Electrical Gurus: Check Engine Light Wiring for 91 LX

Husky44

10 Year Member
Sep 27, 2006
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OK, sorta for a 91 LX. I've swapped the 5.0 from a 91 LX into my 67. I used the Ford Racing Multiport EFI Engine Management Wiring Harnes (M-12071-A50, Rev 2) for the engine harness. I am installing a Dakota Digital dash. The Dakota Digital control box only has a Check Engine (-) Indicator Input connection (only 1 wire location for CEL wiring).

The Ford Racing harness has 2 wires from the 8 pin connector for the CEL. Quoting directly from instructions:
Pin 1 (Pink/Green Wire) and Pin 2 (Pink Wire): These two wires will need to go to the "Check Engine" light. This is a mandatory connection and is required for the computer to do an initial 'Self Test' upon starting the engine.
On the next page in the pin-out, it shows the Pink/Green wire at Pin 1 is CEL (-), and the RED (vs Pink, as previous page describes) wire at Pin 2 is CEL (+).

My question: If I only hook the Pink/Green (CEL-) wire to the DD control box, is everything going to work? I'm assuming the answer is "no" since the circuit isn't completed. If what would complete the circuit is a light in a normal installation, is there a way to connect the 2 wires without a lamp, to complete the circuit so the self test function works properly? If I do that, how do I get a (-) signal to the gauge control box?

Probably a dumb question, but I'm a pretty dumb guy when it comes to electrical.
 
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Husky44

10 Year Member
Sep 27, 2006
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Tacoma, WA
Have you contacted Dakota Digital and Ford Racing about this?
It was after hours last night. Planning to call today, but I am not sure how, much, hell I can get. DD is pretty clear what they need, and the Ford Racing harness has been NLA for a, while-not sure if they still have a tech support person who knows it.

I guess my question was more generic-if I split a wire off the (-) feed to the DD box, then connect the harness wires to complete the circuit, should that work? Do I need a bulb in the circuit to provide resistance?

Could someone draw a picture (crayon preferred)?
 

7991LXnSHO

wanna catch the space herp
10 Year Member
Sep 1, 2010
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It was after hours last night. Planning to call today, but I am not sure how, much, hell I can get. DD is pretty clear what they need, and the Ford Racing harness has been NLA for a, while-not sure if they still have a tech support person who knows it.

I guess my question was more generic-if I split a wire off the (-) feed to the DD box, then connect the harness wires to complete the circuit, should that work? Do I need a bulb in the circuit to provide resistance?

Could someone draw a picture (crayon preferred)?
A bulb or resistor can keep the computer happy. That does not have to be in view, but it helps. Without the diagrams for the harness and the dash, it’s difficult to tell you what you really want to know - which wire to splice from to the dash and if it will work.
 

Husky44

10 Year Member
Sep 27, 2006
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Tacoma, WA
A bulb or resistor can keep the computer happy. That does not have to be in view, but it helps. Without the diagrams for the harness and the dash, it’s difficult to tell you what you really want to know - which wire to splice from to the dash and if it will work.
I appreciate it. No diagrams for the harness. Basically what I copied above is all I have. The dash does have a diagram, but it just shows where to connect the CEL - wire coming from the harness.

DD tech support confirmed that they only need the 1 negative wire. They weren't any help on the FRPP engine harness side. Kept asking if it was a Coyote swap. :confused:

FRPP tech support wasn't able to help. Guy I talked to didn't know what it was. Said if I'd email him the question, he'd ask some of the old-timers.
 

7991LXnSHO

wanna catch the space herp
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It sounds like DD gave you the hard part. With a meter, you should be able to tell which is negative in the harness.
 

Mustang5L5

Put lubricant all over the balls
Mod Dude
Feb 18, 2001
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This is actually pretty easy.

The way the CEL works on the stock foxbody is that the bulb has constant 12V feed on one side, and the negative side is triggered by Pin 17 on the ECU.

It sounds like the Ford Racing harness is set up the same way with a constant positive (simply to feed the bulb power), and a negative that will ground based on if the computer has a code or not.

If the DD dash will trigger off the negative only, then you do not need the positive wire from the ford harness. It's only purpose was to power the bulb but the DD accomplishes this another way. Just tape it up and tuck it out of the way.

As for which wire is the negative? This is easy if you have a multimeter. Test for continuity from the wire you think is negative on the Ford racing harness to PIN 17 at the ECU connector. Pin 17 is the CEL ground.

Connect your neg wire to the CEL- on the DD cluster. Upon turning the key to ON, the CEL should illuminate for a second and then go off. This is a good way to see if it's functioning.

Good luck and let me know if you have any questions
 
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