1950 Plymouth overdrive transmission



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When I bought my car, I knew that I wanted to use an overdrive, but I didn't know what year they were introduced by the factory.  And I didn't know if they were a straight swap for the original transmission either.  After doing some homework, and searching the internet-listed junkyards, I found a '55 Dodge overdrive/tranny at Freman's junkyard in Montana.  I was told that the unit was a fluid drive version, and that the only difference was the input pinion shaft.  I was also told by the seller that the input shafts are interchangeable and that they simply pull out from the front of the case.  Once the shafts are changed, it is a direct swap.

So I bought the unit and had it shipped.  Once I had the unit, looked it over closely, and had a lengthy conversation with Neil Riddle, it was clear that the input pinion shafts would not interchange.  The original '50 tranny had a strut type synchro and the '55 tranny had a pin type synchro.  And the '50 pinion does not pull out from the front of the case.

At this point, I didn't want to search for a late model tranny for a pinion shaft donor, and wasn't wild about the idea of tearing into a transmission.  So I started another search for an overdrive that I could just install without much work.  I found another one in Arkansas, and when I got it I put it in the shop thinking I wouldn't have to worry about it for a while.  I bought a 12v solenoid from Neil Riddle, and when I took the old one off of the second overdrive, it was not the same as the one I got from Neil, or the same as the one on the first overdrive.  It turned out that the second overdrive was an earlier type, made in 1940.  I had no idea that there were different types, and learned that the early type, the R-7, was similar to the later type, the R-10, but was not as user friendly and very hard to get parts for.  At first glance they looked the same, but upon closer inspection, they were actually quite different.

Back to square one.  I sold the R-7 unit to a Plymouth Owner's club member who was looking for one, for what I paid for it, and was left with the fluid drive unit that I started with. So I started looking into getting the parts I needed and discovered that everything I needed was readily available NOS.  Unfortunately, Idaho Transmission Warehouse no longer carries these parts.  

Now that I have the parts I need, all I have to do is find the time to get them installed.  I will photo the process as I go, and post them here in this page of the site.

Also, I have collected a bunch of overdrive repair literature, so let me know if you would like copies.

'55 Dodge fluid drive
1940 R-7 unit
new pinion
New 12v relay
Fluid drive R-10 unit
1940 R-7 unit
 New pinion shaft
New 12v relay
New kickdown switch
small parts

New Kickdown switch
Small Parts Kit



I finally started my overdrive rebuild.  Below are a few photos of what I found inside after dissassembling the tranny case and overdrive case.

I dreaded taking this apart, having no knowledge at all of the inner workings of a transmission.  But, with the advice of David Pollock, and a half-dozen manuals of various types, I jumped in feet first.  I found that it was not rocket science as I had feared, and that it was really a very straight forward process.  

The first step in taking the units apart was to seperate the overdrive case from the transmission case.  To do this part, first the shift forks have to come out of the gear shift housing hole, as well as the shift fork guide which is a long rod that screws out from the front of the case.  One of the shift rails will pull out from the front of the case, and the other will come out from the rear of the case as the overdrive case seperates.  The transmission main shaft and gears will pull out of the tranny case with the overdrive.  The sychro gear will come out from the rear of the case too.  

The counter shaft gears roll on a shaft inside the tranny case.  This shaft is driven from the front of the case out the back, as there is a key in the rear end of the shaft.

There is also a small shaft that holds the reverse idler gear, and it comes out the same way as the counter shaft gears.

The counter shaft gears and the reverse idler gear have roller pin bearings and several washers at each end, which fall out when the shaft is removed.  I'm told that getting them back together isn't that difficult, by using heavy grease to hold the bearings in place untill the shaft goes back in.  The manual lists a tool for this purpose, but I don't know what it looks like.  I will have a spare shaft from my old tranny, so I may cut it down to match the overall length of the countershaft gear with washers, to hold everyting together.

The only trick to getting the overdrive case apart was driving out a tapered pin that holds the cable controll lever shaft in place.  Once the pin is out, the shaft pulls outward and releases from the rail inside the case.

The rest of the parts are held in place by snap rings here and there.  

So here's what I found inside my tranny/overdrive.  When I pulled the mainshaft and all of its gears out, I looked inside the case at the countershaft gears, and they looked fine.  At first, I wasn't even going to take them out, since all I needed to do was replace the syncho gears and the input pinion shaft.  But upon closer inspection, I noticed some small pitting on one of the counter shaft gears that prompted me to look at the main shaft gears.  I hadn't noticed at first, but the low/reverse sliding gear was missing a few teeth and was very pitted. Then I checked out the reverse idler gear in the case and found that it was torn up a bit too.  It looks like someone had hit reverse while moving forward and chipped the teeth off of the gears.  My first thought was, where did the teeth go?  I assume they went throughout the tranny/overdrive, and bearings.  This was a big dissapointment to me, since I could have had the new parts in and everything back together in an hour.

Next, I got the overdrive case apart, and found bad news there too.  Somehow, the retainer clip on the end of the tranny main shaft that holds the freewheeliing clutch cam to the end of the shaft had come off, and was ground up.  The roller bearing cage on the clutch cam was bent, and badly worn too from the retainer clip parts whirling around.

That's the bad news (other than the expense, this overdrive wasn't cheap to start with).  The good news is that the gears inside my original tranny with 86,000 miles looked like new, with hardly any visible wear.  So it immediately became a donor for the new tranny/od.  The countershaft gears, low/reverse gear, and reverse idler gear are a direct swap into the newer unit.  I will, however, have to find new parts for the overdrive.  I may be able to hone the freewheeling outer race and salvage it, but the roler bearing cage is toast.

I'll post more when I get the parts and start to put all if this back together.  Pete

low/reverse gear, note the missing teeth
clutch cam
pinion cage
pinion cage
low/reverse sliding gear, note the missing teeth .
overdrive freewheeling clutch cam, note the wear in the bearing holes.
 pinion cage, you can see where the retainer clip beat the surface of the cage as it was ground up.  Amazingly, the gears show no wear.
pinion cage, the groove on the bottom is an oil slinger, and it was packed with sludge.
ring gear

this is the pinion ring gear and the freewheeling clutch bearing race.  If I can't find a new bearing race, I will try to hone it a bit.



I finished putting the overdrive/tranny back together over the weekend, and to my surprise, it really wasn't that hard to do.  Below are a few photos that show various stages.

I washed all of the parts in a solvent tank to remove the grime and sludge.  I could easily tell which parts came out of the original '50 three speed tranny, and those that came out of the '55 overdrive/tranny, because it doesn't appear that the fluid was ever changed in the '55.  All of the parts from the '55 are stained a dark brown color, and the '50 parts are bright and clean.  The counter shafts from the '55 were visibly worn too, and I could feel a groove on both of them where the bearings had been.

The next step in the assembly was the counter shaft gears, which came out of the '50 tranny.  I put the small reverse Idler gear in first, as it is the easiest.  I then put in the cluster gear.  I used shaft from the '55, cut down to match the length of the cluster gear and washers, to hold the bearings and washers in place inside the cluster gear while I got it positioned inside the case.  I used a bit of grease to hold the bearings inside the gears.  Once it was in place, I drove the cut-down shaft out the rear of the case with a long screwdriver and then sliped the regular shaft back in its place before the bearings fell out.  It worked great, and was probably the easiest part.

The next step was to assemble the tranny main shaft.  This shaft goes through the OD adapter plate and also holds all of the OD gears.  I replaced the bearing first, as it mounts in the adapter plate and on the shaft with snap rings.  The rest of the gears slide on the shaft and are held with u-clips and snap rings.  I used the syncro gears from the '55, and the low/reverse sliding gear from the '50.  The small parts kit came with new snap rings.

I got a used replacement roller cage for the freewheeling clutch from Idaho Transmission Warehouse, and used a bit of grease to hold the rollers in place while the ring gear/rear shaft was slipped on over the clutch.  

Then the OD case went back on, and this was probably the hardest part for me.  It took me three or four tries to get the od shift rail in the right place and working with the control shaft.  Since it is entirely inside the od case except for the end of the shaft, its hard to tell if it is rotated to the right spot for the control lever shaft to engage.  I noticed that the control lever shaft has a dimple in the side for the electro swtich, so I knew where it should be positioned in the case.  After messing with it for a while, it sliped together and seemed to work like it should.

Then I put the sychro gears inside the tranny case, with the shift fork and shift rail that goes in from the rear of the case, and slid the main shaft into the case.  This part simply fell together no sweat. The first/reverse shift fork and shift rail went in next, with the shift fork guide rod, from the front of the case.   After I had bolted everything down, I noticed that the od shift rail was stuck, and did not spring back out of the case when out of reverse.  I couldn't figure out why until I put the solenoid on.  Apparently the soleoid pawl has to be pulled out in order for the shift rail to move freely.  Now everything seems to work great.  

I'm really happy that I took the time to rebuild this overdrive now, because I have a first hand knowledge of what is in there, and how it works.  Transmissions, especially overdrives, where a complete mystery to me, and was skeptical about being able to do the work myself.  And if I had not had to replace the input pinion shaft, I would have simply put the unit in the car and hoped for the best.  And would have been very dissapointed at the results.

It should last a long time now, and be trouble free, knock on wood!

When I find time, and probably after the engine is re-assembled, I am going to degrease the outside of the case one more time and paint it with the factory silver engine color.

Here's the cluster gear and shafts, about to go in.
This is the reverse idler gear in place
This is the new pinion input shaft, with new bearing.
Here's a shot of the counter shaft gears in place though the shift fork hole.
cage parts
This is the main shaft, assembled and in the OD adapter plate ready to go into the case.  the shaft on the left is the ring gear and drum for the OD.
This is the freewheeling clutch and planetary gears for the OD, with the new roller cage and clips installed.
This shot was taken just before putting the two halves back together.
 These are the latest parts from Idaho Transmission Warehouse, the new cage, rollers, and clips.  Note, there are 12 rollers, but for some reason they only sent 10.  I used two of my old ones that looked fine.


Kickdown Switch and wiring:

Here's how the factory wired the overdrive kickdown switch:

od wiring

The factory kickdown switch is a two circuit switch, the first circuit interupts power to the solenoid, and the second interupts power to the ignition to release the torque from the solenoid pawl so that the solenoid will release. Here's the factory switch, it's big and ugly...

The switch was mounted from the factory to the throttle linkage so that it would operate when the throttle was pushed to wide open. This allowed a full throttle kickdown much like a modern automatic.

I had the factory two circuit switch, so I wired it according to the factory setup, and mounted it under the edge of the dash. Here's where I put it.

To shift out of overdrive, I had to reach down and press the button. The first thing I found was that since I'm using a mini GM HEI distributor wired through a fuse block, when the kick down interupted the ignition circuit, it would blow the fuse to the coil and I'd be dead in the water.

I decided that I didn't really care about full throttle kickdowns, so I removed the ignition circuit from the switch. This meant that in order to kickdown the overdrive, I'd have to release the throttle instead of full throttle, because the overdrive solenoid will not release if there is torque on the tranny from the engine. No big deal really.

After driving the car this way for a while, I decided I really liked the kickdown seperate from the throttle, since it gives the driver more control and you can kickdown without accelerating, which comes in handy at times. The thing I didn't like about the setup was reaching down to the edge of the dash every time I wanted to press the button, and since the button was meant to be pressed by the throttle linkage, it wasn't easy on the finger to push it in.

I'd seen an e-bay auction some time ago for a vintage shift knob that had a button in the end of it, which I thought would be a great place to put the overdrive kickdown on my car. So I went to my local radio shack and looked through the buttons they had, and found a tiny little momentary switch that fits inside the threaded hole of the plastic shifter knob. The switch came in a package of two, one red and one black.

I drilled a hole in the end of the knob for the switch, soldered wires to the terminals on the switch and covered them with shrink tube, and put it back together.

Here's a shot of the knob with the switch inside it.

The shift lever on the car is hollow, so the wires from the switch run through the lever to the column, and then down around the bottom to the join with the turn signal wires that run up under the dash.  The shift lever necks down a bit just below the threads, so I used a drill bit to open up the hole just a bit to fit both of the wires through the skinny part.

Finished install.

I didn't occur to me that the circuit of the factory switch was normally closed, and the new switch is normally open. So, when I wired the switch into the same wires that were on the factory switch, I didn't have overdrive unless I held the button down. DOH!

To solve the problem, I went back to radio shack and bought a 12v relay. I had no idea how to wire a relay, so I got out a tester and messed with it till I figured out how it worked. It helped that the relay has a clear case, so I could see what was going on in there.

Once I figured out the relay, I wired it into the system, and now when I press the button on the shifter, it sends 12v power to the relay, which then opens the circuit in the overdrive solenoid. Now I can kickdown the overdrive without reaching down to the bottom of the dash.

And a real bonus to having the switch on the shifter is that I can "click-shift" the tranny up through the gears, for five forward gears. Third gear on these trannys is fairly tall, so shifting from second over to third over is a big jump. Being able to use third direct from second over fills the gap. I start out in first direct, and shift manually into second direct. Once the car reaches 25 mph, the overdrive relay kicks in, and I let off the throttle to automatically shift into second over. Then, accelerating up to the next gear, I manually shift into third while pressing the button on the shifter, which puts the car into third direct.  Accelerate again and then release the throttle and the tranny automatically shifts into third over.

Now that I have driven the car for period of time with the overdrive, I think adding the OD to the car has been the single best thing I've done for the car.  It makes it far more comfortable on the highway with significantly less engine noise, less strain on the engine, higher speeds effortlessly, and better gas mileage.  And, around town it reduces the amount of shifting because you can shift in and out of overdrive in second gear automatically as traffic speeds up and slows back down.  Worth every penny...




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