20 amp ignition fuse keeps blowing

Tiny
TIMOTHY 68COUGAR
  • MEMBER
  • 1996 MERCURY COUGAR
  • V8
  • 90,000 MILES
20 amp ignition fuse blown, replaced with 60 amp to get me off the interstate hwy, but smoke came from under hood. Turned off car looked under hood and restarted. No more smoke. Got new 20a fuse and blew instantly.
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Saturday, March 28th, 2020 AT 4:07 PM

22 Replies

Tiny
JACOBANDNICKOLAS
  • EXPERT
Hi,

You got a dead short. The smoke was likely the result of the higher amp fuse. I hope nothing was damaged.

Before I jump into the wiring schematics, I need you to confirm I am looking at the correct fuse. See the attached pic and let me know if that is the one you replaced and are having trouble with.

If I am correct, then if you look at pic 2, it shows the fuse and what it powers. Note that the power supply wire is red/ light green. You need to locate that wire under the box and trace it to see where the short is located.

Here are a few links you may find helpful:

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter

https://www.2carpros.com/articles/how-to-check-wiring

Let me know.

Joe

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Saturday, March 28th, 2020 AT 4:32 PM
Tiny
KASEKENNY1
  • EXPERT
We need to start with knowing which 20 Amp fuse this is. I attached the wiring diagram but we can narrow this down with knowing which one.

However, we clearly don't want to put a larger fuse like that in or we will melt some wiring or a component because you have a shorted circuit.

We can figure out which one once we know which fuse this was. Thanks
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Saturday, March 28th, 2020 AT 4:43 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
15 fl looks like EEC. My mom bought car new never wrecked or flooded. I got car after mom passed away. Only 90,000 miles. When I put the bigger fuse in to get me home it smoked from right side of dash. Turn off and restarted with no smoke. Got to parts store and replaced with 20A fuse and car wouldn't start, just blew again. I put 60A back in and same as earlier, smoke on right side. Turned off car waited a minute and drove home with no smoke.
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Saturday, March 28th, 2020 AT 5:31 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. The 60 A fuse is causing the circuit that is the issue to burn and that is where the smoke is coming from. Just because you are not seeing smoke doesn't mean it is not getting hot. So we want to stop putting the 60 A fuse in so we can find what the issue because I would hate to see any damage done to the vehicle.

Can you let us know which fuse is blowing? There are several 20 Amp fuses and we need to start with that info. Thanks
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Saturday, March 28th, 2020 AT 5:37 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Fuse 15 FL in engine compartment fuse box. Owners manual shows EEC.
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Saturday, March 28th, 2020 AT 5:41 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Could a shorted o2 sensor like melted to the exhaust cause this? I had exhaust work done a couple months ago.
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Saturday, March 28th, 2020 AT 6:23 PM
Tiny
KASEKENNY1
  • EXPERT
Okay. So we need to unhook the PCM and plug in another 20A fuse and see if it blows. If it does then unhook the contact control control relay module and see if it blows.

There is a splice involved but hopefully it is one of these items.
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Saturday, March 28th, 2020 AT 6:30 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Okay, I can't do it tomorrow but I'll let you know. Thank you very much.
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Saturday, March 28th, 2020 AT 6:34 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Until tomorrow.
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Saturday, March 28th, 2020 AT 6:36 PM
Tiny
KASEKENNY1
  • EXPERT
Sounds great. Please let us know what you find. Either Joe or I will get back to you, whoever gets to it first. Thanks
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Saturday, March 28th, 2020 AT 6:46 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Good morning, I found burnt wire. Tag on harness f6sb-14401-b4 wire is red and only burnt up to splice into 4 red wires.
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Sunday, March 29th, 2020 AT 8:36 AM
Tiny
KASEKENNY1
  • EXPERT
Yep. That will do it. You need to replace that splice as well. Basically cut out the damaged wire and make sure there are no other compromised wires. So cut the wires where I pointed to and then splice a new wire in its place and then just run it to the connector. You can replace the entire harness if you want but if it is just one wire, then you can just replace that.

Also, make sure you twist and solder them together then use heat shrink to protect it. Don't try to use butt connectors because they will not hold up.

Let us know if you have questions. Thanks
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Sunday, March 29th, 2020 AT 8:49 AM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
After I do that. I still need to find out what is shorted. Where does the wire go?
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Sunday, March 29th, 2020 AT 9:05 AM
Tiny
KASEKENNY1
  • EXPERT
We can help locate where the wire goes but we need more detail about the connectors and location on the vehicle. This is a B+ wire which means it is what supplies 12 volts. Unfortunately we cannot just identify the wire by the picture as there are a lot of them. So do you see the other end of the wire that is broken? If so, can you trace that wire to see where it leads?

However, that is the shorted wire so we need to repair that. Then retest to see if there are any other issues.

Let me know if you can trace that wire to find out where both ends are and then we can identify it and repair it. Thanks
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Sunday, March 29th, 2020 AT 12:49 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Unfortunately I have been working on my second vehicle so I can go to work Monday. The red b+ wire that has a splice is in the harness that runs from the middle of the dash by the console along the bottom of glove box to the blue connector that plugs into the PCM harness. I got some more 20A fuses so I will look at more when I get home. FYI I was a master mechanic in the 1980's and now a electrician at KCC HVAC company.
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Sunday, March 29th, 2020 AT 2:16 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Hello guys. I just took the center console out. And traced where the burnt wire was going and it's going to a harness that leads under the car. I'm thinking it's the hot wire. For the oxygen sensor because I just had some exhaust work done a while back and I bet one of those is melted to my exhaust. I can't get under the car right now. I don't have a jack. I'll have to go to my dad's and get it. And I'll let you know as soon as I find it. Thank you.
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Sunday, March 29th, 2020 AT 4:55 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
First picture from harness and second pic harness goes through floor. Night, I'll let you know what I find tomorrow.
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Sunday, March 29th, 2020 AT 5:56 PM
Tiny
KASEKENNY1
  • EXPERT
Yep. That makes sense. I attached the wiring diagram for this and that seems to match what you are seeing. The red power wire for the o2 sensors run through a number of splices and the PCM but also directly from the 20 amp fuses for the PCM.
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Sunday, March 29th, 2020 AT 5:59 PM
Tiny
TIMOTHY 68COUGAR
  • MEMBER
Thanks so much for your help!
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Sunday, March 29th, 2020 AT 6:05 PM
Tiny
KASEKENNY1
  • EXPERT
You are welcome. Please let us know how it turns out and if you need more help in the future. Thanks
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Monday, March 30th, 2020 AT 3:27 PM

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