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Blown PCM Fuse

Tiny
SNOOPY11895
  • MEMBER
  • 1997 FORD F-150
I have a 1997 Ford F-150 with 132,000 miles, 4.2L 6 cylinder engine. My truck keeps blowing the PCM fuse when I engage the ignition. The relay is fine. It used to finally turn over after changing the fuse 4 times, but now it will not turn over at all. I also have a check engine light that says the upstream O2 sensor is bad. I have no idea where to even begin with this problem.

I tested f24, f30, and the MAF red wire. All three have power with the ignition in the on position.

Any advice would be greatly appreciated.
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Wednesday, May 9th, 2007 AT 7:37 PM

18 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
It sounds like you have a system power wire that is pinched or has rubbed through to ground. Here some guides and some wiring diagrams so we can do some testing and get the problem fixed.

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

What fuse # are you replacing and is it in the eng. Comp? When you say "engage the ignition" is that from OFF to RUN or RUN to START? Does your anti-theft light blink rapidly when it fails to start? Why do you think the PCM relay is fine? Do you have an aftermarket anti-theft or remote start system installed. Is this a man. Or auto trans? Your full and complete answers are needed to accurately diag. Your concern.

The fuse number is 24, and it is in the engine compartment. I believe the PCM relay is good because it does not look blown/bad. What I mean by engage the engine is from run to start. I have no anti-theft (factory or aftermarket) The truck is an automatic.

Please recheck the relay as well.

https://www.2carpros.com/articles/how-to-check-an-electrical-relay-and-wiring-control-circuit

Please let us know what you find.

Cheers, Ken
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Tuesday, September 19th, 2017 AT 11:22 AM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
Just to add to this one I just had a 2002 mercury mountaineer that suddenly blew the ecm fuse which feed power to the heated o2 sensors a lot of the emission controls etc. It turned out the wires on the passenger side o2 sensor wires had shorted to the exhaust pipe up high where it was hard to see.I put a bulb across the the fuse that was blowing with the key in the run position. It was brightly lit I had someone watch the light while I moved wires around the light was flashing when I moved the shorted out wire that's how I found it.
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Tuesday, September 19th, 2017 AT 11:39 PM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
Good find!
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Wednesday, September 20th, 2017 AT 11:26 AM
Tiny
SATURNTECH9
  • MECHANIC
  • 30,869 POSTS
That's a great way to find short circuits.
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Wednesday, September 20th, 2017 AT 1:24 PM
Tiny
JOHN FORTINI
  • MEMBER
  • 1 POST
I had the same problem. It was a coil shorted out. The coil was new had 50 miles on it. After market coil from china. We had replaced all the coils with a kit.
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Tuesday, February 12th, 2019 AT 9:14 AM
Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,363 POSTS
Excellent addition to this thread! Please feel free to help out whenever you are on the site :)
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Tuesday, February 12th, 2019 AT 10:05 AM
Tiny
4BOB
  • MEMBER
  • 11 POSTS
  • 1997 FORD F-150
  • 4.2L
  • 6 CYL
  • 4WD
  • MANUAL
  • 170,000 MILES
Any idea where the engine fuse is located on a 97 f150?
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
PACO8224
  • MECHANIC
  • 117 POSTS
Hello,

Here is a guide and a full fuse panel location and identification so you can run some tests. Check fuse #20 5 amp.

https://www.2carpros.com/articles/how-to-check-a-car-fuse

Check out the diagrams (below).

Please let us know if you need anything else to get the problem fixed.

Cheers
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
4BOB
  • MEMBER
  • 11 POSTS
I believe it is the pcm module. Still trying to chase down that short.
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
PACO8224
  • MECHANIC
  • 117 POSTS
The first one is under the dash, and the second one is located under the hood
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
4BOB
  • MEMBER
  • 11 POSTS
If I unplug the wire harness from the pcm module and test each wire will I be able to confirm if the short is in the wire harness?
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
4BOB
  • MEMBER
  • 11 POSTS
All fuses are good I replaced the PCM out of a used truck and it runs great! Thanks for your help I love this site.
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
DIRK119
  • MEMBER
  • 1 POST
  • 1997 FORD F-150
  • 6 CYL
  • 4WD
  • MANUAL
  • 100,000 MILES
I have recently tried to get my truck running again. I have went through and replaced and diagnosed some things and now I am stuck with a PCM 30amp fuse that blows every time I try to start the truck. It did start the first time and ran for 5 mins and blew the fuse. Since then I havent been able to even start it before the fuse blows. It even blows a 40amp fuse. I am not sure how the PCM works or what could be causing it to blow. Any knowledge on this would be very helpful. Thank You

I just pulled my spark plugs and they were in bad shape I had 3 broken plugs and they all had a 100 thousand gap. I heard that if my coil pack was bad it could blow my PCM fuse. Could this be the problem?
Thanks Again
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
HMAC300
  • MECHANIC
  • 48,601 POSTS
Glad you could get it fixed, that kind of problem can be tough. Please use 2CarPros anytime we are here to help

Cheers
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
DIRK119
  • MEMBER
  • 1 POST
I had the same problem and got is fix by using this post you guys are the best!
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Tuesday, February 12th, 2019 AT 10:05 AM (Merged)
Tiny
RIC1234
  • MEMBER
  • 1 POST
  • 1997 FORD F-150
  • 166,000 MILES
Pcm 30 amp fuse blows
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Tuesday, February 12th, 2019 AT 10:06 AM (Merged)
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
I looked at 4.2l engine early production wiring. My wiring doesn't say pcm fuse, it just gives a number and amp rating. If it is the fuse I'm thinking about, like hunting a needle in a haystack. That fuse supplies many different circuits.

Just a thought, try pulling the fuel pump relay and see if still blows the fuse. That may not be conclusive?
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Tuesday, February 12th, 2019 AT 10:06 AM (Merged)
Tiny
JDL
  • MECHANIC
  • 16,098 POSTS
I just rechecked the wiring, I don't believe pulling the relay will help.
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Tuesday, February 12th, 2019 AT 10:06 AM (Merged)

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