Borg Warner Overdrive Transmission
by Don Frolich

The first production offering of overdrive on domestic automobiles was on the 1934 Chryslers and DeSotos, an event no doubt greatly overshadowed by the radically-new Airflow theme offered that year by those same two marques. Itís perhaps ironic, then, that this option was not available on Plymouths until mid-year 1952 production an, only a few years before its popularity would wane in favor of the ubiquitous automatic transmission and a resurrection of the 4 speed manual box.

Detroit had tried to compensate for the power and RPM limitations of the early thirties engine by means of four speed transmissions, but these were expensive. Moreover, drivers apparently were then unwilling to cope with the additional shifting necessary to take full advantage of them. In the fact, surveys showed that a great deal many 4 speed owners seldom, if ever, used all four ratios in normal driving.

The 1934 Chrysler and DeSoto overdrives (Borg-Warner units, developed in conjunction with Mopar engineers) offered a good compromise solution to this problem, and between 1935 and 1942 eight other marques jumped on the bandwagon with the same Warner overdrives, which soon acquired reputations for both high speed and economy.

Both as originally conceived and in its later generation format on the 1952 Plymouth, the overdrive was essentially a small case bolted to the back of the regular 3 speed transmission having a planetary gearset, which when engaged, reduced engine RPMís by 30 percent at a given car speed.

The first overdrive had no "kickdown" feature; once above the engagement speed (45 mph on the 1934ís) you momentarily let up on the gas to engage the overdrive, at which point the car was locked into that drive ratio until you again dropped below engagement speed. It could be locked out manually by a dashboard cable, but only below engagement speed, thus the engineersí trade-offs in selecting the engagement speed were formidable problems. Too high, it would seldom be used; too low, the car would always be lugging, with very poor responsiveness in traffic, especially at higher attitudes.

The fully automatic electrically operated version was introduced in 1939, and its final form, first appeared in 1946, was operationally unchanged but was simplified and less bulky that its ancestors.

Having pioneered the use of overdrive, itĎs surprising what a meager use Chrysler Corporation made of it over the years. Chrysler and DeSoto offered it from the 1934 to 1940 (and 1941 eights); it next appeared in the fifities on Dodge and DeSoto (until 1956) and Plymouth (through 1959) Dodge trucks could be ordered with overdrives from 1954 to 1956. Most independents relied heavily on the overdrive from the thirties until 1963; Ford and Mercury used it from 1949 to 1963; and Chevrolet (the only GM user) from 1955 to 1963.

The basic element of an overdrive is a single epicyclic or planetary gearset, consisting of: (1) a central, externally toothed sun gear; (2) a set of smaller externally toothed planet gears arranged around (and meshing with) the outside of the sun gear, their "axles" collectively supported by a carrier; and (3) an internally toothed ring gear surrounding (and meshing with) the several planet gears. A planetary gearset is in constant mesh. And it can be made to step rotational speeds up or down, or even reverse direction, by judicious choice of which of the three elements is driven and which is prevented from rotating.

For example, if the planet carrier is made to rotate around a fixed central sun gears, than the planet gears "walk" around the sun gear and at the same time revolve about their own individual axes as well (just as heavenly planets both spin on their own axes and also travel around the sun, hence the nomenclature). The planet gears in turn drive the ring gear (output) at some rate faster that the input speed (1.4 times as fast in the overdrive).

Planetary Gearset

As used in the overdrive, automatic shifting from conventional to overdrive and vice-versa is dependent on an electric-solenoid-operated pawl which can be made to hold the sun gear stationary, an over-running clutch on the output shaft, and three electrical circuits: control, solenoid and kickdown. The control circuit (Fig. 1) closes the overdrive relay when: the ignition switch is on, the dash control handle pushed in (engaged), car speed is above the approximately 25 mph (sensed by a governor switch on the transmission output), and the kickdown switch plunger in not depressed.

Fig. 1 Control Circuit

The solenoid circuit (Fig. 2) simply provides current to the solenoid when the relay is closed.

Fig. 2 Solenoid Circuit

When the kickdown switch (Fig. 3) plunger is depressed, two things happen: (1) the normally closed contacts in the solenoid circuit are opened, de-energizing the solenoid; the solenoid spring attempts to withdraw the pawl but the engine torque holds it in. (2) A normally open set of contacts is closed, which completes a circuit from the ignition coil to the solenoid and thence, inside the solenoid, to ground. This interrupts engine torque so the solenoid can retract, which in turns opens the ground contact inside the solenoid, thus immediately restoring ignition function and thus engine power. On the upshift into overdrive, the free wheeling effect of the over running clutch when the accelerator is lifted unloads the torque path (drive line) sufficiently for the solenoid to engage without ignition interruption.

Fig. 3 Kick Down Circuit

As the interchange list indicates, several fifties era Mopar overdrives can be interchanges directly with the transmission assembly (with extension) on many non- overdrive cars back at least to 1941. The overdrive units themselves are all the same; also the transmission in overdrive equipped cars is essentially the same (as non-O.D.) except for a rail which runs internally from the transmission to the overdrive and serves to lock out the overdrive whenever the car is put into reverse, otherwise the over-running clutch would not transmit reverse torque to the rear wheels.

Thus if you find an overdrive transmission that seems correct in most details, but differs, say, transmission input shaft, you can interchange parts between the two transmissions. However donít try to attach just the overdrive part onto your non-overdrive transmission: it can be done but is tricky, requiring special machining of the case and some other parts.

In addition to the complete transmission-overdrive assembly, the following control parts are required to complete the conversion: the solenoid, control lever, and governor switch, all of which fasten into of onto the overdrive housing: the control cable and dashboard handle; a kickdown switch and bracket, which mounts on the carburetor linkage; and the overdrive relay, which mounts under the hood. Try to get the wiring harness also, either to use of for a pattern for replacement. On my 1949 Plymouth, I mounted the kickdown switch through the floorboard but under the mat just to the left of the clutch pedal pad, so I could get kickdown without dumping the carburetor accelerator pumps, or if I wished, go from second overdrive to high "overdrive" by rolling my left foot outward so as to hit the kickdown as I depressed the clutch for the shift. Later, to accommodate my wife, whose natural action when she wanted more power was to press down the gas pedal, not the left floorboard, I added a second kickdown switch in the conventional location on the carburetor. The normally closed (control circuit) contacts on the two switches must be wired in series, and the two sets of normally open (ignition grounding) contacts connected in parallel.

Anyone who makes the conversion should be aware that a given replacement overdrive (or any other) transmission may have come from a car with a different rear axle ratio, especially since in most years standard ratios were different for cars with and without overdrive. Usually, just replace the speedometer drive pinion in the new transmission with the one from the old to correct the speedometer. Also, be certain that the solenoid circuit is wired with No. 10 wire, and that it is protected with a 14 amp fuse at the battery terminal on the relay.

As to standard rear end ratios, from 1949 through 1952, the 118-1/2 " wheel base cars used 3.9 except the 4 door wagon used 4.1 and the 111" wheel base cars uses 3.73. In 1953 and 1954, non overdrive cars used 3.73 and overdrive, 4.1. A car with 3.73 gears may no work too well with the overdrive: what with Dr. Fedís Magic Elixir, the Great 55 MPH Hoax , you may never drive fast enough to avoid lugging the engine in overdrive. My í49 with 3.9 gears seemed to be just fine, but of course I drove a lot then at speeds that are now illegal.

Note the effect of the 0.7 overdrive ration on overall drive gearing:

Rear Axle Ratio Overall ratio in O.D.
3.54 -> 2.48
3.73 -> 2.61
3.9 -> 2.73
4.1 -> 2.87
4.3 -> 3.01

The literature is somewhat contradictory on transmission interchange, so judgment and especially visual comparison is important. The swap should work on any standard transmission Plymouth 6 from 1941 through 1956. However, it appears from the interchange listings for the short wheelbase P-17, P19 and P-22 cars a transmission with a shortened extension housing was used. So to use the same drive shaft as the larger cars would mean that installation of an overdrive would require shortening or switching the driveshaft. On the other hand, as far as I can tell, the 1952 P-22 was available with overdrive, yet the interchange listings show no short drive shafts to accommodate the extra length. So on this switch, your guess is as good as (probably better than) mine.

One warning after you have successfully completed your conversion: in order to lock out the overdrive while moving, depress the accelerator so as to be sure it is kicked down to "underdrive", then while continuing to accelerate simply pull out the control handle. It can be engaged at any time- just push in the control.

In these days of ever-increasing gas prices, especially on tour cars, this conversion makes a lot of sense. It can also save wear and tear on the engine, and add considerably to the flexibility and thus your pleasure, in driving the car.