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How to Install a Holley Terminator X EFI System on a Foxbody Mustang

By: Wes Duenkel | 10/20/2020 < Back to Motor Life Home

In the age of high-top sneakers and room-sized computers, Ford's EEC-IV sequential electronic fuel injection system introduced on the 1986 Mustang was a revelation. It not only set the standard for sophistication but also the incorporation of a mass airflow (MAF) sensor in 1988, made Ford's EEC-IV extremely adaptable to performance modifications. The MAF-based strategy handled most bolt-on parts' added performance with ease. Even mild lumpy camshafts were no problem.


However, when things got serious the EEC-VI needed to be taught new tricks by using a "chip." At the time there were dozens of chip tuners in the marketplace. However, with the advent of OBD-II port tuning, the number of tuners with the equipment (and the experience) to flash a chip has dwindled. These days you have to find an "old guy" to tune these "old cars."


Luckily today, we have better options. Holley just rolled out a perfect solution for 1987-1993 Mustangs: The Terminator X Fox Body MPFI kit. Part number 550-937F is a "plug-and-play" solution that includes a new harness to replace the crusty wiring in a Fox Mustang. Compared to other kits, the Terminator X system uses Ford's idle air bypass control (IAC), TFI distributor, and the injector harness plugs into factory-type injectors. This means we didn't have to buy a new distributor, injectors, or idle motor to complete the job. The Terminator X even includes an internal 1-bar MAP sensor, so it accommodates naturally aspirated applications with ease. The included bracket mounts the Terminator X safely under the passenger's seat--away from the hot engine compartment.


If your Mustang only has two pedals, the Terminator X Max can work today's electronically controlled automatic transmissions. Both the Terminator X and Terminator X Max systems can do all sorts of cool stuff, including running electric fans, boost control, nitrous injection, and log data.


All this made installing the Terminator X extremely straightforward. Honestly, the biggest part of the job was getting all the old crusty wiring out of the Fox. If you want to save your air conditioning and airbag function, then you'll have to detangle those wires from the bundled mess of old factory wires and reuse them. Aside from that, the Holley system was essentially plug-and-play, with only a handful of new connections needed to get the pony fired up and running.


Note that we said "running," and not "tuning." Because the Terminator X incorporates Holley's killer wideband O2-based self-learning strategy, the system tunes itself while you drive. Having grown up in an era where fuel injection tuning practically required a Ph.D. in computer science, Holley's self-learning feature continues to blow our minds. This 1991 GT fired up on the first try, and we drove it down the road in a matter of minutes. This self-tuning stuff is magic!


But enough talk about what the system can do. Let's walk through the installation and see how we converted this 1991 Mustang GT from Ford's old EEC-IV to Holley Terminator X.

Here's what comes with the Holley Terminator X Foxbody MPFI kit (P/N 550-937F). Trust us: it looks worse than it is. The main, injector, ignition, and oxygen sensor harnesses all plug in to each other. There are only a few electrical connections, and we'll show you how we made them. Note the handy bracket to mount the Terminator under the passenger seat.


After draining about a gallon of coolant, we removed the upper intake manifold to get access to the factory injector harness.

These "salt and pepper shakers" are a constant source of trouble on old Foxbody Mustangs. Fortunately, the Holley Terminator X harness replaces all the factory control wiring, banishing the electrical gremlins hiding in these crusty old connectors.


We began by removing the factory injector harness. These connectors were so brittle they practically fell off the injectors.


Next, we removed the passenger seat for two reasons: 1) to give us more working room when accessing the kick panel, and 2) we needed to remove the seat to install the Terminator X and mounting bracket.


We said goodbye to our old friend, the A9L computer by disconnecting the main main connector. Note that you have to "unbolt" it from the computer--it doesn’t just pull out.


Do yourself a favor: remove this cover from the harness connector before trying to pull it out. We learned this the hard way. (This photo is outside of the car for clarity.)


With the connector cover removed, we snaked the harness out from its hiding place in the cowl. The harness comes out intact if you're patient.


We carefully cut the grommet from the harness and set it aside for later.


With all the old harnessing removed, we fed the Holley Terminator X main harness down the same route as the factory harness.


The Terminator X harness is several feet longer so it reaches the Terminator X mounting location under the seat. We cut a slit in carpet to accommodate the harness and routed it along the rocker panel.


Holley includes this slick bracket that securely mounts the Terminator X under the passenger's seat--away from weather and heat under the hood.


Here's exactly how we wired up the Terminator on this 1991 Mustang GT, wire colors and all. Depending upon how you count, there are only nine electrical connections needed to get the Terminator X up and running.


Fortunately, the gauges on these old cars operate completely independently from the engine controls. Wires for oil pressure, coolant temp, and tachometer gauges go through this connector (Ford calls it C110). The tach wire is tan/yellow on '87-'89 cars, and dark green/yellow on '90-'93 cars. (Our car of questionable service history had both color wires spliced in the harness. Whatever.) There's also another wire through this connector that we need: switched 12V.


We used the factory red/green wire from connector C110 and routed it to the kick panel where we made the switched 12V connection to the Terminator X harness.


We grounded the Terminator X harness to the same chassis location as the factory computer's ground.


Rather than run a wire all the way over to the factory fuel pump relay location (under the driver's seat), we determined that the red wire from connector C216 (right here at the kick panel) already made the trip over to the fuel pump relay. So, we connected the green fuel pump wire to this connector's red wire. (Note that this is also the connector for the air bag sensors, which we detangled from the factory main harness and retained.)


Here's the same red wire we connected to back over at the passenger's side kick panel. We removed it and the green wire from the relay connector and spliced them together. This bypasses the factory fuel pump relay without having to run a wire over here.


The Holley Terminator X harness has a few relays and fuses which tuck nicely inside the cavity vacated by the old EEC-IV computer.


Holley includes a P/N 558-305 ignition harness that plugs directly into the factory TFI distributor and the main harness.


Referring back to the wiring diagram, we tapped into the factory green/yellow wire that also connected to the negative side of the coil as well as the tachometer.


Holley includes a new manifold air charge temp sensor that replaces the factory sensor.


Also included is a new coolant temp sensor for the Terminator X. (Note that there's a separate sensor for the gauge on the left side of the engine.)


Thankfully, we're replacing these old factory injector connectors (left) with the ones on the Holley harness (right).


All the harness ends are labeled to make plugging them so fool proof that even a Chevy guy can figure it out.


Here's where the magic happens. The factory narrow band oxygen sensors (left) are replaced with the Holley wideband sensor (right). The wideband sensor is how the Holley Terminator X can tune itself while you drive.


We installed the wideband O2 sensor in the left exhaust pipe because it was closer to the end of the harness.


Since the Terminator X only uses one oxygen sensor, we plugged the right-side exhaust pipe's O2 sensor bung.


Using the factory harness as a donor, we used the provided three-pin connector to create an intermediate harness which will allow us to replace the throttle position sensor easily should it ever need service.



Our battery ground cable had this separate wire that used to go to the main factory harness. We used it for the battery ground for the Terminator X. The positive lead for the Terminator X should always be routed directly to the battery.


The Holley Terminator X has an internal 1 bar map sensor that's suitable for naturally aspirated applications. We connected it to some 1/4" vacuum hose routed to the engine compartment.


The other end of the hose went to the vacuum tree in the back of the manifold.


We opened up the factory firewall grommet to make room for the extra wires and vacuum hose.


Before starting the engine, we connected the included handheld programmer to the CAN connection on the main harness near the kick panel.


My, how things have changed! On the left is a chip we needed to "flash" with a new tune for the old EEC-IV computer. The Terminator only needs a small touch-screen device to get it set up.


After answering a few questions via the "wizard" on the handheld, our engine was literally up and running in a couple minutes. The Terminator X tunes itself while driving. Impressive!


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