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I have a 46 DeSoto Custom (S11) with fluid drive tiptoe transmission. I recently took the car on a long trip and on the way home the transmission stopped shifting into high gear. Eventually it did shift up enabling me to get home, but since then it will not shift into 4th or 2nd gears. I have gone through every test procedure in my manual including an oil pressure test. I do get about 50 lbs pressure at about 15 MPH, but when I acceletrate further the pressure drops down to about 20 lbs. Is this drop in pressure normal?
All electrical and hydraulic tests indicate the transmission should be operating normally with exception that when I check piston movement through the interrupter switch hole the is little or no movement of the piston as I accelerate. Sadly there is no external access to the filter screen - looks like my next step is to remove the tranmission and check the internal filter screen for blockage.
Thanks for any information provided. 
Gary, I had a similiar problem with my 52 a few years ago. I done all the checks in the manual and all checked ok. I was ready to take the transmission out but decided to try this first. I soldered a piece of 1/8" copper tubing about a foot long into a fitting that screwed into my air nozzle. I drained the transmission, removed the filler plug and worked the tubing through the filler hole back to where the filter screen should be and just blew the heck out of it hoping I could clean the filter if it was plugged. That was several years ago and I am not sure now if that solved the problem because about that same time I found a bare wire that may have been shorting to ground. The transmission has shifted perfectly since. Les
Is the idle speed too high?
Just a guess.
Have a nice day
Steve
I assume you have taken a good look here:
http://www.imperialclub.com/Repair/Lit/ ... /index.htm
The points in the governor tend to gum up and then it sometimes works and sometimes does not...
I have seen people who drain the oil, out in a mix of 50/50 10W oil and kerosene and drive it around the block one...drain and refill with fresh oil.
Generally, if it was working then not working then working again it is NOT in the transmission but in the control systems.
Let us know what you find...
James
Well guys I am feeling a little stupid. I was just about to pull the transmission when I noticed a cover plate on the rear of the transmission that was held on by two bolts. On removing the plate I found inside the oil filter screen; duh! It was so badly plugged there was no way fluid was making its way to the piston. I spent the good part of this afternoon cleaning the screen of baked on debris and sludge.
The picture in my manual depicted a large bolt (approx 1 to 1 1/8 inches) holding the screen in place. I was so focused on looking for this bolt that I completely ignored the cover plate. This led me to believe the filter screen was only accessible by removing the transmission. Thankfully I smartened up before it was too late.
Haven't had a chance to road test it yet but I am reasonably confident the clean screen is the fix I need. I will let you know tomorrow.
Gary 
Gary, I had a similar problem with my 42 custom a couple
years ago at a Desoto convention. After driving half a day,
it would not shift into 2nd or 4th. After I got home I started
checking everything. My 42 has a round solenoid vacuum
valve mounted behind the carburator. It was labeled
#SSJ-4101 Autolite. I found that this controls shift 1st & 3rd
when the valve is closed-stops the vacuum. When the valve
opens-allows vacuum in, it shifts 2nd & 4th. It has two electrical wires connected with screws on top of the solenoid. I was able to take it apart and clean the inside. That fixed my problem.
Some time ago I posted this topic. My last entry indicated my hope the problem was solved as I was able to access the oil filter screen and give it a good cleaning (it was accessible from the outside and not located internally as I originally thought). After adding fresh #10 oil I took the car for a test drive and upshift it did. I was really pleased. Soon after I ventured onto the highway and after driving approximately 4 miles the transmission downshifted from 4th to 3rd when my speed was about 55 MPH. After that it would no longer upshift. A fellow member of the local classic car club to which I belong was kind enough to give me, yes I said give me, a fluid drive transmission he had harvested from a Chrysler (circa 1948). After replacing a leaky seal on the rear of the transmission and swapping the DeSoto brake drum onto the Chrysler transmission I installed the replacement into the DeSoto. Pleased to report the DeSoto is back on the road again. Took a tour for approximately 150 miles yesterday and everything is working fine.
On examining the original DeSoto transmission it appears oil was leaking around rail piston, some of which made its way past the rail guide. I suspect the condition of the filter caused contaminents to make their way around the piston and score the piston walls. Someday, when I have nothing better to do, I may strip down the transmission and have a look.
Thanks to all who assisted me with this problem.
Gary/DeSoto Club Members,
I am experiencing the same upshift issues on my 49 1st series S11 that has been described in this post. It started as an intermittent problem that became permanent.
I have decided to start by draining the tranny and putting fresh 10w in it. This post and the manual refers to a strainer / filter but I cannot find it anywhere near the tranny. Does anyone have pics or a better a description as to location as the manual is pretty vague.
