truck: 1999 f250 v10

Recently developed a miss on cyl#7. Replaced the coil on a whim and it fixed the miss for about a day..drove it a week later and the miss is back. Same cylinder. Pull off the connector, primary wire is 12v, but the command wire is super dim when using testing with a incandescent test light. This all happened after I went through some mud puddles. I assume I got water in some connector but can’t seem to find where. Since this is the command wire, it goes to the battery junction box, not the pcm (per the diagram). Anyone have any tips on how to trace this wire, or have a similar issue they were able to resolve?

 

1. You are likely testing the "command wire" incorrectly.
2. The "command wire" comes from the PCM, nowhere else.
3. How do you now you're focused on the correct cylinder?

 

Coil misfire was first verified with a snap on scan tool, then secondly verified by unplugging that coil while running to see if there was an additional miss (there wasn’t), then thirdly verified by swapping with a known working coil.

im fairly confident I’m testing the command wire, I used a pin in the coil connection to see which had power which ignition on, and which was grounding while vehicle was running. The pin marked as primary was the one I was able to trace back to the ecu, but I could have mixed them up at some point, I’m like 15 hours into this and very tired.

 

It's semantics, but I've never seen the 12v supply lines called "command wire" before.
To me the RD/LG wires are the 12v supply lines and the unique to each cylinder wires to the PCM are the coil grounding lines.
Not sure about your '99, but in our '02 all the RD/LG 12v supply lines for the COPs come from a "Hot at all times" fuse in the CJB, thru the ignition switch to the COPs.
Each COP connector will have a unique colored wire going to the PCM where it will get grounded/ungrounded at the appropriate times.
When the COP connector is pulled off the RD/LG will have 12v on the line when the ignition switch is in RUN or START.
The other COP connector wire goes to a chip in the PCM used to ground that wire completing the circuit thru the coil primary winding at the appropriate times.
There should be continuity between the #7 COP connector pin and the PCM connector pin for #7.
When the COP connector is put back on you should see 12v at the PCM connector pin for #7.
I'm not sure, but I think a Snap-On scan tool can only tell if a cylinder has misfired from the live data, not that the coil is bad.
You might have a flaky/clogged fuel injector causing the misfire or the grounding chip in the PCM for #7 COP has failed or the grounding chip for the #7 fuel injector has failed.

 

sorry - when I say “primary” I mean the 12v constant wire, and when I say “command” I mean the grounded 12v from the PCM.

I considered a fuel injector as a possibility, but with the cylinder not firing, I can smell the raw fuel being exhausted, obviously that’s not a sure fire thing but….what about the test light on the command wire? Although I can see that it is grounding that circuit, it’s signal is very weak. I was able to form continuity from the wire at the COP plug to ECU pin #53

 

What is the exact testing procedure you are using?

Are you using the test light between the trigger wire and and ground (incorrect) or power (correct)?
Is the coil connected or not?

The correct procedure would be to install the test light as you would a noid light. That is, disconnect the coil and connect the test light in its place and then observe it while cranking/running for activity. If in doubt, confirm the behavior by connecting to a working COP circuit so you know what to expect.

Is the scan tool reading any fault codes when tested?

 

I place the test light alligator clip to the positive battery terminal and the prong into the trigger wire. scan tool has a a P0357 code

 

Just got home to test this again, 12v constant is fine, signal wire tests fine in continuity (0.5), yet still no grounding signal when testing with a light probe. Leaning towards needing a new PCM.