Compiled By Jrod-13

This is several different write-ups, all combined. This is a lot of information.


4.0L Conversion (1992 Explorer to 1985 Bronco II)

4.0 Conversion Wiring (1991 Explorer to 1989 Bronco II)

4.0L Conversion (1994 Explorer to 1989 Bronco II)

4.0L Engine Conversion Forum

Why a 4.0?:

The 4.0 has considerably more power, and I actually got better gas mileage with it than I did with my Carbureted 2.8. Switching to a 2.9 Will only give you 10 more horsepower… so it’s essentially not even worth it. The 3.0 has a one piece intake and throttle body assembly, and performance options are very limited on all 3 of these engines compared to the 4 liter.

Common Questions:

Question- Will the 4.0L bolt to my 2.9L Tranny and motor mounts?

Answer- Yes, it will, since the 2.9L and 4.0L use the same block, the 4.0L has the same bolt pattern on both the motor mounts, and bell housing.

Question- What do I need from the donor 4.0L vehicle?

Answer- All you need is the wiring harness, computer, nothing more

Question- What tranny should I use?

Answer- Well, that depends, if you have a 2.9L Bronco II with the Mitsu FM145 Manual trans, the 4.0L will bolt up, but there is not much telling how long it will hold. If you have this trans, swapping to a Mazda M5OD from a 4.0L Ranger is your best bet. If you have the Toyo Kogyo 5sp (86 and 87 2.9L’s, mostly 2wds) this tranny should be able to handle a 4.0L, and unless you really, really want to, a swap to a Mazda M5OD is not needed. As for the A4LD, it’s a gamble, since it’s not the most reliable auto in the world. However, they were put behind 4.0L’s from the factory, so, its your choice.

Question- Should I swap my stock axles?

Answer- Unless your planning on running out of this world tires, the stock 7.5in rear axle should be plenty. However, it is recommended to swap out the D28 Front axle to the stronger D35 front axle from a 4.0L Ranger or Explorer.

Question- What donor vehicles will work?

Answer- Any OHV 4.0L from a Ranger, Aerostar, or Explorer will work. The 4.0L was never used in Bronco II’s

Question- I kept my factory A4LD out of my 88 B2 (I know, it’s not recommended) but my question is regarding the overdrive. I have heard that I have to take a green wire from the donor vehicle and splice it into my brake wire so the tranny knows when to come out of overdrive. Has anyone heard anything about this or is this necessary?? My donor was a 1994 Explorer with a A4LD also.

Answer- BOO (Brake On-Off) should already be wired to your computer and it’s the COMPUTER that controls the Overdrive via VSS sensor and TPS inputs. What the BOO triggers is TCC (torque converter Control) BUT if your auto is the original one to your project that SHOULD already be connected. Don’t go cross circuiting things until you know you have a problem. Unless I’m mistaken, you somehow missed the Main EEC ground at the battery.

Reply- Yep, I did miss that ground. The plug was cut off very closely to the main harness so I never saw it. After doing some digging I came up with the exposed wires. It started right up after they were hooked back up. That is why they always say: Grounds, grounds grounds.

Question-  Just out of curiosity did you donor vehicle have a ground running off the negative battery terminal with a plug on it? It is a thicker gauge black wire and has a plug with two pins in it. I do not remember what this ran to in the Explorer and do not have any other connections hanging that could possibly go to it.

Answer- Yes, it had a two conductor black cable that I recognized as being the main EEC ground because I had had experiences with the similar SINGLE ground wire that was on my ’87 supercab from the factory.


If you’re swapping a 4.0L in to a 1988 or older Bronco II you’ll need the dash from the donor vehicle. If you’re swapping a 4.0L in to a 1989 or 1990 Bronco II you can use your existing dash. However, if the 4.0L and wiring harness came from an Explorer, you’ll need the Explorer dash.

Need a dash swap article? click HERE.

89+ you don’t need the dash. The 90+ 4.0 and 2.9 dash harnesses were the same.

Wiring Harnesses – By AllanD:

Basically my take on dealing with wiring on a Generation 1 to retrofit a Generation 2 engine starts with understand the differences.

Generation 1 (1983-1988)

The main harness goes into the engine bay through a grommet and then there is a cluster of small connectors.

The Generation 2 (1989-1992) has a big bulkhead connector

When putting a 4.0 into a Gen1 start with pulling the old EEC and harness out, installing the new harness and installing the computer in the passenger side then working around the engine compartment
from there back to the drivers side grommet.

There are some things you’ll have to keep and a few serious problems…like where to put the EDIS module…


Engine Swap Experience From Some Of Our Forum Members:

Almost Step by Step

How to Swap a 4.0 Liter into a Bronco II

Step 1. Take lots of pictures of your Bronco II engine compartment and of a Ford Explorer’s engine compartment before you begin.

Step 2. Gather all the parts and tools needed for the swap. One Bronco II, one complete 4.0 liter engine with all of the sensors and air box, complete engine compartment wiring harness, fuel lines, computer, 4 row radiator, etc out of a Ford Explorer.

Step 3. Remove the inner fender wells (this makes it easier to work on) and the Hood.

Step 4. Very carefully, disconnect the wiring and label everything.

Step 5. Remove the air intake tube, pull the radiator and set it a side.

Step 6. Un-bolt motor mounts and exhaust pipe, disconnect the heater hoses, fuel lines, A/C lines, remove the starter.

Step 7. Un-bolt the torque converter from the flex plate, and engine at the transmission and make sure that nothing else is holding the engine in place. At this point remove the Bronco II engine.

In my case, I disconnected the engine from the transmission and left the transmission and torque converter connected to the car (this will help to prevent fluid from spilling when the engine is disconnected), (remember to replace your front transmission seal (for automatic transmissions) before you put the 4.0 engine in place).

Step 8. This is a good time to inspect the engine compartment and to swap the wiring harness that goes to computer and transmission etc. Also, replace the front seal on the transmission (be sure to use lube on the seal rubber and the torque converter neck so you don’t damage the new seal).

Step 9. Place the 4.0-Liter engine into the engine compartment. You may have to jack up the front of the transmission to help line it up to the engine.

Step 10. Bolt the new engine in place (motor mounts, transmission, exhaust pipes and line up and bolt the torque converter to the flex plate.

Step 11. Connect the wiring harness from the computer to the engine (don’t forget to connect any ground wires). You’re half way done!

Please note that the new wiring harness from the Explorer may not just plug into your fuse box at the driver’s side firewall. My Bronco II didn’t have a plug through wiring harness. So, I had to start matching up wires from the power distribution box to the harness going through the firewall on the driver’s side. I’m still working on this one.

Step 12. Connect your fuel lines, heater hoses, and A/C lines.

Step 13. Install your new radiator and connect the hoses. Fill with coolant!

Step 14. Remove old Air box and install new 4.0 air box.

Step 15. Check for anything that I may have forgot to mention.

Step 16. Install the hood and fender wells.

Step 17. Recheck that every thing is connected right and you are done.

Wiring notes from my 2.9 liter to 4.0 liter Swap

The new explorer’s wiring harness will just plug in to the 4.0 liter engine’s sensors, but will have to be spliced into the harness that goes through the fire wall on the driver’s side. Here are some of my splices:

I retained the Bronco II’s A/C harness that comes through the firewall on the passenger’s side.

I retained the Alternator wires that come from the driver’s side and spliced in the alternator wires that come off the explorer harness. This I may redo at a later time, to make it all look nice and neat.

Tachometer…Explorer wiring harness, the color of the wire is Tan/yellow, the Bronco II harness is Green/yellow coming from the driver’s side through the firewall.

Vehicle Speed Sensor…91 Explorer wiring harness, the color of the wires are from pin 6 VSS ground is pink/orange, from pin 3 VSS signal is gray/black. On the 87 Bronco II, pin 6 is Black/white and pin 3 is dark green/white.

Converter Clutch Solenoid…91 Explorer wiring harness, the color of the wires are from pin 53 CCO trigger is purple/yellow, other wire is red (the same red that goes to pins 37 and 57.) On the 87 Bronco II, pin 53 is orange/yellow and other wire is red (the same red that goes to pins 37 and 57.)

As far as the 3-4 shift solenoid goes, the 1987 Bronco II code T A4LD automatic transmission w/OD doesn’t have one. I still get a code 86 because of this item.

Park your Bronco II next to a 4.0L Ranger if possible and compare the engine compartments. Take detailed notes on the location of the ECU, wiring harnesses, air intake, electrical system, fuel line connections, and plumbing for cooling and A/C. Vehicles earlier than the 1994 model year appear to be the best donors for this job since many of the swapped components replace similar or identical items in the recipient vehicle. 1995 and newer models have layout changes that would require extensive adapting.

Take photos of the 4.0L engine with all the components still intact for later reference. Obtaining a copy of Chiltons Ranger/Explorer/Mountaineer 1991-97 Repair Manual #26688 will give you the layout of the engines wiring, component locations, and vacuum line diagrams. When you begin disassembly of the 4.0 remember to label everything you disconnect in a manner so that you can easily reconnect them later. An example would be to use masking tape and start with the letter ‘A’ and label each wire connector (Male and Female sides) as ‘A’ to re-connect later. Go through the alphabet and then start double letters (AA,BB..etc) and continue.


Transmission compatibility is a consideration in this swap. Several transmissions were offered in the Ranger over the years. The one common factor is the engine to bellhousing bolt pattern which is identical for all years. The early model manual transmissions have a shorter input shaft with the pilot bearing in the flywheel, while the later years are longer with the bearing in the crankshaft. Of equal concern is the light duty design of the early model, which makes it more vulnerable to breakage. Newer model transmissions use a slave cylinder inside the bellhousing, so you’ll need those components and the interconnecting tubing if you elect to swap one in.

Automatics are equally varied. Units that normally mate to the 4.0 have electronic controls by the engine computer for overdrive and torque converter lock up. It is possible to swap in an early transmission, but you will have to fabricate a terminator that emulates the auto-trans functions that are missing, or use a computer and wiring harness for a manual transmission set-up. Early trucks use floor shifters, while the explorers and later rangers have column shifters.

Fuel System

The fuel system also requires attention. Later models with EFI will be the easiest. All that’s necessary is the adaptation of the quick-connects for the fuel send and return lines in the engine bay. Both dual and single fuel pump configurations provide adequate pressure and flow for the new engine. If you’re replacing a carb setup in an ’85 or earlier model, you’re facing a complete changeover of the fuel system. You’ll need everything from the tank to the engine. The fuel injected engines use an in-tank fuel pump which operates with much greater pressure than a carbureted engine.

Air Conditioning

Air conditioning systems have some configuration changes between 1992 and 1993 models. Pre-1993 models used the same receiver/dryer and evaporator, so obtain an A/C hose setup from a 1992 or earlier engine. The hose assemblies will have the condenser connection on the left side of the vehicle, which mandates a condenser change. The newer style condenser bolts directly into place, making this an easy change. The compressor should come with the new engine and have no interchange issues.


Different electrical variations were offered over the years. Use either the Chilton book mentioned earlier or the proper Hayes manual for your Bronco II. You’ll need to review the sections appropriate for your vehicle and the donor to determine the correct splices. Fortunately, unique color codes are used, which make the work more foolproof.

Electrical mods can be facilitated by acquiring a few key components. First, use the power distribution system found on the newer vehicles. It simplifies the integration of the charging, power distribution and engine control systems. An added bonus is the 10 to 12 heavy duty auxiliary circuits that can be adapted for accessories such as aftermarket lights, stereo, and other electronics. Verify also that the computer and wiring harness match the transmission that your using. While the different versions may appear the same, there are subtle differences in the wiring harness and programming differences in the computer that will show up later. Obtaining connectors with 12-inch pigtails that mate with the computer wiring harness will smooth the splicing work and maintain the modular nature of the system.
There are also wiring changes to consider. The later model alternators use external regulators that require a sense wire with a current limiting resistor from the main power system. The 4.0 V-6 has a tachometer connection in it’s wiring harness. However, due to the distributorless design, the signal it carries may not work with an existing tach. Finally, determine the correct splices to make for the fuel pump, ignition power, starter, power lead to the underdash fuse panel, A/C cutout relay and ammeter or charge light.

Bolting It In

The only custom work here will be drilling mounting holes or using a hack saw. Use the 4.0L factory motor mounts. Verify that the engine is complete with all belt driven accessories, intake and exhaust manifolds, crossover pipe, catalytic converter, starter, throttle cable, and air induction system.

Take the time to lay out all the parts in their general locations and in relationship to one another. Ensure that everything you have mates correctly before it’s installed. This is the time to service and replace any bad or worn parts.

Remove the engine following the guidelines in your Hayes or Chilton’s manual. Once the engine compartment is empty, this is a good time to drill any new mounting holes and perform any necessary fuel system upgrades.

Install the new engine and transmission. Next, install the new computer and wiring harness. These locate in the same place and follow the same routes as the originals. Mount the fuel pump inertia switch adjacent to the computer, and the power distribution panel near the starter relay. Connect the harness to the engine, power distribution panel, vehicle speed sensor and alternator. Then route the remaining connectors to the general areas of the A/C cutout relay connector, fuel pump connector, ignition and power connectors, and carbon canister valve. Mount the electronic distributorless ignition module on the radiator support between the radiator and the right side headlight; then connect it to the computer wiring. Use the original wiring harness connections for the temperature oil pressure sensors.

Install all the engine components and the exhaust as outlined in the Hayes or Chilton’s manual. Install the throttle cable.

Electrical work comes next. 

Install the color coded wiring using your manual and following the labels you made earlier. Early model Bronco II’s will require the addition of a check engine light. Install any fuses and reinstall and reconnect the battery.

Replace all the fluids and check for leaks. Start the engine. If all has gone well the engine will start on the first try. Check your work and check for leaks. Perform a diagnostic check. Once the diagnostic checks are done and the faults corrected it’s time to have the A/C serviced and recharged. In some states it may be necessary to get an emissions certification before you can register your Bronco II.

Miscellaneous Comments

I took a complete 1992 Explorer and gutted it without cutting any wires. I removed the engine & trans from my 1985 Bronco II [leaving transfer case intact]. I installed a 1992 Explorer engine & trans in not necessarily this order:

Put new rear main seal, oil pump, etc. in 4.0L

Serviced transmission

Used 2.8 mounts on 4.0 block [unmodified].

Explorer a/c condenser, new accumulator, new a/c hose from compressor.

Explorer blower motor [much larger motor-same fan]

Rewired a/c and blower wiring to match Explorer [Bronco II had no high blower relay – Explorer does]

Alternator only needs 1 wire hooked up-hot in run, no dummy light required. [Bronco II had ammeter]

Only wiring that’s confusing is the neutral safety switch – Explorer has an extra wire that goes to the electric transfer case shift module. 1985 Bronco II is manual shift.

Fuel pump and filter won’t fit in stock Bronco II location due to brake lines/proportioning valve. Used 1988 Bronco II sending unit and fuel lines to surge tank. From the engine back to the surge the lines are a mix of shortened [with 5/16 comp. fittings] stainless Explorer lines and Bronco II plastic lines [shortened by cutting heating and reinstalling ends].

1988 Bronco II Bosch main fuel pump. 1992 Explorer Hedman headers required custom shift linkage for a4ld made from an idler pulley and rod ends…very tricky to adjust.

Cut off converter and stuffed 3-inch ovaled pipe into converter flange-perfect fit. Basically made the Bronco II look like an Explorer under the hood from condenser to trans. tailhousing.

Spent 2 months of spare time and probably $2,000 of my boss’ money. Nope it ain’t even mine! Definitely a worthwhile swap. This beast isn’t a hotrod but it does smoke the tires and runs 90 at 1/4 throttle in overdrive.

Misc Notes:

Not all motors mounts will bolt up. I recently did a swap from a 1996 ranger 4.0 in my 1988. The stock mounts WILL NOT WORK. I have to get everything from the 1996 and drill new holes in the cross member – posted by samsonitesamsonite