later, Gary
If you truly have the M6, it self shifts when the throttle is lifted. 1-2 in the lever 'up' position, 3-4- in the lever 'down' position. If your engine stalls when stopping, then I think you are correct to assume the fluid coupling is defective (seized/binding/locked up internally). As I recall, the fluid coupling has it's own internal oil supply. I believe it is checked by removing a small panel in the floor hump and rotating the engine until the FD unit filler plug is in position to be removed for checking/filling. If you coupling is seized, I suspect it's a bit late for fluid now... In reply to Gary's comments above, there is a variant Fluid Drive that utilizes a manual three speed transmission coupled to the engine via a fluid coupling and manual clutch, in that order. A driver could use the clutch to engage 3rd gear, release the clutch, and when ready to drive away, just step on the throttle. Acceleration was not brisk, to be sure, but no further clutch action was required unless changing gears. Ray
I appreciate the input guys. And yes it is the M6, it shifts between 1-2 and 3-4 but only if i press the clutch otherwise it just grinds. and it will stall if i dont use the clutch when i come to a stop. I was fairly certain that the coupling it self was locked up. I have no idea what was done to it by the previous owner, he could have filled it with ATF or ran a lag bolt all the through it for all i know. I checked the fluid and it was not reddish but certainly did not look like clean hydraulic fluid. Would draining and refilling possibly help? Is there some way to flush it? I know that i can get a rebuilt one from Andy Bernbaum but not sure what they charge, and i dont want to pull it if i dont have to. At least not yet.
It seems to me you have accurately assessed the situation. I suppose your choices are a) drive it like a manual shift car or....b) bite the bullet and replace the fluid coupling. Ray
Thanks Ray and Gary, i appreciate the affirmation. I will stick to driving with the issue for now. I will probably try and find a new coupling in the future but not until im ready to pull and rebuild the flathead. I just dont feel right about pulling apart a motor that runs like a top.
It should drive like an automatic except for using the clutch to put it in gear. After that you should be able to drive all day, even in traffic, without touching the clutch. What you describe sounds like the fluid drive unit is locked up. I never heard of this happening before and wouldn't have believed it possible but I suppose after 50+ years anything can happen. Normally the only wear point is the seal. If you find a used one and the seal is good just put it in. or if you want to spend the money get one from Bernbaum or other rebuilder. The seals can be replaced.
Never heard of one go bad, I would think one of those could be found, and still be good. Drove an old 53 New Yorker with that semi-auto fluid drive, also had a 52 Dodge pickup with mentioned 3 speed manual and fluid drive, two way different animals. Like already been said though, yours sounds locked up. Never took one apart before but doesn't that coupler come apart like a hydro-matic torque converter?
From what i can tell the only way to take it apart is to split it on a lathe, im pretty sure i read that in the reprinted repair manual i bought or possibly over on the imperial site. I have seen the seals for sale around before but every M6 trans i see for sale never has the coupling with it but ill keep a keener eye out for one. I may try to drain it and refill with Hydraulic Oil and see what happens. Worst case is it does the exact same thing. I have been told that putting ATF in them destroys the internal seals and like i said i have no idea what was done to this car by the previous owner. Let me just say he only sold it because he was moving and he had plans to make it an electric car. I was very happy to save this survivor from that crazy Honeywell engineer. Obviously a high education does not directly equate to smart decisions. Not only would this have been a horrible atrocity, but who would use a 4000lb car to make in to an electric vehicle?
does anyone have a method they could recommend to flush the coupling while it is in the vehicle? I know what to fill it with but im not sure what chemical i could use to flush it and how to get it to drain fully before refilling.
In about as many man hours as it will take to try and drain, flush and refill this coupling (likely with no improvement), you could replace it with a known good one. It's not THAT daunting a task. Get the car on a lift (ideal), or jack stands (acceptable), drop the driveshaft, pull the trans, remove the lower tin cover, remove the manual clutch assembly, unbolt the coupling from the crankshaft and set it on the floor. Assemble in reverse order. Done. Car will now drive as intended. Ray
...actually, it is not a 'torque converter', but rather a fluid coupling. There is no torque multiplication. Not sure that a 'rebuilt' unit is needed (you might even ask what constitutes a 'rebuild'...) and finding a good piece should be easy. The question then is the interchange concerns between the several variations of the trans. .
Looks like an M6 could use either a fluid coupling OR a torque converter. http://www.allpar.com/mopar/m6.html Some period factory info - http://www.mymopar.com/index.php?pid=451 http://www.mymopar.com/downloads/mtsc/049.pdf http://www.mymopar.com/downloads/mtsc/024.pdf http://www.mymopar.com/downloads/mtsc/023.pdf
Procedure to drain and refill- Remove access plate from bottom of bellhousing. Rotate until fill plug comes in view. Remove plug, drain oil. Replace plug. Roll back carpet and remove access plate from floor, on right side of driveshaft tunnel, below dashboard. Pry out knockout plug from bellhousing. Rotate engine until plug comes into view. Stuff a rag around the hole in case you drop the plug. Remove plug, fill with tractor oil, TDH type, ISO32 grade. The fill hole is located so there will be the right amount of fluid and the right air space, there must be an air space for it to work. The chance of this fixing the problem is practically nil. There is only one moving part inside and it runs in an oil bath. If it has failed and jammed changing the oil won't help. On the other hand if bits of metal come out when you drain the oil, you will know what is wrong.
Torque converter may have been available on your car as an option. If your car was so equipped it will say Fluid Torque Drive on the shift quadrant. You can also tell by looking under the car. Torque converter equipped cars had an extra oil supply for the torque converter. There was an oil pan under the bellhousing like on an automatic trans. It is very unlikely your car had a torque converter but they were made that year and could have been ordered as an option.
...in the OP's first post he indicates 'Fluid Coupling" and 'Fluid Drive' so I'll assume that he found a label on the car.... Who wants to venture a guess on interchange? Hollander anyone? .
If idle is too fast it will not shift up. I don't know if it would grind when you stop with clutch out when you stop. If you change fluid it takes a good while to fill it back up. You could probably flush it out with kerosene then refill worth a try I guess. Mine didn't shift right had to change solenoid not it works great and runs great.
Just for fun... our 53 Firedome 276 Desoto in Hawaii early 1960s. We had it for about 11 years and I think it was white over maroon when new. I was too young to then to drive. How can you grow up with a grille like that around all the time and not love hot rods and customs? Gary
The Jalopy Journal
