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

oFinding 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 safety switch.
o 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
o 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.
o 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.
o 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.
o 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.
o Install the transmission
There are a few problems to overcome during the installation:
  1. 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.
  2. 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.
  3. 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!)
  4. 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).
  5. 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.
  6. 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).
  7. 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.
  8. 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.
  9. 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.
  10. 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 fine.
  11. 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 good shape.
  12. 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.


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Last modified: Thu Oct 17 19:29:09 CDT