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Mod Motor Wiring in General

FuturaGuy

Well-Known Member
Staff member
A number of people have asked me, via PM's, how to wire a Mod Motor into a Fox body. I don't claim to be an expert but if it's of any help to anyone, I'll try to pass along some of the things I learned from experience.

Fuel injected Foxes have a pretty much separate wiring harness for the computer. The big difference in the SN-95 Mustangs with Mod Motors is the computer wiring is integrated with the rest of the wiring in the engine compartment. The Mod Motor has 5 wiring harnesses under the hood. There are two on the engine, the engine harness (P/N 12B637) and the starter harness (P/N 12A690). The computer harness (P/N 12A581) is on the right side of the car, and the fuse box harness (P/N 14290) is on the left. The transmission harness (P/N 15525) plugs into the computer harness.

The starter harness has the battery terminals, connecting to the battery mounted left front. The starter harness connects to the fuse box harness thru two connectors (#112 and 113) and with a cable to the stud on the fuse box. The starter harness also connects to the engine harness thru an 8-pin connector (#1005). The fuse box harness connects to the engine harness thru a two pin connector (#100) and a fuse link to the stud on the fuse box. The fuse box harness also connects to the computer harness thru a 16-pin connector (#108) and a 4-pin connector (#109) both behind the right headlight. The computer harness connects to the engine harness with two connectors, a 40-pin (#104) and a 4-pin (#101). The computer harness has 4 connectors (#202, 213, 216, and 259), and the fuse box harness has two connectors, a 24-pin (#105) and a 16-pin (#107), all six of which eventually connect to the dashboard. The attached sketch below of the harness interfaces spells it out more clearly. Everything to the left of the vertical dashed line is in the engine compartment, and everything to the right, the main harness and instrument panel harness, are unique to SN-95's and don't apply to Fox body cars.

If you're putting a Mod Motor in an injected Fox, what you need to do is break into the bundle of about 40 wires that comes from the dashboard and thru the firewall with a big rubber grommet. A non-injected Fox is somewhat different because the battery is on the right and there is one harness that forms a large "U", connecting at the center to the dashboard wiring, and both ends coming down opposite sides of the engine compartment. I recommend mounting the battery on the left (or in the trunk, an easy modification), using the Mod Motor starter harness, keeping as much of your Fox wiring as possible, and removing what you don't need (lights, windshield washer, ABS, air bags, etc) from the Modular computer and fuse box harnesses. The fun part will be matching the wires coming out of your dash to what's left of the Modular harnesses. It's not difficult, just tedious, and the best investment I made besides the wiring diagrams for both the donor and receiver cars was a $15 multi-meter from Radio Shack.

There are a few differences worth noting. On Foxes, the fuel pump relay is near the tank while on SN-95's, the relay is in the kick panel on the passenger side and plugs into the computer harness. Bypass the trunk-mounted relay and use the one on the Modular computer harness. Much easier that way. Also on the computer harness under the right front fender are a fuel pump resistor (it's made of white clay or porcelain and looks like something from the '20's) and the Constant Control Relay Module. You need both to make the Mod Motor run. If you're going to have cruise control, grab the Modular electronic unit from under the left front fender. The cruise control from a Fox is a real Rube Goldberg device that used vacuum diaphragms, resistors, chain linkage, and other strange stuff. The electronic unit is more reliable and can be connected using the wiring that goes to the relay boxes under the Fox dash.

I've counted about 16 different combinations of Mod Motors/transmissions and Fox body cars, even without counting Mark VIII's or 3-valve Mod Motors. I don't know the specifics of every possible combination but I hope what I've written here will be a starting point.

Good luck with your conversion!
 

Attachments

Kurt

Active Member
There are a few differences worth noting. On Foxes, the fuel pump relay is near the tank while on SN-95's, the relay is in the kick panel on the passenger side and plugs into the computer harness. Bypass the trunk-mounted relay and use the one on the Modular computer harness. Much easier that way.
A few inaccuracies here. Fox fuel pump relays are not mounted in the rear of the car. They are either under the drivers seat (early) or under the hood on the passenger side fender apron (late).

SN-95's with return fuel systems have the fuel pump relay integrated into the CCRM. Not in the kick panel.
 

FuturaGuy

Well-Known Member
Staff member
Thanks for your input.

The wiring system I have is '98 non-California and the fuel pump relay is separate from the CCRM, with the connector on the passenger side of the firewall (or kick panel) grommet. I'm sure Ford made changes along the way that they didn't tell us about and I would prefer to have had the fuel pump relay with the CCRM. Much better accessibility. If I ever have to replace the relay I'll have to take the dash part way out. Bitch of a job.

As to the location of the fuel pump relay on '87-'93 Foxes, I'll accept that on some models it is under the seat. My statement that it was near the tank was an assumption since it is shown adjacent to the inertia switch in the schematic. My information is from the 1988 EVTM. There are two wires, #361 (red) and #97 (tan/light green) that go thru connector C228 in the driver's kick panel and continue rearward in the connecting harness to the fuel pump relay. Since these wires were unused when I connected an '88 dash to a '98 underhood wiring system, I used them to supply power to and from the back-up light switch which, on an SN-95, is in the transmission harness and connects to the PCU harness under the hood.

I apologize for causing any confusion.
 
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FuturaGuy

Well-Known Member
Staff member
It's time to do an update on this thread. I've been corresponding with "WhiteponyGT" and he has found some differences in the 1999 and later wiring. The diagram shown in the first post applies to 1998 and earlier Cobras.

Here's the difference. For the '98's and earlier, the computer harness 12A581 has 4 connectors going to the dash, C202, C213, C216, and C259. On the 99's and later, C202 is not there at all, and the 8-pin C259 is replaced by 14-pin C200. Up thru 1998, a signal ran from the gas gauge to the computer, via Circuit 29, on a yellow wire with a white stripe. From 1999 on, there is no signal from the gauge to the computer.
 
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FuturaGuy

Well-Known Member
Staff member
Another update as a result of my running correspondence with "WhiteponyGT". Automatic transmission cars have a second connector, C119, between the computer harness and the transmission harness. They also use a different transmission harness, 7C078, in place of 15525 on the stick shift cars. On automatics, there are several more wires that go to the transmission and they're routed thru C106. The wires for the O2 sensors behind the cats are in C119. On stick shifts, there are fewer wires to the transmission and everything, including O2 sensor wires, runs thru C106.

You're never too old to learn something new.
 
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EvinH

Member
This is really great info and can't wait to be able to see the diagrams.

I, too, want to wire up the Fox interior/dash with a later model PCM and wiring. I think i jumped the gun on my conversion and got a set of complete 03 Cobra heads without actually looking into the swap before hand. Thanks for all those contributing. I hope to one day be able to do the same.
 

bd93cobra

Member
I am gathering information on whether or not to swap the drive train from an '03 Cobra into my '93 Fox. Thanks.
 
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Notch50

Member
Sounds like a lot of useful information here now. Just need to get 10 post so I can view it all.

I did a 2v swap using a wrecked supercharged 01 saleen back in 05, had it running but never completely finished it. It's been sitting in the garage since 06, got married and having kids put it on hold.. Got that itch now where I want to finish it and get it back on the road.

I have one problem tho with the car, I can't get it to fire up, actually haven't been able to get it to fire for several years now. Fuel is good, seems like spark is my issue, I have the entire harness from the 01 in the car and a chip to disable the pats but i seems like the pats is the problem. I had the chip checked just to make sure it was still good. May need a new PCM, which would suck because the one I have is already tuned for the supercharged setup.
 
A number of people have asked me, via PM's, how to wire a Mod Motor into a Fox body. I don't claim to be an expert but if it's of any help to anyone, I'll try to pass along some of the things I learned from experience.

Fuel injected Foxes have a pretty much separate wiring harness for the computer. The big difference in the SN-95 Mustangs with Mod Motors is the computer wiring is integrated with the rest of the wiring in the engine compartment. The Mod Motor has 5 wiring harnesses under the hood. There are two on the engine, the engine harness (P/N 12B637) and the starter harness (P/N 12A690). The computer harness (P/N 12A581) is on the right side of the car, and the fuse box harness (P/N 14290) is on the left. The transmission harness (P/N 15525) plugs into the computer harness.

The starter harness has the battery terminals, connecting to the battery mounted left front. The starter harness connects to the fuse box harness thru two connectors (#112 and 113) and with a cable to the stud on the fuse box. The starter harness also connects to the engine harness thru an 8-pin connector (#1005). The fuse box harness connects to the engine harness thru a two pin connector (#100) and a fuse link to the stud on the fuse box. The fuse box harness also connects to the computer harness thru a 16-pin connector (#108) and a 4-pin connector (#109) both behind the right headlight. The computer harness connects to the engine harness with two connectors, a 40-pin (#104) and a 4-pin (#101). The computer harness has 4 connectors (#202, 213, 216, and 259), and the fuse box harness has two connectors, a 24-pin (#105) and a 16-pin (#107), all six of which eventually connect to the dashboard. The attached sketch below of the harness interfaces spells it out more clearly. Everything to the left of the vertical dashed line is in the engine compartment, and everything to the right, the main harness and instrument panel harness, are unique to SN-95's and don't apply to Fox body cars.

If you're putting a Mod Motor in an injected Fox, what you need to do is break into the bundle of about 40 wires that comes from the dashboard and thru the firewall with a big rubber grommet. A non-injected Fox is somewhat different because the battery is on the right and there is one harness that forms a large "U", connecting at the center to the dashboard wiring, and both ends coming down opposite sides of the engine compartment. I recommend mounting the battery on the left (or in the trunk, an easy modification), using the Mod Motor starter harness, keeping as much of your Fox wiring as possible, and removing what you don't need (lights, windshield washer, ABS, air bags, etc) from the Modular computer and fuse box harnesses. The fun part will be matching the wires coming out of your dash to what's left of the Modular harnesses. It's not difficult, just tedious, and the best investment I made besides the wiring diagrams for both the donor and receiver cars was a $15 multi-meter from Radio Shack.

There are a few differences worth noting. On Foxes, the fuel pump relay is near the tank while on SN-95's, the relay is in the kick panel on the passenger side and plugs into the computer harness. Bypass the trunk-mounted relay and use the one on the Modular computer harness. Much easier that way. Also on the computer harness under the right front fender are a fuel pump resistor (it's made of white clay or porcelain and looks like something from the '20's) and the Constant Control Relay Module. You need both to make the Mod Motor run. If you're going to have cruise control, grab the Modular electronic unit from under the left front fender. The cruise control from a Fox is a real Rube Goldberg device that used vacuum diaphragms, resistors, chain linkage, and other strange stuff. The electronic unit is more reliable and can be connected using the wiring that goes to the relay boxes under the Fox dash.

I've counted about 16 different combinations of Mod Motors/transmissions and Fox body cars, even without counting Mark VIII's or 3-valve Mod Motors. I don't know the specifics of every possible combination but I hope what I've written here will be a starting point.

Good luck with your conversion!




I may have jumped the gun and installed the full 03/04 harness and wired up what was needed in the ignition switch and lights and what I had presumed was everything to maintain all fox interior. I recently started to get prepaired to plug into the battery and decided to check some continuity before I just jumped in. I am getting continuity from neg to positive lead at the relay box where cables hook up. I put a light in line on the positive to see if I had a short and it looked bad as the light is bright. I am confused as it seems as though there is a positive in the system that is hooked straight to ground or something. I seemed to have isolated it to the two runs in the box that are for ignition and starter circuits. I have a power probe and I have been trying to get it to help me but without being fully hooked to the battery it is being a bit difficult. Do you have any ideas as I am confused at this point and could use some help. I wish that I would have caught this post earlier as it appears to be much less involved or at least I seem to understand it a bit better. I need to fix this as I am realy excited with the direction that I have been going up until this wiring situation. Thank you for your time and any assistance is appreciated.
 
I may have jumped the gun and installed the full 03/04 harness and wired up what was needed in the ignition switch and lights and what I had presumed was everything to maintain all fox interior. I recently started to get prepaired to plug into the battery and decided to check some continuity before I just jumped in. I am getting continuity from neg to positive lead at the relay box where cables hook up. I put a light in line on the positive to see if I had a short and it looked bad as the light is bright. I am confused as it seems as though there is a positive in the system that is hooked straight to ground or something. I seemed to have isolated it to the two runs in the box that are for ignition and starter circuits. I have a power probe and I have been trying to get it to help me but without being fully hooked to the battery it is being a bit difficult. Do you have any ideas as I am confused at this point and could use some help. I wish that I would have caught this post earlier as it appears to be much less involved or at least I seem to understand it a bit better. I need to fix this as I am realy excited with the direction that I have been going up until this wiring situation. Thank you for your time and any assistance is appreciated.
I also showed it drawing 2.6 amps when hooked up which seems to be quite high which means to me that it has a direct short of some sort.
 
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