Number 30 (30 amp fuse) blowing

DAVID MORTIMER

MEMBER
2001 FORD F-150
5.4L
V8
4WD
AUTOMATIC
360,000 MILES

The number 30 (30 amp fuse) is blowing when the truck gets hot. Replaced all coils and spark plugs a week ago. New parts (auto zone), truck was good for about three days. Then we were at a store, with A/C on (I have a wife), and it just shut off. Got it home, started looking, never found anything. Checked under the truck, all wires and O2 sensors, then looked up top and still looking. Odd thing, when I turn on the switch, and leave it on, say, maybe a minute, the fuel gauge, oil gauge and check engine light come on, no other gauges, with the fuse blown but the key on. Almost like it's back feeding. Replacing the fuse is a no go when it is hot, fuse just blows, checked relays when it happened last time and they were very hot. I don't know if they get that hot or what, but I could not keep a finger on them. After it cools down, runs fine. Flip flopped the relays, still the same thing. First thing I thought was, pinched wire while doing coils, I can find nothing. Kind of need the truck, any help please?

1 Reply

KASEKENNY

MECHANIC
18,907 POSTS

Any time we have a fuse that is blowing as soon as the key is turned on, it means some component on that fuse is shorted. If the fuse blows as soon as the key is on and the engine does not have to be running then we need to disconnect the items that are powered at key on.

The likely cause of this would be the coils even though they are not firing until the engine starts but if you replaced them then the next likely cause would be the capacitor that eliminates noise from the coils to try and cut down on misfires.

Basically you need to unplug this and then replace the fuse and see if it blows again.

If not then that is the issue. If it does then we need to move onto the next item which would be the PCM. However, it looks like this fuse does not directly feed the PCM. It powers the diode which supplies the power to the PCM power relay which is energized as soon as the key is on. So this would explain the back feeding that you are thinking.

Basically a diode prevents power to from coming back in the opposite direction.

Please take a look at this info below and let us know what you find with this. The diode is in the battery junction block. Please let me know if you have questions. Thanks

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