I thought I would write up a bit about the 4R70W swap I completed a couple of months ago now that I have a few hundred miles on it. BLUF: It's one of the best mods I have done on the Bronco. It did turn out to be a little more expensive than I thought but its ok in the end.
I bought a 99 Mercury Mountaineer transmission off of ebay for $250 without knowing much about it's condition or mileage. I was planning to rebuild it anyway so as long as the case and most hard parts were good then it didn't matter. I went into it with the idea that I was going to do other mods as well including twin sticks, gears, radiator and 3G alternator. I went from 4.10 to 4.56 gears as I thought they would work better with the transmission, and I am glad I did. I am running 35's on a 3.5" SL and 1" BL. I pulled the old C4 out and decided that I would modify the existing crossmember and build new brackets. Both of these jobs were pretty easy with only basic skills needed. I am using the AA adapter to bolt to the D20. When fitting the adapter to the crossmember, I did not need to expand the bolt holes like some people had mentioned. I modified original style poly mounts to fit the new adapter and the bolts lined up well. I made new crossmember brackets out of 3"x1/4"x8" long angle iron and used the original brackets to mark the frame bolt holes. The new transmission pushed the crossmember back about 1.5".
I tore the transmission down and found a couple of issues that are not uncommon in these. The outer 1-2 accumulator spring was broken. It only caused minor scoring in the sleeve that I was able to sand out using very fine grit. Finding new springs was difficult but ebay had some. The reverse drum tabs and the corresponding slots in the sun gear shell also had more wear than I liked so I got a rebuilt reverse drum and a new sun gear shell (magnetic). I ended up ordering the rebuilt kit and parts suggested by Transmission Bench. Their videos are also very helpful. I also got a complete new set of thrust bearings as sometimes they can be difficult to know for sure if they are good as well as a rebuilt torque converter. Following the videos and the instructions with the adapter, I reassembled the transmission, double and triple checking the order and air testing each drum. It's really not a difficult job, you just have to be clean and methodical.
I got the transmission in and then learned that I would need both new driveshafts, different headers (shorties), a column shifter adapter kit and an extended speedometer cable. There were also a number of small parts here and there that are needed. I made the cooling lines out of 3/8" Cu/Ni tubing and used the stock fittings in the transmission. There was enough room to make the bends clear of the exhaust. I decided on the US Shift controller for the transmission along with their TPS harness to work with the Holley Sniper. Wiring was fairly straight forward as I didn't need quite a number of wires that came with the controller. I drilled a hole in the front part of the tunnel to run the wiring through and there is probably 4-5' of extra wiring that I had to make room for under the dash but it wasn't bad. I could have cut the harnesses to shorten them but didn't trust myself to get them all back together correctly.
Open to any questions you may have, I will edit if I think of anything else to add.
I bought a 99 Mercury Mountaineer transmission off of ebay for $250 without knowing much about it's condition or mileage. I was planning to rebuild it anyway so as long as the case and most hard parts were good then it didn't matter. I went into it with the idea that I was going to do other mods as well including twin sticks, gears, radiator and 3G alternator. I went from 4.10 to 4.56 gears as I thought they would work better with the transmission, and I am glad I did. I am running 35's on a 3.5" SL and 1" BL. I pulled the old C4 out and decided that I would modify the existing crossmember and build new brackets. Both of these jobs were pretty easy with only basic skills needed. I am using the AA adapter to bolt to the D20. When fitting the adapter to the crossmember, I did not need to expand the bolt holes like some people had mentioned. I modified original style poly mounts to fit the new adapter and the bolts lined up well. I made new crossmember brackets out of 3"x1/4"x8" long angle iron and used the original brackets to mark the frame bolt holes. The new transmission pushed the crossmember back about 1.5".
I tore the transmission down and found a couple of issues that are not uncommon in these. The outer 1-2 accumulator spring was broken. It only caused minor scoring in the sleeve that I was able to sand out using very fine grit. Finding new springs was difficult but ebay had some. The reverse drum tabs and the corresponding slots in the sun gear shell also had more wear than I liked so I got a rebuilt reverse drum and a new sun gear shell (magnetic). I ended up ordering the rebuilt kit and parts suggested by Transmission Bench. Their videos are also very helpful. I also got a complete new set of thrust bearings as sometimes they can be difficult to know for sure if they are good as well as a rebuilt torque converter. Following the videos and the instructions with the adapter, I reassembled the transmission, double and triple checking the order and air testing each drum. It's really not a difficult job, you just have to be clean and methodical.
I got the transmission in and then learned that I would need both new driveshafts, different headers (shorties), a column shifter adapter kit and an extended speedometer cable. There were also a number of small parts here and there that are needed. I made the cooling lines out of 3/8" Cu/Ni tubing and used the stock fittings in the transmission. There was enough room to make the bends clear of the exhaust. I decided on the US Shift controller for the transmission along with their TPS harness to work with the Holley Sniper. Wiring was fairly straight forward as I didn't need quite a number of wires that came with the controller. I drilled a hole in the front part of the tunnel to run the wiring through and there is probably 4-5' of extra wiring that I had to make room for under the dash but it wasn't bad. I could have cut the harnesses to shorten them but didn't trust myself to get them all back together correctly.
Open to any questions you may have, I will edit if I think of anything else to add.
