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Coil On Plug Fuse Blowing


ab_slack

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Last night my 2002 Explorer died on the highway.

It has a 4.6L V8.

I found it had blown Fuse 38 in the under hood fuse box which is listed as "Coil On Plug"

I replace the fuse, vehicle started and I managed to get a couple miles with engine seeming to run fine before the fuse blew again. After that fuse pretty much blew with seconds of starting the engine.

It didn't appear to blow when just turning on the ignition, but I did make the mistake once of having the ignition on when I was plugging in a new fuse. In that instance I got a spark and fuse blew immediately.

Is there any most common problem that could cause this fuse to blow?

The electrical diagram shows this fuse as only feeding the 8 ignition coils and two "ignition transformer capacitors".

Other side of the ignition coils is the PCM.

Based on the diagram I am thinking one of a few causes.

1. An ignition coil has shorted so when engine runs it draws unusually high current. Not sure how probable this is, they all were replaced 6 months ago.

2. One of the two capacitors has gone bad. I think one was already disconnected. Not sure what harm that causes other than electrical noise. Couldn't find where to connect.

3. The PCM has a problem and is not just pulsing the ignition coil but holding it on resulting in too much current. I tend to think because it started after the first fuse replacement and ran without issues for about 5 minutes that this seems unlikely, but don't know.

4. The wire from the fuse to the ignition coils and capacitors has a short someplace.

5. One of the wires between an ignition coil and the PCM is shorted someplace.


Any suggestions appreciated. Thanks
 


ab_slack

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Tracked it down to a harness issue on the coil on plug power feed back by cylinder 8. Wires were compressed really hard in the harness and some factory shrink tubing had bit insulation of adjacent wires allowing for wires to short when they got warm
 

Doofy

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Thanks for posting the "Fix". It may help someone else with a similar problem.
 

Mark_88

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I saw this earlier but didn't respond because I was researching something else...glad you found the problem...it sounded like a dead short but finding that is not always easy with the way the harnesses are covered over and sometimes inaccessible due to routing.
 

ab_slack

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Turns out what I found, while suspicious wasn't the root cause. It lasted me 2 1/2 months and issue re-appeared.

I did get to the root cause finally. It was another 20 inches further up the harness from where I was working. I apparently shifted it enough to avoid the problem for awhile.

I found the harness was sitting on one of the EGR pipes in the band of the engine. The thermal insulation around the pipe wore thru and the pipe melted into the plastic wrapping. Fortunately the wires didn't all melt, but the one in question had melted/rubbled thru and was shorting to the EGR pike.

In looking at it, I saw that it was supposed to be routed against the firewall. I tied it back up there after fixing the few damaged wires and re-wrapping. It should be good now.

Looks like I can blame this on the shop that replaced the engine 100K miles ago. It is clear that the harness was simply left there. It could not have fallen from the original/correct position against the firewall because when I pulled it back, I realized it was supposed to pass behind one of the other harnesses. Only way it could be in the wrong spot is if the whole harness had been disconnected. Only time that happened was at engine replacement.
 

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