Gen 2 Coyote engine light always illuminated

[Alphamacaroon]

I’m really excited to fire up my Gen 2 Coyote for the first time tomorrow, but I’m having a strange issue with the check engine light!

The background is that I ended up cutting the stock check engine light that came beside the OBD port and soldered in my own 12V LED which I’m mounting in the dashboard.

Tonight I connected the battery for the first time and I noticed that the check engine light illuminated right away.

My first thought was maybe I had the ACC and ignition turned on or wired incorrectly, but I checked and there is no power anywhere else in the car. So I’m pretty sure that the power going to that light is coming from the PCM clock power/HAAT (or whatever you call the power that you supply to the computer at all times). I can turn on the ignition switch and I can hear the (PCM controlled) fuel pump turn on, so I think the ignition power is wired correctly.

Question: should I assume it’s not normal for the check engine light to be on when the car is just sitting there with all power turned off? (other than the PCM HAAT power)

I’m wondering if maybe I need to wire a resistor in my LED?

[wareaglescott]

Have you plugged in a code reader to see if you have any codes triggering the check engine light?

[edwardb]

No, that's not normal. It should flash very briefly when you first turn the key on. Along with hearing the fuel pump run briefly and pressurize the system. Then, assuming you don't have any codes and "MIL ON" showing in the code reader as wareaglescott suggested, should go to a dim glow. When you turn the key off, it will stay glowing for 5-10 seconds. Then you should hear a relay click in the PDB shutting the system down and it should also go off. I've not heard of one staying on all the time even without the key on. Maybe post more details about how you have the Coyote wired.

One other thing. There has been quite a bit written about how the MIL stays lit at a low glow all the time (with the key on/engine running) even when there are no codes. Several have tried to defeat this without any luck. Mine has been doing it for three seasons. Barely can see it in daylight. A little more obvious at night. I've found it isn't nearly as obvious with the Ford supplied LED. With a different one like you did (I did too...) might have some different electrical characteristics and be more obvious. You might be able to tone it down be adding some resistance in the positive lead. But you won't be able to get it to go completely out without affecting how it works when the PCM actually switches on the MIL. FWIW, the Gen 3 Coyote does exactly the same thing.

[Alphamacaroon]

I thought about using my code reader, but since it was illuminated without ignition power I didn't think I would see much. Plus my wireless code reader is having trouble connecting to my phone right now so I might have to run out and buy another one.

I went ahead and cranked it over this morning and the engine purred (roared) like a kitten (after a brief head scratching moment when nothing happened, but remembered I forgot to depress the clutch switch)— so it appears everything is at least wired correctly.

So I figured I would debug a little more:

1. Engine is wired pretty standard as per FFR and Ford instructions. The engine fuse box is connected directly to the battery (via the big fuse). The engine pigtail connector is properly wired to the Ron Francis harness (starter engage and ignition power wires). Like I said, the engine started right up, so I think it's wired correctly.
2. When ACC and IGN are switched off I can measure the voltage to my new MIL and it shows 2.5V— which seems to coincide with the "dim" state that you and others describe. The only difference is that it seems like the dim state on mine is constant as long as there is power to the PCM.
3. My control pack came with the MIL wired in in a bracket next to the OBD port. Most everything I've read online and even the Ford instructions mention that you have to wire the MIL light yourself into the pigtail. Maybe I have a newer version?

My guess now is that maybe this is normal behavior and that my LED just works at much lower voltage? Maybe that when the PCM is in its "sleep" state, it does provide a small amount of power to the MIL, but that the light they provide has higher resistance (maybe it's incandescent?) and that it's just not enough for it to light up. The fact that it does have a dim state in the first place probably means that the factory light is incandescent (or maybe less-likely a dimmable LED). I'd be curious if anyone else has easy access to their MIL light to see if they also read a small voltage on it when the car is turned off (but battery is still connected to PCM).

I guess what I'll do next is measure the resistance on the old MIL (assuming I didn't throw it away) and see if maybe that is the difference between the two. Other thoughts?

[edwardb]

Early Gen2's (like what I have in #8674) didn't have the MIL pre-wired and in a bracket with the ODB2 connector. The light was provided in the controls pack (I can confirm it's an LED, not incandescent) and instructions were given on how to wire it up along with everything else. The early instructions were confusing and actually contradictory. But we figured it out. Later Gen 2 versions (and the Gen 3 BTW) have the MIL pre-wired and mounted with the ODB2 connector. Best I can tell, however, there's no difference electrically between the two versions. I can confirm the voltage on the MIL leads is zero once the PCM shuts everything down 5-10 seconds after the ignition sense wire goes to zero volts, e.g. key turned off. As I mentioned in my previous post. At least in my experience. I personally doubt Ford intends for the MIL to be on all the time. If yours still has voltage after the time period I described, either something is wired wrong (and I don't know where to suggest to look) or Ford Performance has changed something. You might want to call them and ask.

[Erik W. Treves]

I never hooked my light up :) nothing to worry about that way.... I will hook up my code scanner up to the car from time to time to see if anything is there.

[Alphamacaroon]

Was finally able to debug this and fix it. It looks like the problem was that I had the black ground wire to the MIL connected to the chassis/common ground as well. The black wire looks like a ground because it measures +12v across the two leads, but I think it is an isolated/floating ground and should not be connected to chassis ground. Disconnecting the MIL/LED ground from the common/chassis ground fixed everything. So I guess it was technically a wiring error, but a bit of non-intuitive one.