Ignition coil fuse blows when key is turned on

[AJB0009]

2004 Ford Explorer XLT, 4.0L SOHC, 2WD, 150k miles
Drove it a few miles this morning, no issues. It sat for an hour or so, then would not start. Cranks fine, I have fuel. In the fuse/relay box near the battery in the engine compartment is a 15 amp fuse labeled Ignition Coil. It was blown. Put a new fuse in, blew it again. I had a known-good spare coil pack (long story), so I swapped them out. No change. As soon as the key is turned on, the fuse blows.
Any thoughts or suggestions?

 

[Rand]

need a wiring diagram.

 

[mk378]

There's generally a lot of things connected to that fuse. As said, you'll need to look up a diagram and isolate each part, and also consider that the short may be in the wiring harness itself. You can use an ohmmeter from the dead side of the fuse socket to ground to tell if the circuit is shorted without wasting fuses.

 

[mk378]

Actually looking at the diagram that circuit only feeds the coil, and the coil capacitor nearby. A short in the PCM could pull enough current through the coil to blow the fuse. If that's the problem, the short will go away with the coil unplugged.

 

[AJB0009]

Actually looking at the diagram that circuit only feeds the coil, and the coil capacitor nearby. A short in the PCM could pull enough current through the coil to blow the fuse. If that's the problem, the short will go away with the coil unplugged.

That is very good information. It's interesting that you bring up the PCM, I was not thinking that. My brain was just thinking of a wire rubbed through somewhere.
I installed a used PCM in back in mid-May or so. It hasn't missed a beat since then until now.
I may try what you suggest and unplug the coil pack and see if that removes the short. If so, I have my original PCM yet, which should still work to try.
~
You were able to look at a wiring diagram somewhere?

 

[chemman]

Be sure to check the coil capacitor to make sure that it has not shorted out. A good capacitor should show a very high resistance. If the capacitor suffered a failure and shows 0 ohms, it could be the cause of the fuse blowing (assuming one side of the coil capacitor is connected to ground).

 

[FowVay]

Here is a simple diagram that might be useful. It shows 3 15 amp fuses and three separate wires with distinct color coding leaving the battery box. Do a bit of digging before you condemn the PCM.

 

[clinebarger]

Actually looking at the diagram that circuit only feeds the coil, and the coil capacitor nearby. A short in the PCM could pull enough current through the coil to blow the fuse. If that's the problem, the short will go away with the coil unplugged.

Would it blow the fuse right away? (Coil charge & overheat)

 

[zzyzzx]

Look for rodent damage on the wiring as well.

 

[oil pan 4]

Look for rodent damage on the wiring as well.

Yes detailed visual inspection for mice/rat damage and as a shot in the dark try pulling power from all the ignition coils, see if the fuse still pops. Doing that eliminates the coils as the possible short circuit. If the fuse pops with the coils disconnected then the problem is somewhere in the minusha of the wiring harness and may god have mercy on your soul.

 

[AJB0009]

So, low and behold, it seems it was the PCM. I checked a few things with my meter this morning and everything seemed fine (coil, capacitor, wiring, etc.). I pulled the used PCM out (which I installed mid-May) and put my original one back in. No more short. Hooray!
Now it will not turn over.
After I changed PCM's back in May, I sent my original up to SIA Electronics to see if they could repair it. They played with it for a while and said no, the main chip acts flaky when it heats up, so no, it can't be fixed. They sent it back to me.
Now when I turn the key on, the red light on the dash blinks fast (indicating a PATS system issue, I believe) and it will not turn over.
I don't know if PATS disables the starter (I thought it just disabled the ignition, but I am really not sure). Or I wonder if perhaps when SIA was messing with it, the program got erased altogether.
It is always something with this vehicle.

 

[Rand]

PATS: in some its injectors, in others it disables the starter

 

[michaelluscher]

So, low and behold, it seems it was the PCM. I checked a few things with my meter this morning and everything seemed fine (coil, capacitor, wiring, etc.). I pulled the used PCM out (which I installed mid-May) and put my original one back in. No more short. Hooray!
Now it will not turn over.
After I changed PCM's back in May, I sent my original up to SIA Electronics to see if they could repair it. They played with it for a while and said no, the main chip acts flaky when it heats up, so no, it can't be fixed. They sent it back to me.
Now when I turn the key on, the red light on the dash blinks fast (indicating a PATS system issue, I believe) and it will not turn over.
I don't know if PATS disables the starter (I thought it just disabled the ignition, but I am really not sure). Or I wonder if perhaps when SIA was messing with it, the program got erased altogether.
It is always something with this vehicle.

FORScan can help you DIY the PATS programming
If you have two good keys, you can handle it yourself with a 10 minute timeout