Tech article by Tim Bowsher (Broncobowsher)

Selecting a new transmission.
It started with when I got the latest Bronco. I didn't want the old 3 speed anymore. I have heard about the NP435, so I got one. I ran along happily with 33's, 4.10's, and the NP435 behind the warmed up 351. It worked ok. The granny was great, but the rest of it was still just a 3 speed. Actually a little slower then the old 3 speed. It really sucked for acceleration.

Rev to the sky. Click. Click. Clunk. Bark. Bog. Repeat.

The crawl ratio was just fine. There could always be more, but it was satisfactory. The clutch chatter was always a problem. Then came the blown D30 front end. Stripped the side gears, chunks took out the ring gear. Driveshaft, U-joints, and finally all the parts fell out of one of the kingpins. The rear posi was not locking like it should.

Came time for a driveline rebuild. Keeping the engine, that runs fine, just need a pan gasket.

I wanted an automatic. That will stop the clutch chatter. I knew I liked the low gears, but would like to get a little better highway cruise. I looked at a lot of options. C4, C6, AOD, E4OD, 700R4, 4L60E, 4L80E, then finally 4R70W. C4 would be an easy swap, but lacked the spectrum of gearing I wanted. C6 was stouter, but same gearing. Looked for something with an overdrive. AOD, It would work, but not the best reputation, put that thought on the back burner. Found AA kit for a 700R4 behind a small block Ford. That looked intriguing. I know that a 700R4 can be built real nice. But then again it is a GM transmission, and the life expectancy is almost nothing. That too hit the back burner.

While killing some time at Driven Auto Parts, I was discussing the options. Why not a 4L80E behind the AA Chevy trans to Ford engine adapter? What about the computer? 4L80E has (at least the option of having factory) a duel stator torque converter. Think of the torque! Ok, time for some more research.

Found transmission computers really expensive when Looking at companies like JET. Overall transmission lengths were getting ugly. Adapters were nonexistent, or rare. Looked at the option of swapping everything. Things looked bad, thought there wasn't anything on the market that would make me happy. Too expensive, too long, poor gearing, poor history, something was wrong with almost everything.

Then I found a nice transmission controller at a decent price. Baumann Engineering made a nice little controller that solved almost everything. I took a tape measure to an E4OD. With a 20" long PAN length, I knew that wouldn't fit under a Bronco. I was pretty much down to the 4R70W. The 2.84 first gear is lower then the AOD/AODE 2.40. The Overdrive at .70 was nice. Not as wide a ratio spread as a 700R4, but it was pretty much a bolt up. Time to start rounding up parts.

Look for a transmission.
Back to Baumann Engineering with the history of upgrades to the AOD/AODE/4R70W line. Sorted through the info and came up with a '98 or newer V8 Explorer transmission as the perfect base to start with. Soon afterward, Randy at Driven found one. But it had a broken tailshaft and output shaft. That was no problem, I need to replace those parts when I install the adapter. Get it out and find a broken case and a few other wasted parts. But I now have all the upgraded guts and low mileage too. Later I find an AODE from a 3.8L V6 Mustang. Now I have a case, and a spare close ratio gear set in case I ever want to try that.

With the basic ingredients starting to come together I find I still need more before I begin. I order up the transmission controller, specing it out for the digital range sensor. Had a custom made harness that was 4 feet longer then normal at no extra cost, just a little delay in time. That isn't much of a factor, I have been delaying this for over a year now, mostly because of the new house. A trip to the local Ford dealer got the parts outlined in the AA catalog for making the AOD adapter work for the AODE/4R70W. Oh, I need the AA adapter as well.

Now that I have the big stuff accumulated, I need a few more little things. B&M flexplate, Starter from Towers & Harwood in Manhattan, KS (deal mainly in Ford roundy round racing stuff, aka lots of horsepower and reliable too). Some hardware like flexplate nuts and bolts, Lokar shifter (nice, but a real pain, more later). And a big cooler from an F550.

While collecting parts for the ailing Bronco, I needed to do something about the wasted D30. The answer is obvious, A D44. I wanted it fresh, might as well do it right to begin with. What gears to run? I figured I would start with highway gears and work my way down from there. It is a lot easier to go lower (Terra, cold duck, Marlin, Rock Eaters, Atlas) then it is to go higher (bigger tires, that's about it).

The current 4.10 gears would come out to about a 2.87 in OD. That was looking a little too tall for tires in the 33 to 35" range. Looked at 4.56. That came out to about 3.20 in OD. That would be nice on the highway. 4.86 and 5.13 were looked at as well. Those came out to 3.40 and 3.60 respectively.

Then I found Keith Jones from Protofab had a 9" 31 spline Detroit with 4.86 gears and a D44 housing and a set of 4.89 gears for that. Coughed up the dough and got the bounty. 4.86 gears it will be. Cruise in OD lower the running 3.50 gears. Perfect. Those go under the truck and while I am at it, replace the front springs with some CITI spring from the 2000 Bronco Stampede (only been sitting on those a year and a half). Rears get a set of used 10 leaf Nationals. 2" block brings up the back to level it. Later a torque arm solves the axle wrap problem. Drove the truck a couple of times with the 435 and 4.86 gears. Wicked. Sucked over 50 MPH.

With the axles ready, time to swap transmissions.
I rebuilt the transmission while waiting on axle parts, so when it was ready as soon as the axles were done. With luck, I had access to a copy of a factory Ford 1997 Expedition service Manual. I copied the 4R70W section and followed it. Out came the front pump, and the rest of the guts. Learned that I needed to mark the location of an anti rattle spring and a snapring. Got the good guts sitting out, time to do it again. Pulled the AODE apart. Set everything aside. Drilled and tapped the vent hole for a remote breather. Washed out the case. The AA instructions suck, but I still managed to get everything back together. While I was at it, I went ahead and replaced the clutches. I didn't mess with taking the pistons apart, I didn't have the right tools to put it back together right. Besides, the seals are almost new. Small gamble, but it worked out. All the easy to replace seals got replaced. Carefully assemble the guts in the case and make sure I don't have any left over parts. A couple of calls to Greg at Baumann Engineering got me all straightened out. The valve body kit went in on the "Moderate" setting with increased torque capacity. After a little though, decided to keep the stock convertor. If I don't like it, then I have a spare fromt the AODE that I will have reworked later.

Going into this swap I had some concerns about the exhaust clearing the transmission and the transfer case clearing the floorboard.

Then it came the day of no return. After idling into the shop in 4L granny thinking of how much I like this creeper gear. I drained fluids, dropped exhaust, unclipped the fuel injection, and pulled everything out of the way. Got the engine/trans/transfer out in one piece, just like it went in 3 years ago. Set everything on the ground and eyed thing up. Cracked off the old 4 speed and compared it to the auto. The auto was just a little longer then the NP435. Everything looked great. Pulled the slipping clutch to find the input shaft seal on the transmission was leaking gear oil directly onto the disk.

I should mention that the roll of film I burned doing all this didn't come out, and I am not about to take everything apart just for some pictures.

Time to trial fit some parts.
Transmission to engine went together. Now for the questionable stuff. The transmission dipstick. Neither the Mustang or the Explorer dipstick will fit on the first try. I have done some in vehicle checks before the engine came out to see what might work. I decided to go with the Explorer dipstick. It required a new bracket welded to the tube. Originally it bolted to the head, but the 351 is taller then the 302. I made a new bracket the bolted to the bellhousing bolt.

Hooked up the Exhaust. Wow! It cleared. Not by much, but it cleared. The headers clear, even the muffler side clears. Looking good. I clean up a few little things on the engine that need attention like the leaking dipstick on the side of the engine oil pan.

While I have the transfer case out I got the wild hair that I wanted lower gears. In went a set of Tera lows. I though that AA had bad instructions. The Tera's had no instructions. I am not sure if mine got lost, or if it never had them. Any way, the D20 is an easy rebuild. There is an extra ring that I had no idea where it went. Called up Tera and found it goes under the snap ring holding the input gear on the spud shaft. The Tera kit comes with new seals. Considering the cost of a reseal kit, the Tera's look like a good deal.

The following weekend I attempt to install the new creation. 351, 4R70W, and a Tera lowed D20. Stripped of easily damaged parts (headers, dipsticks, shifters, throttle body, etc. It drops in nicely, but it won't line up. Everything hits. The valve covers hit the master cylinder (Ford motorsports tall, new item). Heads hit the firewall. Engine too far forward at the mounts (odd since it can't go back because of the firewall clearance issue?). Transfer hitting the floor. After jacking the transfer into the floor real hard and finding things line up a little better, I cut out the floor just behind the tunnel. At last, clearance. As the transfer case comes up, the heads roll away from the firewall and allow the whole engine to slip back a little until the engine mounts line up. That was the trick. Either a body lift of cut a hole in the floor.

Now for the transfer case crossmember (images 1-4 above). After looking at the factory one, and trial fitting out of the vehicle, it was set aside and a custom one made. The drivers side perch was flipped over because I always have trouble with that bolt. I am making a custom part, why make it harder then you need to? Passenger side stayed because I can reach the top of that bolt and it would get in the way of the exhaust if I flipped it.

For bushings, I found some Jeep torque arm bushings in urethane. Drilled some 1" holes in some scrap angle iron. Bolted it to the transfer case. Dug through the scrape pile and made a new crossmember. It goes under the front drive shaft. A little different then the factory over the driveshaft style. Measurements were made and the rear drive shaft was sent out to be shortened.

Next drop in the headers.
The drivers side, the one that is a pain every time I have to put it in or pull it out. It just fell into place. All the room you could ask for. The Passenger side, you know, the easy one. It would not fit. After hours of trying. Pulling engine mounts, transmission mounts, lifting, lowering, cursing, everything but pull the body. I was pissed. Chopped off #1 pipe. Got closer. After 5 minutes of wiggling, cut off #2 pipe. Then figured it out. #1 goes in loose. Set it off to the side. Then goes in the remnants of the header, also loose. Then #2 pipe goes in. 1½" tailpipe splice kit from Checker patches it back together and sheet metal screws hold it in place. I don't like it, but it works for now. (image 5 below) A big hammer clearances the drivers side exhaust where it interferes with the new crossmember. (image 6 below)

Little parts go right in. The speedo cable has enough slop to stretch and fit. Starter goes in, etc. The rear drive shaft if fitted in the front. No way is the 2½" tube going to clear the pan. Mark where it is close and off to the drive shaft shop. As I cruise the freeway in a borrowed company car I look in my mirror to see flashing red and blues come down off the entrance ramp behind me. Damn it, I was just keeping up with traffic. Start pulling over to the right. Lose sight of cop in the mirrors, he is going for the car pool lane. Feeling lucky for a moment. Then he is behind me again, but further back. What is going on? Another swerve across the whole freeway (I-10 under the deck park tunnel in Phoenix if you know the area). This one cop is shutting down the entire freeway, and I am the last car to get by. Now I am paranoid. Is someone coming the wrong way on the freeway? Chemical spill ahead? Fatal accident? Someone forget to take their Alzheimer's medicine and driving right for me? Full alert. Nobody behind me. Take the next exit. Life returns to normal. Get downtown and get a custom made 2" tube front driveshaft made and make sure the balance weights are not going to be in the way, that's what the marking on the rear shaft were for. It was also the right size so I could tell him that this is what size ends I want on it.

I now have all the hard parts in place and hooked up. Check the wiring from Baumann and the factory Expedition manual, and what I have. I get about half of it, the output shaft speed sensor and the digital range sensor. Another call to Greg at Baumann Engineering (this guy is great) and get the valve body wiring straightened out. A couple more connections for power and ground. A splice for TPS off the fuel injection. Pour in some AFT, start the engine for a moment, more ATF, start again, more ATF, start and let idle. A few more quarts and it is finally visible on the dipstick. A quick run through the gears. Reverse, engage. Drive, nothing. Reverse again, nothing. More ATF. Through the gears again. I am getting excited, it might actually work. I finally get drive and reverse. Pull it out of the shop. Check the fluid again. More ATF. I have completely lost track of how much ATF I have used.

OK, now is the time for the first drive around the block.
A quick run down the street and back, check the fluid one more time. That was the plan anyway. Started going down the street, then waited for a shift, and waited. Backed off the throttle, engine returned to an idle. On the gas again still in first gear. Great, I screwed up the tranny! Turn around and watch the computer monitoring the transmission controller. Take off, shifts 1 to 2 then 2 to 3. At least that is what the computer says. Too bad the transmission isn't doing any shifting. Back in the driveway. As I am turning it off I realize exactly what I did. Don't even get out of the truck. Reach down through the hole in the floor left from the NP435 and plug in the wires going to the valve body. Wow, the on screen trans temp now works. Time for another drive.

This time I aim for around the block. It works. Pulls a lot harder now. I had been driving in 2nd gear before thinking it was 1st. Those low gears really pay off. The shifts are late and hard, but it shifts. I get down the road hit the 2 to 3 shift. Again, late and hard. Get up to cruise and it shudders as it goes into 4th. A little more driving confirms everything works. Transmission fluid is a little low, but still on the dipstick.

Then the Crane PS91 coil goes out taking the Unilite module with it. Rebuild the ignition with a good old duraspark and a Jacobs CD box firing a stock coil. Now is the time to take care of the 3 to 4 shudder.

What was happening was that 4th gear (OD) was trying to engage before 3rd gear completely released. When I did the valve body kit I did the 3 spacers on the 3-4 modulator valve. The instructions stated 2 spacers may cause a binding shift (like what I am getting). 3 Spacers may cause a flair (momentary neutral) during the 3-4 shift. I was getting the bind with 3 shims. I had 1 extra spacer left. I decided to put it in and have 4 shims.

That required me to pull the valve body. Unfortunately for me, that is a real pain with the current exhaust. Pull the muffler and the right header pieces out of the way. Then drop the pan. The anti drain back is in the valve body so when you pull the valve body, the torque converter drains on you. Added the 4th spacer to the 3-4 modulator valve and put it together.

Time to try it again. Add fluid again back it out of the driveway. Start to go around the block. Something isn't right. It feels like there isn't any line pressure until you rev the engine. There is a shudder as the clutches engage and it starts pulling forward. Damn it! Back it up and pull it in the garage. No reverse. I am revving it as high as about 2200 RPM in reverse, but only have neutral. Fearing a reverse neutral drop if I rev it any higher, I shifted it back into drive and went around the block. The revised shift points I put in while waiting on ignition parts show itself through the slipping. Back into the garage.

Time for a little trouble shooting.
There is enough oil. It did work. The only part I played with was the valve body. Out it comes again. This time I pull the drain plug on the converter, much better. Careful examination shows that the gasket had a minor blemish. NEVER reuse transmission gaskets. Especially when there is a whole stack of them. One stack from the rebuild kit and another stack from the shift kit. New gasket and all is good.

There is a lot of heat coming in through the floor. Maybe I need to cover the hole left from the NP435. A section of not too molested floor comes from Randy at Driven Auto Parts. A Bronco floor to fix a Bronco floor. Self tapping sheet metal screws make it a quick release panel for easy access. A little cut and patch takes care of the relocated transfer case shifter hole. A trip to Wally World net a stout baking tin that covered the hole above the transfer case. Some trimming and some more sheet metal screws and the floor is sealed up.

Let's put some miles on it. About 100 miles round trip to work, a friend's house and home. This is in July is Arizona. Trans ran good and cool, about 160°. Probably the only reason it was that warm was that I was warming the fluid through the radiator cooler after the monster F550 cooler. But, the engine was running warm. Around 210 to 215°. That was on a flat level road. It didn't run hot before the swap to the automatic. Maybe it is that giant tranny cooler blocking airflow. After all, it is larger then radiator.

Pulled the big cooler out and ran just the radiator cooler. Made another 100 mile drive to Hooters and back. Engine temp 185°, trans temp 213°. Time to compromise. Get the largest B&M cooler (about a 1/5 the size of the F550) and try that. I just put that in. Why haven't I driven it? I had to take care of another problem. Apparently there is a little problem with the AA adapter for the AOD. The seal between the transmission and the transfer case has a tendency to fall out. I have heard of several incidents of this, but AA doesn't know anything about it. The last drive where I was getting the higher transmission temps probably caused the aluminum bearing retainer to grow a little bit and the seal just fell out. Put a little tranny fluid in the transfer case. Got a new seal and glued it in place. Let's see how well loctite works.

Another annoying part of the conversion should have been very simple, the shifter. I wish I just got a simple column shifter. But not having one, I did a floor shifter. I didn't like the standard mega shifter type of floor shifter. While working on getting the shifter the way I wanted it, I did a temporary shift cable. It was real simple. I took an old shift cable, hooked it to the transmission and welded a bolt to the other end. Simple T handle, push and pull. (image 7 below)

I picked up a Unimatic, but didn't like the feel and got rid of it. What I did get is a Lokar floor mound shifter. (image 8 above) I got a C4 kit instead of the AOD kit. I wanted the correct pattern on the knob. Nice, simple, small, no big bulky cheap looking plastic housings, but not that easy to put in. The kit came with a solid shift rod. There is no way that is going to work. The transfer case just isn't going allow it. I got a push pull Morse cable 6 feet long with 3 inch stroke. Hooked one end to the transmission and wrapped it around back to the shifter, which is mounted between the seats. The shifter lever is spun 180°, Shelby Cobra style. Had to do some playing to get the stroke of the shifter to match the stoke of the transmission. I still wasn't done. After all that, the gates in the shifter were not where I wanted them. Lokar likes to lock the shifter in neutral, I don't like that. I want a stock shifter feel. Maybe I should have used an old Mustang shifter? I had to take the shifter apart and file the shift gate to get the action I wanted. The first time I fixed the flaws I knew I didn't like. A second trip with a file cut the gates to match the detents on the transmission. Maybe I should have done a Lokar Nostalgic double bend shifter for an AOD and just changed shift knobs instead? Too late now.

I have done a half dozen revisions to the shift tables to get where I am now. Much easier then setting the fuel injection. It took a little work to clean up the early and late shifts and set the line pressure. I had a flair going from 3rd to 4th at light throttle, I though it was the extra shim in the valve body to cure the before mentioned bind shift into 4th. I discovered that I was locking up the torque converter in 3rd. The flair was when the converter unlocked just before the shift to 4th gear. A quick change of software took out the 3rd gear converter lockup, and the flair.

I have only done a little off roading so far. A few hundred incident free street miles and it will ready for a little light trail work. Then we will see how much fun I can have with it. There is a trip to the dragstrip planned soon. It ran a 16.3 weighing 4000 LBS with a bad carb and the NP435. No telling what it will do now with the lower gears, automatic, winch, very heavy bumper, rock skis, fuel injection and larger tires.

Do I like the transmission? Yes. I have dropped about 600 RPM on the freeway, even though I jumped from 4.10s to 4.86 gears. The simplicity of just drive and go is great. Shifts are firm and quick. The adjustability of the transmission controller has allowed me to set the shift points and firmness where I want it.

Would I do it again? Yes.

Did I gain gas mileage? Don't know. Still playing with it and haven't checked yet.

Would I have done a different transmission? Probably not. I would liked to have done an E4OD, but the bronco doesn't have enough wheelbase.

Any thing that I might have done differently? I would liked to have done an Atlas 3.8 instead of reworking to D20. Budget didn't allow everything.

Costs for the transmission: (not including axles, lift, or the Tera low set):
  • Broken 4R70W: lucky freebie
  • AODE: $350
  • Baumanator: $406
  • Rebuild and valve body: $250?
  • Adapter: $500 ?
  • Starter, flexplate, shifter, cooler, fluid, misc.: $500 ?
I tried to spread the costs out over a long time and I don't really want to know what I just spent on it.

  • Transmission and other Bronco stuff: Driven Auto Parts
  • Transmission Controller, rebuild parts and a lot of good info: Baumann Engineering (Try the middle of the week, they are often out racing on the weekends. Take Friday and Monday off too)
  • AOD adapter: Advance Adapters
  • Ford engine parts and starter: Towers and Harwood Performance (785)833-6093
  • Flexplate, shifter, hardware, and miscellaneous speed parts: Summit Racing