View topic - Copy from Tech Articles: M6 Transmission Question

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PostPosted: Thu Sep 07, 2017 2:07 am 
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Reposting from Tech Article page. Submitted by "jimyates".

Finally drove my 48 club coupe with the M6 transmission. It was working well with smooth almost imperceptible up shifts. I drove about 3 miles and turned around for the ride home. Then it hung in low and didn't up shift. I released the gas pedal again and then it shifted. It did this twice. My father had a Dodge with the M6 transmissions and I remember him opening a panel on the transmission hump and cleaning something. I was a kid of about 10 and didn't understand what he did. But after he worked on it the up shift would be really crisp. What did he clean that would do that? What else should I check to make it up shift?

Mark


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PostPosted: Mon Sep 11, 2017 4:25 am 
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Location: Ann Arbor, MI United States
Hi Mark,

I've had several of those transmissions through the years. I'm probably the only person in the world who actually likes them. To be able to diagnose one you have to know how they work, so my answer is kind of long.

On postwar cars, the transmission shifts hydraulically using pressure from a pump on the output shaft. On the right side of the transmission is a solenoid valve that controls hydraulic pressure. When that valve is de-energized, it allows fluid to go to the upshift piston, so when you back off and unload the synchronizers, the piston pushes the synchronizer forward and it shifts. When the valve is energized, it dumps pressure and a spring returns the piston moving the synchronizer back forcing a downshift. On the S11, power to that valve is controlled by a relay mounted near the air cleaner bracket. S13 and later cars did not use a relay.

In order to upshift, the transmission has to have the solenoid de-energized, and the engine has to be idling low enough (500 RPM) so that the synchronizers unload during the back-out. The relay that controls the solenoid is itself energized by either a switch on the carburetor activated at wide-open-throttle, or a set of governor contact points on the side of the transmission. Those points should close on coast at about 9 MPH, and open on acceleration at about 12 MPH.

So- the conclusion of the long answer is that you might want to check your idle speed to make sure it is around 500 RPM, and then check the contact points in the governor to make sure they are clean and adjusted so that they reliably open at around 12 MPH. The transmission section of the shop manual gives pretty good instructions for accessing that governor (through the trans access panel just like your dad's Dodge) and for cleaning and setting the points. You might also want to check the oil level in the transmission. Low oil can starve the pump and prevent upshifts. The shop manual recommends SAE 10 oil.

Hope this helps! Let me know what you find!

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Brent Jacobsen
Owner of a 1952 Desoto Firedome


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PostPosted: Tue Sep 12, 2017 12:39 am 
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I'm confused - but what else is new.

When I drive my '48 Custom 4-dr sedan, I ALWAYS have to let up on the gas in order for the Fluid Drive transmission to shift up. (And if that doesn't work, then putting in the clutch usually does the trick.) Is Jim saying that he usually does NOT have to let up on the gas for his transmission to shift up? Granted, cleaning the points and/or topping off the transmission fluid shouldn't hurt his transmission, but I'm not sure it's going to change anything. Thoughts?

Bill


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PostPosted: Tue Sep 12, 2017 3:15 am 
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Bill,

I think Mark is seeing something I saw on my 1949 S13 after I first got it back in 1973. It would not consistently upshift on throttle back-outs at lower speeds. I reset the contact points (they were dirty and too tight), and reset my idle (it was too fast). That helped fix the inconsistent shifts, but I still had a periodic case of pump cavitation noise when the car was cold, and some incidents of inconsistent shifts still occurred. I ended up draining the SAE 10 oil out of it, filling it with Dexron automatic trans fluid and cleaning the pump inlet filter (easier on your cars than on my S13). I drained and refilled the trans with fresh Dexron after about 500 miles on the first fill of Dexron. Honestly, switching to Dexron did more to improve the trans function than anything else I did. The pump cavitation noise vanished, and upshifts were quicker and very reliable. It was still shifting great when I sold it in 1997.

I have been running ATF +4 in my '52. It shifts very fast and reliably too.

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Brent Jacobsen
Owner of a 1952 Desoto Firedome


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PostPosted: Tue Sep 26, 2017 1:36 am 
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Thanks for the replies about my tranny. I have been on vacation for 2 weeks. That is why I haven't responded. The idle is set to about 480. When I drove the car (only about 6 miles) the transmission shifted well although my memory of Dad's Dodge was that there should be an audible "clunk". It shifted with no sound at all. But after 3 miles it didn't shift until I ran it up much faster and let off the gas then it shifted. It downshifted every time I stopped.
I bought some sae 10 non-detergent motor oil today. Is that the right oil although I see that auto transmission fluid will work too? I removed the panel above the transmission and I can see 3 plugs. One large one about 1 1/8, a small pipe plug almost level with the large plug, and a lower plug that appears to be the drain plug. So how can I tell when the tranny is full?
I can see a round cover with 4 screws and a wire running to it so I assume the points are under this cover. Do I just remove the screws and lift the cover? If I clean the points is there a gap or other adjustment I should make?
I have an old Motor Repair Manual but it doesn't show much detail regarding filling the transmission and nothing at all about the points. I plan to order a shop manual, but would like to drive it while waiting on the manual.


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