M6 Transmission question

[ezrstl48]

48 DeSoto Coupe, just finished going through the engine and have it up on jack stands to test everything. Up on jack stands the transmission is not shifting by itself into the high range from 1st or 3rd. I have 134 tractor oil in the Fluid Drive convertor, 10W in the trans and the pressure checked good at 40 to 45 psi at the trans. The Relay on the inner fender checks good and sends power to the different terminals as rpm comes up. I can hear the solenoid on the trans as I put elelctrical power to it. I am going through the troubleshooting portion of the manual and will be doing the Mechanical Inspection portion this evening.

My question; is it possible that the transmission would need the weight on the car going down the road to allow the transmission to shift properly? As it is right now it does not make the 'click' or 'clunk' into the high range as I bring rpm up and let off the throttle.

I have a couple other interesting things to add about the adventure that this has been and look forward to posting those soon.

BTW, the 134 in the trans case doesn't seem to work as well as the 10w oil. Seemed much tougher to get into gear...

Tim

[55 Fargo]

Well this trans must have the engine rpm drop to 450-500 rpm, before it shifts up. Sounds like you are covering all the diagnosis, I would contiue, and hopefully someone will chime in with more experience to share and help solve the problem.

BTW, how did it shift prior to your work on it, and is this trnas not a sotck 46-48 M5 trans which is a little different than the later M6.....

[Plymouthy Adams]

it is my understanding that the universal 134 hydraulic fluid and the iso32 are one and the same fluid..both being a 10W oil

[ezrstl48]

I think I found the problem. Will know for sure when I get home this evening. I mistakenly did no fill the fluid drive to the proper level, my feeling is that his is it.

Secondly, I found an older post of someone mentioning that they thought that the 134 might also be a good choice for the gear box with straight 10w oil not being super easy to find. I tried it; no go. At least in my application, the transmission quickly got stuck and would not manually shift into either low or high. Back with the 10w 30 synm and after about 30 seconds of running the engine and getting it into reverse, it all 'broke loose' and smoothly went into gear.

To answer how the trans shifted before all this is that we were told it was good. We bought the car with the engine out of it.

Will post an update soon.

Thanks for the help.

Tim

[greg g]

Keep in mind that the fluid drive unit and the transmission are not interconnected, and the performance of the fluid drive unit will not have a lot of effect on the way the transmission shifts, assuming it is transfering power to the trans.

[ezrstl48]

Adding the correct amount of oil to the Fluid Drive made a positive difference as the speed comes up much easier but still no shift. Tomorrow night I will be doing the mechanical inspection per the manual.

[Dodgeb4ya]

Pull the fuse on the trans relay on the left fender well and drive it. Report back.

[captden29]

i always used hydraulic jack oil in the M-6 in my 1951 windsor. everything worked just fine. it had a leak and would slip when it got low, so i would pull over, add oil, and be on my way. capt den

[Dodgeb4ya]

I've run 10/30 in old and tired M5-6's (low oil pressure or worn input shaft and blocker ring and sleeve) and they shifted 100% better.

[ezrstl48]

Pull the fuse on the trans relay on the left fender well and drive it. Report back.

Willing to do this, but what do you think will be deteremined by this? Will try this tomorrow (Sunday); it will actually be the first time that I have driven it down the road. All my testing has been done on jack stands.

[Dodgeb4ya]

It will eliminate an electrical wiring circuit issue from preventing an upshift. Doing this will cause the trans to upshift imediately-even at 4 miles an hour the second you let off on the gas. It won't downshift at a stop unless you push the clutch in and wait a few seconds. The trans will also not downshift at higher speeds once into an auto upshift- you can manually shift into the lower range with the lever though.. This procedure will quickly prove if the trans will upshift correctly. 1 block of driving and you will know!

Bob

[ezrstl48]

Finally upshifted. My guess is that after the 30 plus years of sitting, the trans just needed to have the oil 'slung' everywhere inside. After about 2 trips around the neighborhood, it finally 'clicked' into high range. Much more quiet than I expected...

I removed the Interrupter Switch to make sure the Piston was moving forward like it should, and it was, even when it wasn't upshifting. So, am I right in thinking that the Direct Speed Fork and Rail were not moving; keeping the trans from upshifting? And that the Engaging Spring was just getting compressed when the Piston was pushed forward...

Thanks for the help!

[Dodgeb4ya]

Hard to say why it wouldn't upshift if your wiring was still hooked up. Thats why I disconnect the fuse and do a quick road test-if it then shifts I can take care of a wiring or contacts/solenoid issue.

Sometimes these M-5 and M-6 tansmissions after sitting will not upshift the 1st time after sitting all winter. Try it again and it shifts. Goota make sure its full of oil and idle speed is 450.

Glad all is OK!