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fluid drive downshifts


Royal Duke

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I have a 1941 Chrysler Royal with a Fluid Drive transmission. It shifts from the high range to the low range on its own even though I am travelling on the a level or near level road and am not accelerating.  Though I have read the various manuals, nothing addresses this occurance. I can hear and feel it "searching" for which range to be in, and ultimately, the lower range wins out. Seems to me to be, either a pressure problem (but the oil levels are full), or the diaphram is shot and not holding the vacuum. I would appreciate some assistance troubleshooting this problem. If it is the diaphram, can I disassemble the can and replace just the diaphram, and if so, what do I use?   Duke

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Since the transmission upshifts and downshifts, it's the semi-automatic. You mention a vacuum diaphragm.  The 41 used a vacuum system to make the downshift.  It sounds like you have the M4 transmission.  Some of the guys can chip in.  Also, get a shop manual to see how the transmission works, or is supposed to work.

My '47 has the M5 transmisson, which uses a governor and solenoid to control oil pressure to make the downshift.  Unintentional grounding of the downshift circuit would cause it to downshift unintentionally.   Something similar is happening with your vacuum controls. 

(Additional information:)  My transmission has reverse in the usual location, towards the driver and up, and the driving range is away and down, like third gear for a manual transmission.  (The car takes off in third gear, courtesy of the fluid coupling, and upshifts to fourth gear.)  There is a power range,  away and up, like second gear in a manual transmission. (The car takes off in first gear, and upshifts to second gear; seldom used; called the "stump puller" gears.)  There is no to-the-driver and down position, as for first gear of a manual transmission  

Edited by DonaldSmith
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Sounds like the Vacamatic  has troubles with , as mentioned above, a leaky diaphragm or a vacuum leak somewhere.

First thing to do is put a vacuum gauge on the engine to monitor what is going on.  If there is a leak then vacuum will be abnormally low. If the readings fluctuate while the trans is attempting a shift then a closer look at the transmission vacuum unit is called for.   Mechanically, these transmissions are almost bullet proof.

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i have a 41 Windsor with exactly the same  transmission ,i changed the vacuum diaphragm recently .You have to remove the floor ,only about 12 bolts and then its relatively easy to swap for the NOS unit ,mine was from eBay and cost $50.Could also be the wiring is grounding ,its a confusing wiring diagram to use!

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3 hours ago, normanpitkin said:

i have a 41 Windsor with exactly the same  transmission ,i changed the vacuum diaphragm recently .You have to remove the floor ,only about 12 bolts and then its relatively easy to swap for the NOS unit ,mine was from eBay and cost $50.Could also be the wiring is grounding ,its a confusing wiring diagram to use!

I also have the vacamatic in my '41 Windsor. The diaphram went out on mine a couple of months ago and I locked out the underdrive and have been driving it as a manual with 2nd in low and direct in high. With the hole in my diaphram the transmission would not upshift from 1st and 3rd, when it happened to me I was driving fine then it would it would just not upshift no hunting as in your case.

The easiest way I found to check if you have hole in your diaphram is to follow the metal tube going down to the transmission from the intake manifold at the end there will be a section of 5\16" hose that attaches to the vacuum marked in red in picture. Check condition of hoses and clamps, disconnect from the metal line and suck on the hose to check if you can draw any air from the vacuum you should not be able to if the diapham is intact.

Do a search for Vacamatic transmission this site (thread by Norman I believe) and you will find a very detailed posting (by Don I believe) of the wiring and actions in the underdrive mode and the direct gear mode.

NOS diaphrams will be around 76 years old as this vacuum system was only used 41-42. It sould like people have been successful in making a diaphram out of rubber from an inner tube, I was going to try that.

I went to change the diapham on my unit, but some nice person drilled through the body and post and put in a roll pin without any purchase room. A job for the winter.

20170522_103558 (1400x788).jpg

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Are you sure it downshifts and not just slips the fourth gear? What I mean I had same experience when accelerating fast in fourth (like driving down a ramp onto a highway) , it slips the gear and engine revs until I let off the accelerator and it jumps back in fourth.

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13 hours ago, Royal Duke said:

Guys, i have been away for a week.  Chrysler1941, what did you do to correct your problem?  Duke

If your car is slipping I would check for proper clutch pedal play, if that does not help, possible low fluid level in the fluid drive or worn or contaminated clutch.

Edited by 1949 Goat
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6 hours ago, 1949 Goat said:

If your car is slipping I would check for proper clutch pedal play, if that does not help, possible low fluid level in the fluid drive or worn or contaminated clutch.

Yes 1949 Goat is right but this wasn't my problem. I cleaned the pilot valve as mentioned in the tech book. See attached

Furthermore I swapped the SAE 10W recommended oil to ISO 32 hydraulic oil and the flutter between high and low disappeared when hard accelerating.

Diagnosing The Hydraulically Operated Transmission.pdf

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  • 3 weeks later...

I put a voltage meter on the solenoid and it was bouncing around. I bought a NOS one off of ebay. The ohms reading for the new solenoid is 1.8.  When I removed the old solenoid, it read 2.2 ohms. Does anyone know what the resistance of the solenoid should be?  Also, the spring that holds the pilot valve down is nothing more than what you would find in a ball point pen. Is that correct?? There is little to no resistance to compress the spring. I am going to siphon out the 10W motor oil in the fluid coupling and replace it with ISO 32. I am also in the process of installing new rear rubber motor mounts, and so have not driven the car since changing the solenoid. I will let you all know how it works out, but since the car is still on jackstands, perhaps you could answer my two questions.  Wayne  

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21 hours ago, Royal Duke said:

I put a voltage meter on the solenoid and it was bouncing around. I bought a NOS one off of ebay. The ohms reading for the new solenoid is 1.8.  When I removed the old solenoid, it read 2.2 ohms. Does anyone know what the resistance of the solenoid should be?  Also, the spring that holds the pilot valve down is nothing more than what you would find in a ball point pen. Is that correct?? There is little to no resistance to compress the spring. I am going to siphon out the 10W motor oil in the fluid coupling and replace it with ISO 32. I am also in the process of installing new rear rubber motor mounts, and so have not driven the car since changing the solenoid. I will let you all know how it works out, but since the car is still on jackstands, perhaps you could answer my two questions.  Wayne  

Uncle Tech says

"The ball valve seat spring,the heavy spring that holds the seat down, is approximately 55/64 of an inch.

If it has taken a set so it is shorter, it' ll let the seat lift, and you'll have trouble with upshifting.

The spring under the ball is 1 1/8 to 1 ~ 1 1/4 inches long."

I remember it as a  strong spring that can be compressed with your fingers and about 3/4 size of the ball. See pic

Regarding the solenoid, I wouldn't worry. It's only an electromagnet that has to lift it's own plunger during low shift, so 1.8 or 2.2 ohm is only a difference between 3.3Amp and 2,7Amp at 6 volts.

Uncle Tech says:

"Connect a lead from the negative terminal of the battery
to one terminal of the solenoid. Ground the other terminal
against the battery hold-down bolt. Hold the solenoid upside
down plunger pointing up. When current passes through
the solenoid the plunger should move out. It should drop
back in when the circuit is broken."

Capture.PNG

Edited by chrysler1941
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"Mr Tech"can only help diagnose and help with service on the 1946 and later Chrysler  M-5 transmission as Mr Tech started teaching in 1947.

He has no helpful info on the earlier1941 chrysler M-4 transmissions.

All his training books and films he put out can only advise on the 1946-48 M-5 and later 1949-53 M-6 transmissions.

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21 minutes ago, Dodgeb4ya said:

"Mr Tech"can only help diagnose and help with service on the 1946 and later Chrysler  M-5 transmission as Mr Tech started teaching in 1947.

He has no helpful info on the earlier1941 chrysler M-4 transmissions.

All his training books and films he put out can only advise on the 1946-48 M-5 and later 1949-53 M-6 transmissions.

Yes you are right, sorry about that.:wub: I haven't taken mine apart and my experiences is with my friends M5.

Although the solenoid works in the same way, the spring is different and smaller.

But diagnostics for it is still the same, it's and electromagnet.

I have an extra transmission laying around. I'll see if it still has a spring and let you know over the weekend.

Edited by chrysler1941
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Some more tid bits.....

1941-42 chryslers and Desoto's  used both a M-4 and a M-5 transmission.

The M-5 transmission came late in 1942 and continued up through the end of 1948.

The M-4's came in two different electrical /mechanical control types .

1941 to early 1942 M-4's used the some what problematic diaphragm shift control and later M-4's used a improved "piston" type shift control.

Vacuum plumbing and electrical controls were some what different too between these two "SIMPLIMATIC"  M-4 transmissions..:)

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Yes it was vacuum.

It's noted in the parts books .

I have a couple new ones  too.

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So I looked at my spare transmission and no spring, but measured the resistance in the solenoid and it's about 2 Ohms.

Looking at the hollow opening for the spring, you are right. It very narrow and long. It's only suppose to push back the plunger after de-energizing.

 

1.jpg

 

2.jpg

Edited by chrysler1941
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I put a voltage meter on the solenoid and it was bouncing around. I bought a NOS one off of ebay. The ohms reading was 1.8.  When i removed the old solenoid, it read 2.2 ohms. Does anyone know what the resistance of the solenoid should be?  Also, the spring that holds the pilot valve down is nothing more than what you would find in a ball point pen. Is that correct. There is little to no resistance to compress the spring. I am also going to siphon out the 10 weight motor oil in the fluid coupling and  to change the fluid to the iso 32  

9/16/17:  I have read your helpful hints.  I have attached a photo I took looking down onto the transmission. Perhaps the photo will better help you in knowing which unit I have. I have yet to take it off the jack stands to take it for a test drive after installing new rear motor mounts and the solenoid which I purchased off of ebay.  Duke

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