Borg Warner Overdrive Transmission
Webmaster Note: This article, by Don Frolich, was first published in The Plymouth Bulletin
and is used with permission. Additional information and photos have been added for
clarification.
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. It’s 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 RPM’s by 30 percent at a
given car speed.
The first overdrive had no "kickdown" feature; once above the engagement speed (45 mph on
the 1934’s) 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
engineers’ 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, it‘s 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 fifties 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.
Overdrive Schematic
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
don’t 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. Fed’s Magic Elixir, the Great 55 MPH Hoax , you
may never drive fast enough to avoid lugging the engine in overdrive. My ’49 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.
Old
3.54
3.73
3.9
4.1
4.3
->
->
->
->
->
New
2.48
2.61
2.73
2.87
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’s were 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.
Additional Reference Information Added 12/2001
The current guru of overdrive information is Neil Riddle at Riddle's in Washington State
(206-285-6534). He sells a package of 3 mopar factory Service Reference Books including:
"Automatic Overdrive Operation", "Automatic Overdrive Controls" and "Automatic
Overdrive Maintenance". He can also help on locating parts and repair services, but doesn't
usually have complete units available. Rhode Island wiring sells a replacement wiring
harness. The only downside to the overdrive is finding one. As the article noted Chrysler was
sparse in their sales effort and units are not easily located in junkyards. Don't be surprised if
units for sale are approaching the $1,000.00 dollar range. Sellers know the units are rare and
in high demand. One bargain resource are the guys that build hot rods. They often will part
out the entire driveline for next to nothing and do not realize the value of the overdrive unit.
P15 Plymouths being judged in Plymouth Owners Club sanctioned events will receive a
deduction in scoring for having an overdrive unit installed as it was not an available
Plymouth option from Chrysler.