How to use a 3.0 wire harness on a 4.0


cp2295

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Okay so I swapped my 3.0 with a 4.0 recently. I was forced to use the 3.0 harness as a 4.0 auto harness wouldn't crank the starter and because the 4.0 had a manual tranny and computer, but no harness. The computer was from a 4x4 but I don't think it matters (at least for 98-00) because there's another module behind the dash that controls 4x4. Anyways, my 3.0 was a 99 with a 5 speed (also 4x4). I searched up and down at the junkyard for a 4.0 with a manual tranny and no luck. You need the harness that your computer came stock with, so an auto computer needs an auto harness (engine and the transmission pigtail) and of course a manual needs a manual harness. Otherwise the starter doesn't crank because the wiring is different, and I never figured out how to convert a 4.0 auto harness to work with the manual computer

The 3.0 and 4.0 harnesses are very similar and only a few modifications and some basic electrical knowledge is needed. I want to point out I did this using a 99 3.0 5 speed engine and tranny harness connected to a 98 4.0 5 speed computer. The harnesses will most likely be different for other year ranges. But if you do this to a 98-00 you should encounter no issues.
Read over this whole thing before proceeding

The only issues are needing to extend a few sensors and rewire the cmp sensor, and changing the crank sensor.

What I did to extend the sensors was use some 16 gauge wire, wire nuts, wire loom, and a bunch of electrical tape. Just connect the same color wires together with the wire nuts, electrical tape over the connection to make it nice a weather proof, then wire loom it and electrical tape the wire loom onto the connection you made, and you'll be golden with weather and it will never come apart.
You can use butt connectors with heat shrink attached to them (at hardware stores auto parts stores) which will create a nice seal as well. I just have an issue with butt connectors, they seem to unplug themselves.

Just extend the wires as far as they need to go to reach their proper sensors.

1) you will notice the fuel injectors connectors look different but they plugin just fine and work great, I prefer the 3.0s plugins anyways because they are easier to disconnect.

2) you will notice that you need to extend a few sensors wires:
A) oil pressure sending unit, needs to be extended a few feet, on the 3.0 harness it's located near the cmp/passenger o2 sensor, it's the single green/white wire. You can use your 3.0s oil pressure sending unit, or the 4.0s. The 4.0s oil pressure gauge plugs in underneath the power steering pump.
B) the crank position sensors wires needs to be extended about a foot. Make sure when you extend this sensor that you wrap your wire in aluminum foil so it doesn't make your radio buzz. You also need to use the 4.0s connector as the sensor is different, however the wires are routed the same and are the same color
C) the egr vacuum solenoid sensor wires needs to be extended too, about 1.5-2 feet. On the 3.0 it's mounted on the ignition coil. On the 4.0 it's mounted on the driver side of the intake manifold. No need to change the connector
D) extend the tps sensor wires about a foot, just to give you enough wire to work with; same connector
E) the dpfe sensors wire needs to extend 1.5 feet approx. (same connector)
F) the iac valve wires need extending a 1-1.5 feet. (Same connector)
3) you must change the ignition coil connector, it's wired differently, but the color of the wires are the same, and the way it routes to the computer is the same as well, just different how they plugin on the connector.

4) A) now for the camshaft position sensor. You will notice that the 3.0 and 4.0 have different cmp sensors. The 3.0 has a 2 pin, while the 4.0 has a 3 pin. No big deal though. Cut off the 3.0s connector, and then wire cap and electrical tape the red and grey wire, this one is unused in the 4.0s cmp.
B) take your 4.0s cmp connector, and then attach about a foot of wire to the orange and light blue wire. Then attach that wire to the egr vacuum solenoids orange and light blue wire (just attach it into the part you extended at the beginning). That is the easiest way to tap into that connection, all the orange and light blue wires in the harness are all spliced together at splice 109. But by connecting it at the vacuum solenoids wire it makes it much easier and does the same thing.
C) take the black and white wire and extend it about 2 feet. Then you want to route that wire to the mass air flow sensor. Cut the MAFs black wire approx 8 inches away from the connector. Then just wire nut all three wires (the black and white from the cmp, and both sides of the MAFs black wire). This ties the CMPs black and white wire into the ground connection at splice 106 which is where it would be from the factory in the 4.0 harness.
D) all that's left is to splice together the orange and dark blue wires
from the 3.0 harness and the 4.0 cmp sensor

Doing this has allowed me to run a 4.0 manual tranny PCM and a manual tranny! Without spending $430 for a new wiring harness.

It'll be easier if you grab a wiring harness off any 4.0 within the year range, that way you can use it's wire for extending the connectors and also so you can use it's connectors on the ones that are different.

I've been running this setup for a hundred miles no issues. If you get a check engine light for the canister vent solenoid, cmp sensor, o2 sensors, egr vacuum solenoid you more than likely have a blown fuse like I did. It's in the power distribution box in position 13, it's a 15 amp mini use. Just replace it and your cel light should turn off. You probably won't have that happen though, since you'll wire it correctly the first time. I had to figure out that I wired it wrong, rewire it, and then replace the fuse.

Hope this helps anyone trying to do the 3.0 to 4.0 swap and putting a manual tranny behind the 4.0. Also if you want to swap trannys with your 4.0 (auto to manual) this should work just fine for you too. I haven't done it though so don't count on it! I know that computers are easy to come by on ebay and whatnot but harnesses for a 4.0 manual are not even on the market from my findings..

If anyone needs pics or has questions let me know and I'll help you out! If you need a wiring diagram too I have one that helped me out
 
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stmitch

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This is for a 4.0 OHV right? Not a 4.0 SOHC? Also, you may want to mention the years of the equipment involved.
 

Mac

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Well I'm stumped as to why the 4.0 engine harness didn't work. You did use your existing 3.0 trans harness I assume. The usual cause for a no-start like this is not having the Clutch Petal Position switch jumpered. Not in your case since it worked before and after. No difference in the auto and manual engine wiring I can think of except at the trans connector (if that). The difference in the number of wires at the CPP is due to the different years, not engine. A lot have just used the auto harness, just including the few needed. Got to take a look at the diagrams in the morning to check.
Dave
 

adsm08

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Wire nuts are fine for a house, but are not appropriate for use on a car. They tend to come lose when they get knocked around too much.
 

Mark_88

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Wire nuts? Are we talking about marettes? I've used those on my truck and they worked OK...think I still have some on my blower fan wire splice...but they are more prone to moisture than a good sealing joint...
 

cp2295

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Yeah I was eventually gonna do heat shrink butt connectors, just used wire nuts to check if it all worked and then electrical taped it as I wanted to drive it :D. And I did mention the years: 99 3.0, 98 4.0 OHV
I jumped the clutch pedal before I did all this and when that didn't work I just modified the 3.0 harness. And yeah I was stumped too. I didn't think it would matter about the auto harness because the manual PCM wouldn't be looking for park or neutral to start in. And I wouldn't think it would be routed any different to the PCM but i modified the 3.0 harness and it worked so what the hell right? I probably overlooked something, and I didn't have a very good diagram to work off, (didn't show much difference with auto and manual harnesses).
 
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merelyjoey

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Cant find 4.0 OHV engine harness

I bought a 98 4.0 OHV with an automatic transmission that some one stuck a Mountaineer engine wire harness in. Its causing all kinds of issues. I'm trying to get the right one (part #F87Z-12A581-FD) but its discontinued. I can find some 3.0 automatic transmission harnesses on eBay. Any Idea if I could get it to work? Or any other options?
 


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