Complete Overdrive Conversion for ’78-’79 Ford Bronco

By Danny Cabral

The objective of this article is to provide you with the knowledge and
identity of components needed to perform an automatic overdrive
transmission conversion on our ’78-’79 Ford Broncos. We will be
converting from a Ford C-6 transmission to a Ford AOD (Automatic Over
Drive) transmission. Furthermore, this AOD transmission, from Art Carr
Performance Products, will be able to withstand 800 horsepower! Every
transmission they build is dyno tested. Some of their more reputable
modifications include; tighter tolerances, 8 Hi Pro direct friction disk
clutch pack with kolene steel plates, Hi-Tech valve body shift kit, 2"
HD overdrive band, needle bearing thrust washers, overdrive "A" servo,
zero leak down accumulator, "wide ratio" planetary gearset, one piece
300M input shaft and higher stall HD Super Torque converter. Art Carr
Performance Products calls it their "Extreme AOD" and they’re world
renowned for their bulletproof transmissions.


I highly recommend purchasing a performance rebuilt AOD transmission, since
the factory
stock AODs were notorious for their weakness. Aftermarket transmission
companies (such as Art Carr, Jet Perf., Baumann Engineering, TCI, B&M,
FB Perf., etc.) correct and modify every inherent flaw or weak point in
their high performance transmissions. If you insist on rebuilding an AOD
yourself, despite my advice, then be certain to find an ’88 or later
year transmission core because these had the enhanced lubrication for
the rear output shaft (major improvement) as well as other less
significant improvements. Obviously, you will also need to find a four
wheel drive model AOD. Ford used the AOD in their F-series trucks and
Broncos from the mid ’80′s to the early ’90′s until the AOD-E
(electronic version) was introduced. The AOD (when rebuilt with
performance modifications) has several advantages compared to a C-6. The
AOD is more compact and lighter in weight than the C-6, however, it’s
overall case length is half an inch longer at 20.5 inches. We will NOT
be replacing or relocating the transmission crossmember for this reason.

The AOD has better shifting characteristics because it employs a
throttle valve cable to control shift points in relation to throttle
position. This function is used instead of the vacuum modulator and
kick-down rod that a C-6 uses. The AOD is less leak prone because of the
lack of external case protrusions such as an intermediate band
adjustment, servo cover, vacuum modulator and the neutral safety/reverse
switch is mounted higher above the fluid level. In addition to this, the
gear shift lever and throttle valve lever have their own seal. (The C-6
uses one seal for the gear shift lever and an o-ring for the kickdown
lever.)

The AOD "wide ratio" planetary gearset gives you a 2.84:1 first
gear ratio, the lowest of any Ford transmission and a 1.55:1 second gear
ratio. (Just use your imagination for what this did to my Bronco’s
acceleration with a built 460 engine.) Acceleration and towing power is
greatly improved. Speaking of towing power, Art Carr Performance
Products claims their extreme transmissions are so strong, that they
allow you to tow in overdrive. (I can’t imagine doing so, since most of
our engines wouldn’t be in their powerband.) Last but not least, the
.67:1 fourth gear gives you a 33% overdrive factor. This is probably the
most significant reason most of us want this transmission. Fuel economy
is improved and interior engine noise is decreased.

Let’s begin with the front of the AOD and work our way to the rear. The
first implication is the bellhousing bolt pattern. The AOD was only
produced with the small block bolt pattern, so unless you’ve installed a
302 or 351W into your ’78-’79 Bronco, you will need an adapter plate.
The adapter plate will mate an AOD transmission to a 351M, 400M, 429
(1.375 crank pilot hole) or a 460 engine. The adapter plate is a 3/8"
thick aluminum plate that is shaped identical to a big block on the
outside and shaped like a small block on the inside. It also positions
the big block starter (351M-460) in the correct location. Your starter
will need the threads drilled out of the top hole, making the two bolt
holes on the starter identical. The reason for this is, the starter
mounting holes on the AOD case are both threaded. You will need two 3/8"
x 2" long grade 8 bolts to mount the starter. (A word of caution: check
the clearance between the teeth on the flexplate and the nose cone of
the starter because I had to grind a little off the nose cone in this
spot since there was slight contact.) This adapter plate is used in
place of the factory engine plate and is from Art Carr Performance
Products part #22600. A custom built torque converter must be used in
conjunction with the adapter plate.

The torque converter is
dimensionally a C-6 on one side and an AOD on the other, for obvious
reasons. Art Carr Performance Products will discuss your needs and
vehicle specifications with you before building you a suitable stall
speed torque converter. They will build you a non lock-up torque
converter with a solid one piece input shaft made from 300M steel,
virtually unbreakable. (The lock-up torque converter with a two piece
input shaft is the weakest link of an AOD.) The AOD gear shift pattern
is as follows: P-R-N-OD-D-1. Ford deleted the number 2 manual shift and
put D (drive) in it’s position, so you can still shift out of overdrive
if you desire. Since there are still only six gear shift detents, we can
retain the ’78-’79 Bronco automatic column shifter. We will be retaining
the column shifter, linkage rod and frame mounted bellcrank without any
modifications to them. For the AOD, we need to modify the C-6 bracket
that the frame mounted bellcrank rod pivots on. Unbolt the bracket (two
bolts) from the C-6. You’ll notice the AOD only has one threaded
mounting boss and a 3/8" projected hole down near the corner of the pan.

Fortunately, the single threaded mounting boss on the AOD is in the
exact same position as the lower threaded mounting boss on the C-6. We
only need to fabricate a means of attaching the upper hole of the C-6
bracket to the lower projected hole on the AOD. I (being a skilled
fabricator) welded a rod (from the top hole) to the head of a 3/8" bolt
fitted through the projected hole with a lock washer and nut. (This is
easily done after the transmission is installed in the vehicle, so
you’re able to see what alignment/position the bracket needs.) We will
also be retaining the C-6 linkage rod (about 5" long) from the modified
C-6 bracket to the gear shift lever on the AOD. The AOD gear shift lever
needs to point just past the one o’clock position. Ford used a few
different gear shift levers that pointed in a few different positions,
however, the one that points just past the one o’clock position, matches
the one used on the C-6 equipped Bronco.

Unfortunately, for those of you
who choose to rebuild an AOD, I only have the engineering number (cast
into the lever) for the correct AOD gear shift lever and it’s #BBEOWP.
Actually, this might even be easier, since it’s cast into the lever in
plain view. If you purchase your AOD from an aftermarket performance
company, they will discuss this with you and use the correct gear shift
lever. As mentioned earlier, the AOD uses a throttle valve cable to
control shift points, instead of a kickdown rod and vacuum modulator.
Lokar makes a stainless steel throttle valve cable kit specifically for
conversions and supplies everything you’ll need to hook it up to a
carburetor or throttle body. It’s a great versatile kit and it’s Lokar
part #KD-2AODHT. The neutral safety/reverse switch is still a four wire
switch, but a better design, because it threads into the case up higher,
with an o-ring. I purchased a pigtail sub-harness from Windsor-Fox
Performance part #HRN-AODA. Simply connect the four corresponding wires
to the ’78-’79 Bronco chassis wire harness.

Since the AOD was never an option in our ’78-’79 Broncos, there isn’t a
factory formed dipstick that will fit. Lokar makes a beautiful,
stainless steel braided flexible dipstick assembly that works perfectly.
I especially like the double o-ring seal, where it fits into the
transmission case. It’s Lokar part #TD-3AODFM. The transmission oil pan
always has a simple purpose, however, I prefer to have a drain plug and
extra fluid capacity for increased cooling. I purchased an Art Carr
Performance Products part #10500 aluminum deep pan which holds 2 more
quarts of fluid, has a drain plug and does not leak because the rigid
aluminum flange resists warpage. The output and return ports for the
transmission oil cooler lines are 1/4" pipe thread. I simply obtained
brass barbed fittings (from my local auto parts store) and six inches of
5/16" transmission cooler hose for each line. You then need to cut and
flare the existing lines, toward the ends, and make the appropriate
connections. The return line is connected to the bottom port. As always,
install a good quality oil cooler – every aftermarket performance
transmission company makes them.

I stated earlier that the case length is only a half an inch longer
(20.5"), so we will be retaining the factory crossmember in its factory
location, although two minor modifications are necessary. First, the
slots in the crossmember for the transmission mount need to be elongated
about half an inch rearward. The reason for this, is the half inch
increase in case length and the 3/8" thickness of the engine adapter
plate move the mount rearward a little. Second, a notch needs to be cut
in the middle front edge of the crossmember to access the drain plug
which is located toward the rear on my oil pan. You will not need to cut
this notch if you use the factory pan or you find an aftermarket pan
with the drain plug centered in the pan. We will be retaining the C-6
transmission mount and bolts without any modifications. The case
dimensions pertaining to the transmission mount are identical for both
AOD and C-6.

Ford always used the same circular six bolt pattern with a 31 spline
output shaft on their fullsize Bronco and truck transmissions. Excluding
driveshaft length issues, as long as you have the matching extention
housing for a particular output shaft, you can retrofit different
transfer cases on the C-6. This is even easier on the AOD because
there’s only one four wheel drive model. This enables us to retain the
New Process 203 full time transfer case or the New Process 205 part time
transfer case that our ’78-’79 Broncos were originally equipped with. If
you have a NP203, you may procede with bolting on the transfer case. If
you have a NP205, you have an additional modification to make. We need
to grind or cut a notch on the AOD extention housing for the hi/low
range shift rail on the NP205. This is the upper shift rail shaft that
actually cuts into the circular six bolt pattern. The extention housing
on the C-6 (and NP435 manual transmission) is cast this way from the
factory for the NP205. The NP205 was never used behind the AOD, hence
the reason for this modification. Use the gasket as a template to
transfer the outline of the notch onto the AOD extention housing. Drill
two holes for the inside corners and then cut the notch out. There is
also a rib that is aligned within this notch that will need to be ground
flush (2" long from the transfer case mounting surface) with the outer
surface of the extention housing. This is to allow the hi/low range rail
to move in and out as you shift the transfer case.

The AOD extention
housing has two threaded mounting bosses for the transfer case shift
lever. The mounting bosses project outward too far, so you will need to
cut a 1/2" off each one. This is done to center the shift lever through
the hole in the floor. You will also need to cut 1/2" of threads off of
the NP205 shift lever mounting bolt itself. We will be using the front
mounting boss only to attach the NP205 gear shift lever and bolt to the
extension housing. The link between the shift lever and the NP205 shift
rail shafts will need to be lengthened or shortened depending on the
length of the C-6 extention housing you had. If you have a NP203, you
will not need to grind or cut the notch (and rib) on the AOD extention
housing, however, you will need to modify the two threaded mounting
bosses to work with the different type of gear shift lever mechanism. My
Bronco has a NP205, so I don’t know what exactly needs to be done to
attach the NP203 shift lever, but I am certain it can be done quite
easily.

If you own a ’78 Bronco, now is the best time to upgrade to the ’79
Bronco driveshafts, if you haven’t already done so. They are 3 7/8" yoke
width with 1330 series u-joints, as opposed to 3 1/2" wide with 1310
series u-joints. We will be using the larger CV (double cardan) type
driveshafts front and rear. The rear driveshaft we need is from a ’79
Bronco equipped with a manual (NP435) transmission and NP205 transfer
case. It is a 37 1/4" collapsed length driveshaft and fits our
conversion correctly – JBG part #2020. (If you can’t find one or just
choose to purchase a new one, as I did, Jeff’s Bronco Graveyard (JBG)
sells new Spicer driveshafts and flange yokes that are drop shipped from
the Dana Spicer plant to your home residence.) The front driveshaft we
need is from a ’79 Bronco equipped with an automatic (C-6) transmission
and NP205 transfer case. It is a 27 1/4" collasped length driveshaft and
fits our conversion correctly – JBG part #2003. (These are the
applications described in the JBG catalog – the measurements are
accurate.)

I realize the cost of this AOD conversion is very expensive (I spent
$3500 total on this conversion – $2500 for the Art Carr transmission
alone), however, I wasn’t concerned with the initial cost and I’m not
looking to recoup my investment with fuel economy. This AOD conversion
became a personal vendetta against transmission/driveline shops and off
road/four wheel drive magazine publications who say it can not be done.
Well, contrary to popular belief, I now own a highly modified daily
driven ’78 Bronco Ranger XLT equipped with a bulletproof AOD
transmission and enjoying the benefits; awesome acceleration, reduced
interior engine noise, increased fuel economy and better all around
driveability. I personally performed this entire AOD conversion over a
year ago and I absolutely LOVE IT!! I have not had any problems at all.
Disclaimer: I strongly recommend that all work be performed by a
competent and licensed mechanic or automotive repair shop. I will not be
held liable for poor workmanship or inferior quality components.

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