1979 F100 EFI Wiring
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EFI Wiring

Here's a bunch of stuff for the EFI wiring harness along the frame to the gas tank(s) and to the external fuel pump. I started with the frame wiring for a 1987 F150 and pulled out just the wires I needed to build my own custom harness - I did not need all of the wiring for the ABS (I'm not swapping it onto this truck) or the rear taillights (already wired and working in the original wiring on this truck). What's nice is that I did not have to cut or splice anything to do this, I have an extra ground connection at the rear of the harness should I need it later, and I still have the connector for the second fuel tank if I ever decide to add it. And it will use all factory Ford weatherproof connectors - pretty slick, or at least I think so. I ran the new EFI wiring harness through the existing "large" plastic hoop clips used for the 1979 wiring, so it all looks reasonably factory and will work well.

I un-taped the original harness to get at the individual wires and separated out just the fuel pump, fuel sender, and related ground wires. The ground wires are spliced midway in the harness, so I just left those pieces taped up and re-used them as-is. I left the factory harness alone for the short sections of the harness that go up and on top of the gas tank to the the flat four place connector for the low pressure fuel pump (power + ground) and fuel level sender (gauge + ground). These sections had decent split loom on them and good fabric tape, so I left it alone. There was an extra ground wire going to the rear round connector - likely a ground for the taillights or something - and rather than cut it, I just taped it to the side of the harness and left it alone. It exits the taping around where the wiring goes from the side frame rail towards the center of the truck to connect to the rear gas tank. Since there are wires for two tanks, there is a second tank connector part way down the frame. I left this also and just taped it to the side of the harness to keep it out of the way.

These two photos are of the connectors that were at the front (square connector) and rear (round connector) of the harness and that had wires I needed connected to them. I carefully removed all of the wires from each of these connectors (see here for more details) to make it easier to separate the wires I needed - it's normal for wires to be twisted and intertwined in the harness, and this made it a breeze to separate them. First, you pull the red/orange piece from the "inside" of the connector, then you wiggle the rubber gasket out of the back of the connector (for the weatherproof style connector), and then you can depress the little plastic clips that hold each terminal in place so you pull the terminals out of the back of the connector. The terminals can be pulled through the rubber gasket for the weatherproof style connector without too much trouble. After the you get the connector disassembled, you have individual wires with terminals still attached so they can be re-used later.

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Here's the wires laid out alongside the truck. The rear fuel tank is already installed and the harness is connected to the tank. I started re-taping with vinyl harness tape (the non-sticky kind) right where the factory split loom ended. There are close-ups are, in order, for the external high pressure fuel pump, forward fuel tank + extra ground wire, the extra ground and the wiring heading up the rear gas tank, the wires and factory ground splice at the tape-out point for the forward fuel tank wiring, and the terminals at the front of the wiring harness. Also, you'll note that Ford used orange for one of grounds coming from each tank. It's really odd to see orange and black wires spliced together, but it's "correct" from the factory. Go figure. I also found out that the VSS sender on Mustangs has the orange and white wire connected to ground also, so apparently the "orange is a ground" idea pervades Ford's electrical engineering department, or at least is did in the '80s and '90s...

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Here's the final harness taped up and ready to be installed. I had one wire that ran long as I got up towards the front of the harness, so I doubled it back on itself a bit to make sure that the terminals all ended up in about the same place when I was done. The final harness will look much nicer if all the terminals are roughly lined up.

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Here's a close-up of the ford fuel tank wiring and the extra ground wire taped to the outside of the harness to keep them out of the way.

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Here's a close-up of the front of the harness. Note the small bit of fabric tape used to keep the vinyl harness tape from unwrapping. The wire colors and functions for all six wires that come to the front of the harness are listed below for reference. The last two are specific to the front tank and I'm not using them, but they're in the harness so I'm noting it here. The ground wire is slightly larger than the others since it carries the return current from multiple things.

  • Black - Ground for everything, all three fuel pumps plus both fuel level senders
  • Pink with black stripe - High pressure fuel pump feed
  • Brown with white stripe - Rear low pressure fuel pump feed
  • Yellow with blue stripe - Rear tank fuel level sender
  • Red - Front low pressure fuel pump feed
  • Blue with yellow stripe - Front tank fuel level sender

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These are the Ford fuel and brake line clips - on the left are the 1979 versions and on the right are the 1987/1989 versions. I used the 1987/1989 versions - in particular, the triple place one does a nice job of holding the brake line plus the fuel feed and return lines, all using the original mounting holes in the 1979 frame. There were enough spare holes to make it work out, and most of the original clips were in a decent position to be used with the new EFI hoses. To remove the 1979 clips, you need to drive the center pin out from the "back" side (I used an appropriately sized drive pin punch and a small hammer), then the entire clip can be easily removed by pushing on it from the back with just your finger. The 1987/1989 clips need to be slowly twisted and pulled out of their mounting holes if you want to re-use them. The lines simply push into the 1987/1989 clips once they are fully pushed into the mounting holes in the frame - it's very fast assembly for that part.

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Here's the paper template I made to locate the new mounting holes I had to drill in the frame rail for the reservoir. I just made a rubbing of the mounting area and then drew the basic outline of the unit onto the paper, then cut it out and punched out the holes for the mounting bolts. Then I held the paper template up to the inside of the frame rail (make sure it was facing the correct way), center punched the mounting holes, and drilled pilot holes, then drilled full size holes from the outside of the frame rail. It fit perfectly on the first try. I located it forward and aft based on the length of the EFI fuel lines to the rear gas tank. Up and down it was a pretty snug fit into the frame, so I just centered it as best I could and it worked fine.

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The original engine wiring harness on the truck terminates in two connectors near the driver side hood hinge, one four wire wedge lock style connector (with only 3 wires used), and a second four wire weatherproof connector for the Duraspark system. On the engine, this harness connects to the temp sender (1 wire), oil pressure sender (1 wire), coil (two wires), and the distributor (three wires). The Duraspark connector is of no use to us for the EFI conversion, but the 4 place wedge lock connector is - it has the following wires coming out of it (engine harness side), two of which we'll use below. On the factory diagrams, the 4 place wedge lock connector is connector C220, and it has these wires in it:

  • White with red stripe - connects to the oil pressure sender (use for EFI conversion)
  • Pink with no stripe - connects to the temp sender (use for EFI conversion)
  • Red with green stripe - ignition power to the coil (goes through a resistor wire, see below, can't use for EFI conversion)

Next to the 4 place wedge lock connector on the firewall, there is an separate 3 place wedge lock connector that is used for cruise control, if the truck is equipped with it (at least according to the factory wiring diagrams). This is important because it contains a fused ignition power wire that does not go through the ignition resistor, and thus gets a full 12V. Everything I can find about the Mustang EFI setup shows that the coil can/should be wired directly to a full 12V feed, not a reduced voltage feed (via a resistor or resistor wire) like the old coils were. This is much like a GM HEI system. On the factory diagrams, the 3 place wedge lock connector is connector C327, and it has these wires in it:

  • Red with yellow stripe - ignition power for cruise control (full 12V feed, use for EFI conversion)
  • Red with green stripe - ignition power to the coil (jumpered from C327, see notes above, don't use for EFI conversion)
  • Black - ground (don't use for EFI conversion - we need a better/bigger ground)

Here's the Mustang EFI harness I'm using in the engine compartment. There are more details on my Ford EFI Swap page, and the brief listing of the wire colors and purposes is below. Basically, I untaped the entire harness to trace wires for the external connectors (grey and black in the engine compartment, green inside the passenger compartment), to move the main power feed to the passenger's side, and to re-tape the harness in a slightly different way so I could move the place where it goes through the firewall. I wanted to preserve room on the passenger's side for the AC unit to go there later, so this will be going through the firewall in the middle area. I had to get enough length in the harness to get it to double back and reach that area by pushing the large rubber firewall grommet/seal towards the computer connector a few inches. I won't have as much harness inside the cab this way, but that's what I want, so it worked out OK for me. I basically pushed is as far as I could towards the main EEC relay - even inch counts in the engine compartment for this job.

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Here are descriptions of the wires in all three of the main connectors that need to be wired up to stuff on this vehicle, with the items in italics in the "connect to on vehicle" column being what I'm doing on this specific vehicle.

Grey Connector (in engine compartment)

Wire color Purpose Connect to on vehicle
Black with yellow stripe Not connected in EFI harness n/a
Pink with light blue stripe Not connected in EFI harness n/a
Dark green with yellow stripe Tachometer signal wire Connect to tachometer if you have one
Connect to existing green aftermarket tach signal wire
Orange with black stripe Not connected in EFI harness n/a
White with pink (or possibly red) stripe Oil level sender Not normally used in most retrofit applications, if not, ignore
Route inside cab, then terminate - may hook up to a warning light at a later date
Green with purple stripe AC request AC switch under dash if you have AC, if not, ignore
Terminate at a one wire connector in engine compartment- no AC at this time
Red with blue stripe Start power/signal Starter solenoid trigger wire
Connect to starter solenoid wiring at closest possible/reasonable place
Red with green stripe Ignition power Connect to ignition powered wire, usually previous coil power feed
Connect to red with yellow stripe wire in original three wire cruise harness connector

Black Connector (in engine compartment)

Wire color Purpose Connect to on vehicle
Red with white stripe Temp gauge sender Connect to original temp gauge sender wire
Connect to pink wire in original four wire engine harness connector
Blue with yellow stripe NDS (Neutral Drive Switch) sense Automatic transmission only. If automatic trans O2 harness is used, comes from pin #30 on the 60-pin EEC connector, the manual trans O2 harness connects this to nothing. The computer can tell if the car is in neutral or drive via the resistance to ground on pin #30. The starter solenoid has a small but noticeable resistance to ground when the NDS switch is in park or neutral, and it's an open connection in reverse or drive. This tells the computer how to idle the engine and react to certain off-idle and dead throttle conditions. Connects to the ignition switch side of the NDS through a diode to prevent back-feeding. (Diode is shown in various factory diagrams)
Ignore for this conversion - manual transmission.
Yellow with red stripe Not connected in EFI harness n/a
Pink with black stripe Fuel pump monitor Comes from pin #19 on the 60-pin EEC connector, connect to main fuel pump feed wire after relay. EEC uses this to monitor the fuel pump power to see if the relay is OK and the pump has power.
Make up a new custom fuel pump wiring harness for this with inertia switch and relay
White with red stripe Oil pressure gauge sender Connect to original oil pressure gauge sender wire
Connect to white with red stripe wire in original four wire engine harness connector
White with pink stripe NGS (neutral gear switch)/CES (clutch engagement switch) sense Manual transmission only. If manual trans O2 harness is used, comes from pin #30 on the 60-pin EEC connector, the automatic trans O2 harness connects this to nothing. In my case, goes to O2/oil level harness as a white with purple stripe wire, and in my 02 harness it loops back to the main harness without connecting to anything else in the O2 harness. The NGS and CES switches are wired in parallel and prevent engine from starting except with clutch pedal depressed or the shifter in neutral, and help the computer tell if the transmission is "in gear" so it knows how to idle the engine and react to certain off-idle and dead-throttle conditions. The other side of the NGS and CES switches connects to pin #30 of the 60-pin EEC connector. Connects to the non-ground side of the NGS and CES switches.
Route inside cab to a two pin connector with black with white stripe wire from green connector - the two pin connector will be a connection point in the wiring inside the cab so appropriate NGS and CES switches can be hooked up later if desired - leaving these disconnected will make the computer think the transmission is in gear and the clutch is not engaged
Green with white stripe VSS positive (+) Connect to positive wire from VSS and to cruise VSS feed (if needed)
Connect to F150 VSS harness going down to the VSS on transmission
Orange with yellow stripe VSS negative (-) Connect to negative wire from VSS and to ground
Connect to F150 VSS harness going down to the VSS on transmission and to cruise VSS feed inside cab

Green Connector (inside cab)

Wire color Purpose Connect to on vehicle
n/a Not connected in EFI harness n/a
Black with white stripe NGS (neutral gear switch)/CES (clutch engagement switch) ground reference Manual transmission only. Comes from pin #46 on the 60-pin EEC connector, this wire is the ground reference for NGS and CES switches in the factory harness. The NGS and CES switches are wired in parallel and prevent engine from starting except with clutch pedal depressed or the shifter in neutral, and help the computer tell if the transmission is "in gear" so it knows how to idle the engine and react to certain off-idle and dead-throttle conditions. The other side of the NGS and CES switches connects to pin #30 of the 60-pin EEC connector.
Route inside cab to a two pin connector with white with pink stripe from black connector - the two pin connector will be a connection point in the wiring inside the cab so appropriate NGS and CES switches can be hooked up later if desired - leaving these disconnected will make the computer think the transmission is in gear and the clutch is not engaged
n/a Not connected in EFI harness n/a
Tan with no stripe MIL (Malfunction Indicator Light) also known as the "check engine" light Connect to negative side of MIL, positive side of MIL goes to fused ignition power
Wire up to a custom MIL light under dash
Grey with yellow stripe HEGO (oxygen/O2 sensor) heater power Connect to fused battery power through a relay triggered by an ignition feed (aka, the red wire from the green connector). Having the HEGO relay inside the firewall is funky, but it's the way Ford did it.
Connect to fused battery power through a relay triggered by the red wire from this green connector
n/a Not connected in EFI harness n/a
Tan with green stripe Fuel pump relay control Comes from pin #22 on the 60-pin EEC connector, connect to negative (ground) side of fuel pump relay coil to allow the computer to turn the fuel pump on and off.
Make up a new custom fuel pump wiring harness for this with inertia switch and relay
Red with no stripe Fuel pump relay feed Comes from pin #57 on the 60-pin EEC connector, connect to positive side of fuel pump relay coil and HEGO relay coil.
Make up a new custom fuel pump wiring harness for this with inertia switch and relay

Here's an initial drawing I made up to detail the small custom adaptor harness I need to build to connect the grey/black connectors to the frame harness (to the fuel tank and pumps), the VSS harness, and then into the cab to connect to the green connector. There is also wiring in here for pre-existing work on this truck such as the fog lights and some extra add-on back-up lights to make loading and trailer hookups easier in the dark. I decided to incorporate it into this harness to make it easier - the original wiring for those things was pretty lame and done before I knew how to wire things "correctly". It was also just run through the open hole around the old speedo cable, which was messy at best, and hazardous at worst - the wires were rubbing directly on the edge of the hole in the firewall without a grommet. Like I said, it was done before I knew how to wire stuff correctly. Now that American Autowire sells special bulkhead connectors (see part #'s 500548, 500564, 500547, 500566, and 500666 in their catalog for more details) and firewall grommets (see part #'s 500181, 500182, 500183, 500185, 500186, and 500187 in their catalog for more details) to make this easy, it's just dumb to not do it right the first time. Being able to unhook and extract the wiring later on for service or repair without cutting it is kind of a nice thing, especially since you'll most likely be the person doing the extraction, repair, and cutting of the harness down the road...

Here's a more updated copy of that same basic wiring diagram, and this one prints much more easily/clearly. I also filled in a bunch of previously missing details on the existing custom wiring, plus some other bits. I did this after I got started the EFI wiring work and realized just how messed up the older custom wiring was... I'm kind of a nit-pick for wiring stuff. Keep in mind the wiring colors on these reflect the actual harnesses and connectors I used, which were a mixture of Mustang and F150 pieces that I had laying around. I tried to avoid splicing stuff and extending wires, so I used wire stubs with terminals already attached, and thus this diagram is a bit funky in places, particularly the VSS hookups. For some reason, I could not find the F150 VSS harness connector to use, so the wiring stub and connector I used were for something completely different, with completely different wiring colors. That's why the colors are all over the map.

Here's the VSS and harness I used - I think it came out of a 1987 F150 that I parted out. I found the VSS harness in my pile of Ford stuff when looking for a VSS connector to use as a basis for a custom wiring harness and it was the perfect length to get the wiring up and into the engine compartment, so I just used it as-is. I even found a spare connecter that fit the top end of the VSS harness so I could connect the VSS harness to the custom adaptor harness detailed above. The speedo cable is one I had in the garage - again, likely from a 1987 F150. The original speedo cable was about 77" long and the one I used was about 75" long. I did have to buy a new VSS - the one I had laying around was missing it's mounting bracket. I swapped the original speedo gear from the original speedo cable onto the VSS so the speedo would be the same as it was before - not entirely accurate due to the rear axle swap, but not too far out of whack, either. It looks like Ford used pretty much the same speedometer connection for years and the VSS mounting into the transmission is a standard Ford thing. And, the VSS hookup to the cable is standard as well for all the VSS applications I've seen in the 80's and early 90's. For a speedometer cable, as long as the end connections match your needs, the only other important thing on a speedo cable is length. If you can't find something in your stash of parts that will work - remember, grab everything imaginable from the donor vehicle for any EFI swap project - then go buy a new speedo cable. You can get them in all sorts of arrangements and lengths - go visit NAPA or check out the stuff online at RockAuto.com to find something that will work for you. They're not a terribly expensive part, so spending time fretting over what to use can be easily solved with a quick trip to the parts store.

It took me a while to figure out how to get a Ford speedometer cable out of the VSS. There is a small metal clip in there, and the cable pushes into place and then the clips snaps into place. To get the cable out, you have to gently spread the open end of the clip while pulling on the cable and it will come out. Pushing the clip's ends apart with the end of the finger works, but the clip ends are prone to dig into your skin in a moderately painful fashion. With all the grime that accumulates on these things under the car, my first attempts to remove the cable were, well, frustrating. I think I ended up completely mangling and removing the small metal clip with a pair of needle nose pliers. It wasn't pretty. Ah, the joys of learning on junkyard parts...

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This is the original engine harness after un-taping it. The four wire connector on the right of the first picture (which only has three wires in it) is the one I need to salvage and re-use, since this gets some important connections to the factory wiring that the EFI needs to hook up to - the ignition power source, as well as connect up the oil pressure and temp gauge senders to the original gauges. The second picture is a close-up of that connector showing where the three wires exit the connector and which wire location is unused.

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Here's the connector by the brake booster that the factory engine harness wiring connects to. The adaptor harness will plug in here, among other places.

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Here's the other connectors by the brake booster. The round weatherproof black one at the bottom went to the Duraspark box (there was a second round weatherproof one connected to the engine harness), the green one is the main connector for the original frame harness and various other stuff (NOTE: this is not the same as the green connector for the EFI harness, though it is physically identical - the EFI one is inside the cab near the 60-pin EEC computer connector on the EFI harness), a small 3 pin connector that was not hooked up to anything on this truck, but is used for cruise control if the truck is so equipped (at least according to the factory wiring diagrams), and the 4 pin connector that went to the original engine harness. The round weatherproof connector will remain unused after the EFI conversion. The 4 and 3 pin connectors will be used in the EFI conversion, and I'll also tap into the wiring at the green connector for the fuel gauge wiring coming from the new frame harness and for the backup lights (improving on the pre-existing custom wiring). I'll have to put a small connector on that to allow the harnesses to be separated in the future if needed, or possible a jumper harness if I can find a pair of matching connectors to use for it. I think I have some in my spares pile, and they appear to match the green connectors used inside on an EFI harness...

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This is the factory connector for the neutral start and backup light switches on automatic transmission trucks. This truck is a manual transmission truck, so there is no neutral start switch and the wire for the neutral start switch just doubles back onto itself. This is where I'll tap into the start circuit for the EFI adaptor harness. I can easily splice into this wire and this connector will provide a handy disconnect point for the adaptor harness. The backup light wiring on my truck does not go through here - the switch is down under the truck on the shifter linkage, and it uses a separate custom harness to get to it. I'm not sure why Ford did it this way since it's wasteful, but it is what it is.

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I finally remembered to install the short bit of smog pump hose and diverter valves onto the passenger's side of the motor. I also installed the vacuum harness for this so that the vacuum lines would be connected. There's vacuum harness connects to the vacuum lines behind the upper plenum, and has one vacuum line for each diverter valve (2 of them), one to the EGR valve, and a connector to go the vacuum valve assembly that's not installed yet. The three connectors at the bottom left are for those vacuum valves, and the rubber hose under them goes to the exhaust to allow the smog pump to pump air towards the catalytic converters. I'll have to custom make a hookup for that later on - for now, it'll dump to atmosphere and be a bit noisy.

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To get to all the wiring inside the cab, and to be able to have room to make the large hole in the firewall for the EFI harness, I had to yank the entire dash out. I was originally trying to figure out how to do the work without removing the dash, and after some time, I realized it was just easier and smarter to yank the whole thing and work on it that way. Sometimes a bit more work up at the start and end of a job equates to less work overall, and a better/easier/more correct job as well. Once the instrument cluster was removed, the dash itself came out surprisingly easily - a few screws on top held it to the metal lip under the windshield, a few bolts at the steering column to free that up, one bolt on the lower edge on each side - and it was free. I had to remove the headlight and wiper switches from the dash to un-do things at that end (it was easier than fighting with the wiring plugs on those things), disconnect the two cables for the heater (temp and air outlet control) down at the heater box, and unplug a few wiring bits where they plugged into various things (like the blower switch, radio, aftermarket gauges, etc.), and that was it. I took some photos of the wiring, especially the aftermarket gauge wiring, to help me remember how to put it back together later. I also removed the defroster ducting to get good access to the spot I wanted to put the hole for the EFI wiring harness. I checked for adequate computer room in the area of the hole I would be making, and it looked good, so I proceeded with the plan to put the hole between the wiper motor and the wiring harness grommet on the passenger's side. This would clear the wiper linkage and give me some room to mount the computer in this general vicinity.

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I also removed the heater assembly so that I could remove the firewall insulator so it would not be destroyed while drilling the EFI harness hole. Also, I may decide to upgrade to the AC-style setup I have in the shed - the Deluxe Hi-Lo heater. The stuff inside the firewall is almost identical to the AC stuff, with the only difference being the control panel and some wiring. The engine compartment side changes a bit, mainly by installing the evaporator in the same housing the Deluxe Hi-Lo heater uses and hooking up the AC hoses to it. The bit deal is making the required huge hole in the firewall for the thing...

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Here's the location of the hole for the EFI harness. It requires an oval hole, 1.5" x 3". You can make it by using a 1.5" hole saw to drill two side by side holes, and then removing the two volcano-shaped bits in between to make it one large oval. You can see this marked out in white marker in the second and third photos. I did this to verify spacing and such by tracing around a 1.5" hole saw and then drawing two straight lines to make the oval. After drilling the holes, removing the two volcano-shaped pieces and carefully deburring it on both sides with a file, I painted the surrounding area with some black paint to prevent rusting. After that dried, I pushed the EFI wiring through the new hole in the firewall, and the grommet slid right into the hole like it was meant to be there. My only possible complaint (other than not having green paint for the engine side of the firewall to make it look better) was that the hole was a tad loose for my tastes. A hole saw tends to wallow out the hole slightly, so instead of a 1 1/2" hole, it was probably closer to a 1 5/16" or even 1 5/8" hole. Basically, 1/16" to 1/8" too big. I need to get some scrap steel of the right thickness and try this with a 1 3/8" hole saw to see how that works. It worked with the 1.5" hole saw, and it's fine for this job, but when I get around to doing this on same basic EFI conversion my Ranchero, I want to do a better job than this.

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I had to do some repair work on the EFI harness at the AC WOT cutout relay. For some reason, one of the original wires was frayed badly and another was frayed near the connector. My guess it that the harness got too close to the exhaust or the AC compressor clutch was drawing too much current or something else odd. For the two wires in question, I pulled the wire + terminal out of the relay connector and wrapped just that wire with a couple of good layers of electrical tape, and then put the wire + terminal back into the connector in the original position. Then, I wrapped that section of the harness in vinyl harness tape to protect it and keep the electrical tape from coming loose. I would have preferred to use a new wire section, but I didn't have the right terminals to do that, so I went the tape route.

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Here's the roughly final routing of the harness on the passengers back side of the engine compartment. The power connector is hooked up at the starter solenoid, the AC WOT cutout relay is mounted just to the rear of the already existing fog light and backup light relays, and the MAF sensor is hooked up, and the harness is draped over an existing harness support above the fan area. I have not yet tied up the EFI harness near where the bundle of connectors in the middle of the first photo. I have to figure out where the EGR and two smog pump solenoids will mount at before I can do the final harness tie-up here. Also in the first picture, you can clearly see how the main harness doubles back on itself to get to the firewall grommet. The second photo shows the relays clearly, the wiring over to the solenoid, and the round connector (under the relays) where the O2 and oil level sensor harness will be hooked up at. That harness will run over to the back of the engine and down to the exhaust, as well as over to the passenger's side of the motor for the oil level sensor.

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This is the speedo cable going to my shiny new VSS, complete with a blue anodized aluminum housing. Ooh, shiny... (Whatever.) The speedo cable will have to be carefully routed at some point to keep it from coming into contact with the exhaust after I do that. I will have to build some sort of clamp or holder off the trans crossmember to keep it from moving around and rubbing on stuff.

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Here's the factory under-dash harness unwrapped. I wanted to find out if I could bypass the original ignition resistor, so I opened up the harness to see what I could find. In the end, I decided to not go this route and instead found another source for ignition power that did not run through the resistor wire. It's the cruise control and smog pump feed, which runs through a 7.5A fuse.

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The red loop of wire hanging down with the black fabric covering on it is the resistor wire - it stretches from the ignition switch all the way up towards the main grommet at the firewall just above the parking brake pedal.

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Here's a close-up of the splice near the firewall where the resistor wire is connected to regular wire. The second photo is labeled for make it more clear.

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Here's a close-up of the back of the ignition switch. The resistor wire is the farthest left and is connected into the same cavity as a regular red wire with a green stripe. To remove the resistor wire, I'd have to re-do the connections right at the ignition switch - not a very appealing option... Also, on the left side just below the resistor wire cavity and above the purple with white stripe wire is an empty cavity. This is a second unused connection to the "prove out" option. In the start position, the prove out terminals are connected to the ignition switch body, which is grounded. This is used to "prove out" warning lights and such in the dash. The factory purple with white wire is for the brake warning light, and I'm adding a second purple wire in the unused connector cavity in case I need it later. I'm going to have to re-wrap up the harness anyway, so I might as well do this now. The second photo has these things labeled.

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Here's a close-up of the resistor wire itself. If you look real closely, you can see the green printing on the wire that says "RESISTOR   DO NOT CUT OR SPLICE". The writing is upside down in this picture, but you can read it if you zoom in on it. And, that warning says it all - you can't mess with this length of resistor wire.

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Here's the dash wiring before I added the custom EFI wiring and the updated custom wiring (gauges, etc.). You can see the new bulkhead connectors in the firewall in the first two pictures. The last picture shows the center area, the wires that come off for the radio/ignition switch, as well as the EFI computer's general mounting location.

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Here's a shot of the harness layout board I created to help me build the new EFI under-dash harness without having to lay it all out and assemble it hanging into the truck and on/under the dash. I simply pulled one length of wire through the intended harness route, and marked off all of the important locations with tape - those are the small blue tape "flags" on one of the red wires on the board. Then, I simply drove long drywall screws into a spare piece of plywood to approximate the shape of the harness, using the blue tape "flags" on the red wire to guide where the screws went. I mounted the inertia switch and relays at one end to anchor that end of the harness. For the other end, I drove in one more screw in the middle of the wires at the computer connector to help anchor that end of the harness. I then wrapped each screw in electrical tape to prevent the threads from tearing up the wires as I ran each one. The result is that I can run wires based on the previous diagrams and connect each one to the proper places, and then wrap up the entire harness in the approximately correct shape, all in the comfort of my garage. This is all assuming that I measured correctly with that original wire, but I left a bit of slack in things to have a bit of leeway in how things would be routed. The second photo is the same as the first, just with things labeled for reference.

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Here's a close-up of the inertia switch and relays from both sides. The inertia switch will be mounted to the firewall using long screws and short spacers (made out of straight lengths of metal tubing) so it's out past the firewall insulator (smaller holes are easy to cut), but it will still be solidly connected to the firewall. This will help ensure that in case of an accident, any shock is transmitted to the inertia switch and it can open and shut off the fuel pumps as intended. The relays are mounted to a small square metal plate on top of the inertia switch. One of the relay mounting holes matches the inertia switch mounting hole, and the other relay mount hangs off the lower corner of the inertia switch and is held to the plate with a small sheet metal screw. The other inertia mounting screw is behind the wiring and will not be in the way. The second and last photos are the labeled versions, and are otherwise identical to the first and third photos.

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Here's the wiring harness on the board waiting to be installed and have final connections made at the firewall connector and final connectors installed for the MIL and the Clutch Switch/NGS.

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Here's a close-up of the back side of the final relay and inertia switch area. I opted to put the connector for the fuel tank selection switch here. Since I'm only using a rear tank, it's jumpered to always have that one active. But, the connector is there in case I need/want it later.

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Here's the MIL wires coming out of the harness. I don't know how long or short these will need to be, so I've let them run wild here until I get the harness get in place, and then I can put a simple two-place connector on here to plug the MIL into later. The red wire is a pass-through power connection to other things in the harness, that's why there is a loop of red wire here. The tan wire is the one the ECU grounds to light the MIL/Check Engine light.

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Here's the CES/NGS wires and the wires that will eventually go to the new six-place firewall connector. As with the MIL, I'm not exactly sure where these will end up at length-wise, so I've let them run wild. Once I get the harness installed, I can trim to length and install terminals/connectors on these. The firewall connector is already in place, so all I have to do is push the terminals into it from inside the truck and then seal the back side of each terminal cavity with some dielectric grease. Exactly where the relay and inertia switch end up mounted to the firewall will determine much of this.

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Installing the harness into the truck was relatively event-free. It lines up nicely and had enough slack in key places to get a good harness routing without weird bends or stretching things. One of the wires did pull out of it's terminal in the relay base, so I had to install a new one (soldered this time for security since it was a "two wires into one terminal" arrangement), but that was pretty easy. I mounted the inertia switch and relays next to the fuse block as planned, and I used some 1/2" lengths of steel tubing as spacers to allow the inertia switch to sit out past the firewall insulator. This saved me form having to cut a large sloppy looking hole in it. The tubing bits were cut from a short length of brake tubing I bought at the local parts store. Installing the wires into the bulkhead connector went easily, but getting the bulkhead connector connected for the first time was a bit of a challenge. I ended up pushing the whole thing through the firewall into the engine compartment, connecting it to the custom adaptor harness, and then pushing it into the hole to seat the grommet. Trying to push the connectors together from the engine compartment side just pushed the bulkhead connector and grommet right into the cab. Oh well; live and learn.

This is the O2 sensor and oil level sensor harness connector where it plugs into the main engine EFI compartment harness. You can see where the "loop back" wire is connected at - this determines the "manual or automatic" transmission-ness of the wiring. This should be the manual trans version, but it's been tough to decipher the information in the manuals and out there on the internet. I'll know soon enough after I start driving the truck. :-)

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Here's the final wiring at the computer on the inside of the cab, all taped up and ready to go. After I took this picture, I taped the EFI power relay to the harness so it wouldn't flop around under the dash.

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Here's the routing of the O2 sensor harness in the engine compartment. I used tie-wraps to keep it tight to the rest of the harness, which ensures it was pulled up and out of the way of the exhaust system down below.

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Page last updated 06/26/2009 12:07:54 PM