Blowing fuse

[AA7KN]

My 1996 Explorer xlt now all of a sudden blows the ignition fuse onte side of th dash. It blows as soon as you turn the key to The on position. My mechanic has been tryng to find the short for 1 month now. Anyone out there ever have this problem? Or can give me some advise about how to find and fix this? Many thanks iadvance.

 

[410Fortune]

Which fuse number is it? Without this info we cannot determine which circuits it protects and where to look

 

[AA7KN]

Which fuse number is it? Without this info we cannot determine which circuits it protects and where to look

Fuse #19 I think. It blows as soon as the key it turned to on. I saw in the sschematics that the exm relay, a radio ffilter and one more item seemed to be the only thing it powers, my mechac cntfind the short. Thank you.

 

[J_C]

In the '96 owner's manual, page 383, fuse 19 is listed as ignition coils and PCM.

However the wiring diagram also shows it connects to the radio capacitor, which it should, to put the capacitor in parallel with the ignition coil noise to shunt it to ground.

I might try process of elimination. Unplug the connector to the ignition coil driver module and see if the fuse still blows. If it does, disconnect the radio capacitor and repeat the test. If it still blows, probably a short in the PCM relay which seems less likely but to be thorough, unplug PCM relay and repeat the test. Note that I wrote relay not PCM itself. Fuse 19 appears to only power the relay coil, not supply power to the PCM itself.

However when doing the unplugging, disturbing the wire could temporarily remove a short due to wiring insulation failure, but if there is a short like that, and considering it's a 25A fuse, that type of short and amount of current might create a spark in the engine bay that you might see if it's dark, but more practically speaking, if you have unplugged all 3 things listed (radio capacitor, coil driver module, and PCM relay) and still measure low resistance to ground, a wiring short may be all there is left as a cause.

An alternate approach instead of blowing fuses till you find the fault, is pull fuse 19 ahead of time, then turn the ignition on, then measure for 12V on the fuses box contacts between each and chassis ground to find the upstream battery supply side. When you find which side of the fuse has 12V, next put the multimeter in resistance mode and probe the other side of the fuse contacts, the downstream side to the loads, measuring resistance between that side and chassis ground. From this point forward, once you identify which side of the fuse contacts goes to the loads, you do not need the ignition on for for the following tests:

Presumably there is very low resistance since it's blowing fuses, and you can see if that low resistance goes away when you disconnect the coil driver module, and then the radio capacitor, and then the PCM relay.

Wiring diagram attached. Hopefully it 100% applies to a '96 even though the under hood power distribution box may be a bit different.

 

[AA7KN]

In the '96 owner's manual, page 383, fuse 19 is listed as ignition coils and PCM.

However the wiring diagram also shows it connects to the radio capacitor, which it should, to put the capacitor in parallel with the ignition coil noise to shunt it to ground.

I might try process of elimination. Unplug the connector to the ignition coil driver module and see if the fuse still blows. If it does, disconnect the radio capacitor and repeat the test. If it still blows, probably a short in the PCM relay which seems less likely but to be thorough, unplug PCM relay and repeat the test. Note that I wrote relay not PCM itself. Fuse 19 appears to only power the relay coil, not supply power to the PCM itself.

However when doing the unplugging, disturbing the wire could temporarily remove a short due to wiring insulation failure, but if there is a short like that, and considering it's a 25A fuse, that type of short and amount of current might create a spark in the engine bay that you might see if it's dark, but more practically speaking, if you have unplugged all 3 things listed (capacity, coil driver module, and PCM relay) and still measure low resistance to ground, a wiring short may be all there is left as a cause.

An alternate approach instead of blowing fuses till you find the fault, is pull fuse 19 ahead of time, then turn the ignition on, then measure for 12V on the fuses box contacts between each and chassis ground to find the upstream battery supply side. When you find which side of the fuse has 12V, next put the multimeter in resistance mode and probe the other side of the fuse contacts, the downstream side to the loads, measuring resistance between that side and chassis ground. From this point forward, once you identify which side of the fuse contacts goes to the loads, you do not need the ignition on for for the following tests:

Presumably there is very low resistance since it's blowing fuses, and you can see if that low resistance goes away when you disconnect the coil driver module, and then the radio capacitor, and then the PCM relay.

Wiring diagram attached. Hopefully it 100% applies to a '96 even though the under hood power distribution box may be a bit different.


View attachment 464011

Thanks so much for taking the time to write such an informative reply. I'm sure with the info you gave me my mechanic should be able tofix my car. Thank you again.

 

[DintDobbs]

@J_C I have nothing to do with this conversation, but that reply was absolutely the most comprehensive method for troubleshooting that could be described. I approve and endorse that process.