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Wiring Harnesses:  Jeep? Meet Ford...

Of all the parts of this project that had to be done, getting the wiring sorted out was the biggest unknown of all.  The engine I bought came with a very large box full of wiring harnesses - almost every harness from the donor Mustang.  So when I ordered the Mustang shop manual, I made sure to order a copy of the wiring diagram as well.  A visit to a local library netted me some very useful photocopies of the electrical system connections from the Electrical and Vacuum Troubleshooting Manual, which in hindsight were at least as useful as the actual schematics.  Together, this material made hooking up the 5.0L's extensive electrical system to my Jeep a very do-able job.

In order to make this work, you need the Mustang's main engine harness, the intake manifold harness (for the injectors and sensors), the oilpan harness (for the O2 sensors), the computer, all the modules that attach to the engine harness (like the BAP sensor, ignition coil, WOT relay, air valve and EGR vacuum solenoids), and, if at all possible, the Mustang's main interior harness.  This last item is needed because it has the the mating connectors that connect the engine harness to the car, where almost all the splices need to be made.  It also has the fusible links that will be used to supply power to the Jeep's electrical systems.  Make sure your harnesses are complete, with no missing connectors.

For the most part, the 5.0L engine really has a very self-contained electrical system.  There is one main engine harness made up of dozens of wires, but almost all of them connect directly to the computer.  There were only a handful of wires left over that had to be connected to the Jeep:

1. The battery connects to the battery terminal on the Ford starter solenoid.
2. The engine block is grounded to the battery ground terminal.
3. The Jeep's power input to the ignition switch connects to the battery terminal on the starter solenoid.
4. The power feeds from the ignition switch ("run" position) connect to the power inputs to the engine harness.
5. The harness ground wire connects to the tub or battery ground.
6. The crank (start) signal from the ignition switch connects to the corresponding wire in the engine harness, and to the starter solenoid.
7. The coolant temperature and oil pressure sender wires from the engine connect to the Jeep gauges.
8. The Mustang tach wire from the distributor connects to the Jeep's tach wire.
9. The fuel pump relay connects to the Jeep's new electric fuel pump.
10. The reverse switch connections go to the reverse switch on the new transmission.
11. The alternator "enable" wire connects to a small lamp and from there to the ignition "run" position power.

First, an '89 Jeep and a '90 Mustang use pretty much the same level of automotive technology.  Fusible links instead of fuses, which means that you don't need to touch the fuse panel at all.  In fact, ALL of the wiring splices occur inside the engine compartment, between the Mustang and Jeep engine harnesses.  There's no need to rip the dash apart at all.

Second, ALL of the Jeep's wires that need to be spliced into the Mustang harness pass through one of the two halves of the Jeep's bulkhead connector, beside the brake booster at the top of the firewall.  One half of that connector is for the front end lighting and horn, and the other half has all of the wires we're interested in.

Third, you will end up with one main harness: the Mustang's (harness A).  Spliced into two of its connectors it will be several wires from the Jeep's old engine harness, but when you're done it'll look like one harness, pretty much.  Almost all of the connections to the Mustang harness will be made through one grey and one black circular connector, both of which originally connected to the car's interior/dash harness.  These end up conveniently close to the Jeep's bulkhead connector.

Fourth, you will also end up with a smaller harness (harness B) running alongside the main harness on the firewall.  This second harness will carry power and ground from the battery over to the bulkhead connector, as well as the alternator enable wire, the blower motor wire, the underhood lamp wire, and the reverse switch wires.  This harness is basically just a catch-all place to route all the engine bay wires that don't connect to the engine.

Fifth, there will be another smaller harness (harness C) running from the green 8-pin rectangular connector on harness A (near the computer) inside the cab, through the firewall, and over to the bulkhead connector.  This harness contains the fuel pump relay status wire to the computer, plus the check engine lamp wire, the two Speed Sensor wires, and the HEGO power.  Ideally, this harness would be combined with harness B, but that would require significant rearranging of the main engine harness, so I didn't bother.

Sixth, the only other bit of wiring required is for the fuel pump.  The pump itself has to be connected to the pump power wire on the Jeep's underbody harness (the same one the sender wire is already connected to).  As it happens, all YJs have this wire routed in the underbody harness by default, so all that's necessary is to hook it up to the pump.  I did this by replacing the harness connector with one from a later-model (EFI) YJ, and using a later-model YJ pump/sender assembly with a Mustang pump installed.  After the pump's installed, a relay must be added to control the power to it.  These connections are described in the second table below.

Here's a few photos showing the finished, installed harnesses:

This table details where each of the Jeep's bulkhead connector wires ends up going:
 

And this table details the connection to the fuel pump relay:
 

The pin numbers are marked on the bulkhead connector itself, and are shown on page 8W-160 of the '89 Jeep factory service manual (electrical).  The circuit numbers for the black and grey connectors come from the Mustang wiring diagram and Electrical/Vacuum Troubleshooting Manual (EVTM).  If all else fails, just go by colour, but be careful not to confuse the two red wires and the two green wires!

And here's a brief list of the steps I followed to end up with a complete, working hybrid harness:

1. Lay the old Jeep engine harness out on the floor and remove all the plastic split loom and tape from it.
2. Identify the wires you'll be keeping from the above table, and cut them all to free the bulkhead connector from the harness (as far from the connector as possible - too much length is always better than too little).  Be sure to leave the wires and connectors for the brake warning switch, underhood lamp, blower motor, and reverse switch alone - they'll need to be plugged back into their original locations afterwards.
3. Cut the grey and black connectors from the Mustang's dash/interior harness that are the mates for the two connectors used in the splice table.  Also take the green 8-pin rectangular connector whose mate is right beside the computer connector on the engine harness, and take the four fusible links (YL, BK/O, BK/Y, and O/LB).  These each have ring terminals which will connect to the starter solenoid for battery power.  Take as much length as possible in each case.
4. Splice the two circular connectors and the wires to the rectangular green connector to the Jeep bulkhead connector, with enough length that they will mate with the engine harness without being too long or too short.  Test-fit the harnesses on the Jeep to get the right length, especially the one to the green connector, since it snakes through the firewall all the way to its mate, near the computer.
5. Reposition the power wire in the engine harness (it's the one with the ring terminal) so that it will reach the starter solenoid, wherever you choose to put it.  I located mine on the firewall, right beside the battery.  This required that I shorten the power wire, and loosen the harness wrapping slightly to move it to where it needed to be.  Bear in mind that the Mustang's battery was on the other side of the engine compartment, so this wire need to be moved quite a ways.
6. Leave the ground wire in the engine harness (it's the black one with the single-pin connector) in place.  Rather than reroute it with the power wire, I chose to cut off its connector, crimp a ring terminal onto it, and ground it to the engine using one of the air compressor bracket attaching bolts and a star washer.
7. Splice the crank signal wire from the starter solenoid into the engine harness.  I made this splice right at the harness junction where the distributor wires branch off, since this wire goes to the distributor.  This is the only wire that gets spliced directly into the engine harness.
8. Wire up the fuel pump relay.  My YJ had a pair of relays connected to the battery tray, and I re-used one of the relays in this little relay pack for the fuel pump relay, after remounting it on the firewall near the bulkhead connector.  The fuel pump power wire comes from the bulkhead connector, and the other four connections to this relay are in the table above.
9. If your Jeep was a carbeureted model like mine, the fuel pump power wire will exist in the interior harness (on the cab side of the bulkhead connector) but not on the engine bay side of the connector.  You can move one of the unused pins (with its cut-off wire) in the bulkhead connector into the empty location for the fuel pump power wire (pin B4) to complete the connection.  Moving the pins around in the connector requires that you first pull the plastic pin retainer out through one of the walls of the connector.  It's very difficult to see the retainer by looking at the connector, because the only part that's visible is a long thin plastic strip on one of the side walls that's a different colour from the rest of the wall. It can be dug out with a small screwdriver or knife blade.  Removing this thing is the only way to move the pins around without destroying them or the connector.  Trust me.
10. Connect the alternator enable (exciter) wire to a small light bulb in parallel with a 500-or-so ohm resistor (1 Watt or better) and then to the ignition "run" position power wire as mentioned in the table.  I taped my bulb to the outside of the wire bundle coming from the bulkhead connector, and will eventually move it inside the cab, as a dash lamp where it really belongs.  This lamp/resistor setup duplicates what was originally connected to this wire in the Mustang's dash.
11. Rebundle the wiring so it's neat and tidy (and waterproof!).  Remember: electrical tape is your friend.  Hockey tape works pretty damn well, too.
12. Once the harnesses are all together, position them in place along the firewall, installing retainer clips as necessary to hold everything in place.  Then install the BAP sensor, WOT relay, ignition coil, and other similar parts wherever there's room for them on the firewall or fenders.  The computer can be mounted behind the glovebox, in the same place the Jeep's was originally.  In order to make this work, it's necessary to make a hole in the firewall for the cable to pass through.  You can see this some of this stuff in the photos above.

• The battery ground gets connected directly to the engine block with 4-gauge wire
• The block gets grounded to the firewall with a wire running from an unused bolt hole in the rear of the passenger-side head to one of the mounting bolts for the starter solenoid.
• The engine harness ground wire is connected to the block via one of the air-con compressor bracket mounting bolts.
• The Jeep's electrical system ground wire (from the bulkhead connector) is grounded to the same starter solenoid mounting bolt as the block.

On to the next section, where all the smaller bits and pieces are covered

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