Automatic Overdrive Conversion
I have a 1977 Ford Bronco which I recently converted to a 1991
Mustang GT 5.0 H.O. EFI motor and AOD transmission. I started
this project 3 weeks before leaving for the Easter Jeep Safari
in Moab, Utah, when else would be a better time? Got it
finished the Friday we were leaving, nothing like having all
the time in the world to do shake-down runs and fix all the
little bugs. The engine ran about like what you would expect
of a 5.0 EFI great!! The best is yet to come, the
transmission was well worth the effort involved in doing the
conversion. I have 4.57 gearing with 33" tires. The
overdrive worked out to 2700 rpm at 75 mph and returned gas
mileage in the mid 15s with a high of 17 mpg at the same
speed, no more having to stay in the slow lane being passed by
everything in sight. Are you ready for the good stuff, with
the 2.84 low first gear this transmission is about the best
thing that you'll ever drive with an automatic. It will CREEP
over most obstacles on the trail and you don't have to listen
to all the smart X#?*&! comments from those people with NP435s
about why your brakes lights are on all the time going down
hill - it will crawl downhill.
Installing an Ford AOD (Automatic Overdrive)
Transmission in an early Bronco
- Finding a doner
- The first thing that you need to do is to locate a core from a 1989 or
newer Mustang GT or LX. They have most of the good stuff already in them,
higher shift point governors and usually a larger overdrive servo plus the
rear output shaft and cases on the later transmissions are updated to rear
oiling. Be sure to get the sub-harness for the back-up light and neutral
Obtain an adapter kit
- The only source for these is Advanced Adapters. This will be
neccessary to mate the Ford AOD to your Dana 20 transfer
case Part Number 50-2703
Obtain a low gear ratio set from Ford Motorsports
- This set contains a 2.84 first gear, 1.54 second gear,
1.00 third gear and .67 overdrive. You will also need a
overhaul gasket set and a set of clutches and driven plates
for the forward clutch drum.
Overhaul the transmission
- Have a transmission shop overhaul your new acquisition
using the parts from Advanced Adapters and Ford Motorsport
or do it yourself if you have the tools and ability.
Trial Fit Before Starting
- Before you start to install the AOD in your vehicle,
it is best if you trial fit all the parts together and
install the transfer case shift linkage and make sure that
it works properly before proceeding any further along.
The time spent here will be well worth the effort in not
having to drop the transmission to get your transfer case
shifter linkage working correctly.
Modify Transmission Crossmember
- The transmission crossmember will have to be modified
according to Advanced Adapters instructions and then some.
Be sure you have adequate clearance all away around the
tailshaft housing including between the crossmember and
the tailshaft housing bolts. The crossmember will have to
be relocated rearwards approximately 1 1/2" or simply
re-drill the locating hole in the side supports and weld
an extension on to the supports.
Install the transmission
- There are a few problems to overcome during the installation:
- Depending on your exhaust system most cars with
headers will have clearance problems with the
transmission oil pan, get out the torch heat and bend
the headers out of the way.
- If you have headers you may also have a problem
with the fill & dipstick tube hitting the headers -
heat and bend the tube as needed to clear the headers
(be sure to remove the O-ring before heating). A new
dipstick and tube from a late model Crown Vickie will
work much better than the Mustang tube.
- You should have new driveshafts made but you can
get by for awhile just by switching the slip yokes
front to rear, be sure to check for adequate clearance
between the front driveshaft and the transmission oil
pan. If you do not have much clearance you will need
to have a custom driveshaft made out of 2" diameter
tubing with .120 wall thickness. (hammer works too!)
- Both styles of transfer case shifters will work
but both will need modification. The T-style is the
easier of the two to use. Shifter location will be
back about 1 1/2 inches and to the left by the same
amount (the torch can cure this problem also by
bending the lever arm on the J style shifter).
- You will have to lengthen your transmission shift
lever connecting rod approximately 2 - 4 inches, also
using the shift lever from a late model F-150 with
column shift will make your shift indicator on the
column line up much easier.
- In most cases you will not have to massage the
floor to make it fit nor do you need a body lift, if
you do have clearance problems it will usually be at
one of the body reinforcement support panels (a hammer
will cure this).
- Make sure to replace the transmission vent tube
with one from a 4x4 or fabricate your own. It goes
from the vent in the top of the transmission up to one
of the top bellhousing bolts and then back down to the
side of the transmission, this prevents fluid from
running out of the vent on steep angles.
- Install a new 4x4 transmission oil pan and filter
- this has a deep sump pickup like what was stock on
the C-4 equipped Broncos.
- Make sure you have the correct alignment on the
Advanced Adapter piece where the end of the shift rod
sticks out from the end of the AOD, you may have to
enlarge the hole so that the shift rod does not bind
when the rod enters the adapter. Be sure to use
sealer or locktite on the bolts on both sides of the
adapter or it will work loose.
- Depending on how you drive you may or may not be
happy with the way that the stock valve body performs.
You have several choices at this point: There are many
aftermarket valve body kits on the market, if you go
this route try not to go over-board in selecting
something that is designed to put the shuttle in orbit
when it shifts - you really won't like it! I used the
latest upgraded valve body from Ford and it works just
- Once you have finished modifying the crossmember
and shifter linkage and bolted the transmission up be
sure to use a good quality Mercon style of fluid and a
transmission cooler to keep your new investment in
- Miscellaneous stuff that you don't really need but
will make your job a lot easier consist of: AOD
transmission cooler lines to splice into your original
C4 lines or lines that you have bent from your
radiator or transmission cooler. Now is the time to
replace those worn out rubber bushing on the transfer
case crossmember (much easier to modify these than the
stock ones) with those colorful poly-urethane ones.
Mail questions or comments to Lloyd Matsuda.