Coyote/Ford Performance wiring MIL/Check engine light

[Rian_Colorado]

Trying to wire in dash light for the ford performance wiring harness on my Gen3 Coyote.....

The wiring harness comes with an LED setup right next to the OBDII port. I figured that it would be triggered by a 12v wire, but when I tried to tap into it, I noticed that I have 12v to that light is on 100% of the time (even with the motor off) - yet.... the light isn't on? Is this somehow triggered by the ECU tying the light to ground instead of tying it to 12v?

FYI - I'm not just trying to put an LED on the dash. My gauge setup has a CEL icon built into it, and that is triggered by receiving 12v.....

Rian

[JohnK]

Yes, the coyote MIL is triggered by the PCM through the ground wire. The red wire should have +12V all the time.

[Alan_C]

Yup, the Ford PCMs use a switched ground. I had the same thing with a 4.6L DOHC engine I put in my MK4. The ABS light was a head scratcher for quite awhile.

[facultyofmusic]

Glad you caught this early Rian! I grounded my CEL light to the chassis and blew a fuse! I think the Ford engineers knew this might happen so they put a fuse there ;)

[Mastertech5]

Use a relay that the Ford ECU will ground to activate thus sending 12 volts to the gauge icon light.

[Alan_C]

Why are you suggesting the need for a relay as it just adds unnecessary complexity? Just apply +12V to one side of the LED used for the MIL light and apply the switched GND output from the PCM to the negative side of the relay and you are done.

[Nigel Allen]

Why are you suggesting the need for a relay as it just adds unnecessary complexity? Just apply +12V to one side of the LED used for the MIL light and apply the switched GND output from the PCM to the negative side of the relay and you are done.

It was a solution suggested by Master tech in the case of the OP needing a positive voltage for the MIL in his custom gauge cluster.

Cheers, nige

[Mastertech5]

LEDs act just like a regular diode in a circuit. They are one way "valves", so to speak, and have a pos. and a neg. side and must be wired accordingly. If the gauge is already wired to accept a pos. signal to light then using a relay to activate it is a fairly simple way to make that happen. I noticed you mentioned relay in your solution. ECU grounds relay and a pos. voltage goes to the gauge. 1 B+ wire for relay power jumped to one pass through terminal, 1 wire to the CK engine light on the other pass through terminal and one ground wire for the ECU. Use a small low current relay.

[Ducky2009]

I have a GEN 2 Coyote. The Coyote installation instructions says "Malfunction Indicator Lamp (MIL) BU ---- Provides +12V to MIL when error state is present". This is lead 4 from the pigtail plug.
I attached the blue wire to the MIL lamp and connected the second wire to ground. Works fine. Not sure if a GEN 3 is different than a GEN 2

[toadster]

instead of tapping into #4 (GND) and #16 (12V+) -

wouldn't it be just as easy to tap into the black/red wires on the MIL - seems to be a thicker wire :)

182905

[toadster]

another question - pin #4 is chassis ground and pin #5 is signal ground, assume neither would show continuity unless the ECU closes the circuit - correct?

so to light the CEL/MIL light would it be:
pin #5 and #16 or
#4 and #16?

[toadster]

update - JohnK got back to me and verified that it's pin #4 for ground, and pin#6 for + on the CEL

THANKS!

[dbo_texas]

Just seeing this, and wondering why toadster's post #10 (https://thefactoryfiveforum.com/showthread.php?45304-Coyote-Ford-Performance-wiring-MIL-Check-engine-light&p=523250&viewfull=1#post523250) wouldn't work? I haven't powered my electrical system up but I just tapped into the red & black wires going to the MIL on the Coyote harness (as shown in post #10), and ran those two wires back to my dash check engine light. Wouldn't that work?