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A. Penland
A. Penland, Ford Senior Master Technician, ASE
Category: Ford
Satisfied Customers: 6293
Experience:  ASE Master Certified, ASE Advanced Gasoline Engine Performance
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It is blowing PCM fuses as soon as I turn on the ignition,

Customer Question

it is blowing PCM fuses as soon as I turn on the ignition
JA: Do all the fuses have the correct amperage for the circuits they're in?
Customer: yes
JA: What is the model/year of your Ford?
Customer: 1997 Ford F150
JA: Are you fixing your F150 yourself? What have you tried so far?
Customer: Looking for wiring shorts
JA: Anything else you want the mechanic to know before I connect you?
Customer: Nothing I can think of
Submitted: 2 months ago.
Category: Ford
Expert:  A. Penland replied 2 months ago.

What are the fuse # locations and also are they under the hood or under the dash?

Customer: replied 2 months ago.
Under the hood. It is the PCM Fuse #24
Expert:  A. Penland replied 2 months ago.

Remove the PCM power relay in the same fuse box and see if the fuse still blows.

Customer: replied 2 months ago.
The fuse blows as soon as I turn on the ignition. Shall I still remove the PCM relay?
Expert:  A. Penland replied 2 months ago.

Yes, the fuse powers the PCM power relay. If you remove the relay, and the fuse blows or not, we'll know what direction to go.

Customer: replied 2 months ago.
With the relay out, the fuse did not blow when I turned on the ignition and turned over the engine
Expert:  A. Penland replied 2 months ago.

Ok. Do you have a digital multimeter?

Customer: replied 2 months ago.
Next move?
Expert:  A. Penland replied 2 months ago.

Ok. Look on the side of the relay and the bottom. The pins should be numbered, 30, 87, 87a, 85 and 86.

With the relay out of the fuse box. Check and see what pins in the fuse box for the relay have ground when the key is turned on. There should only be one.

Customer: replied 2 months ago.
How do I check this
Expert:  A. Penland replied 2 months ago.

Set the digital multimeter to volts and place the red lead of the meter on the positive post of the battery and the block lead you touch each pin in the relay socket of the fuse box one at a time. You will get battery voltage on the meter reading 12 volts if there is a ground on one of the pins. Do this with the key turned on.

Customer: replied 2 months ago.
it will take a minute to find my digital multimeter. stand by
Expert:  A. Penland replied 2 months ago.

No problem. I'll be on line till 10:30 PM EST. I live on the East Coast.

Customer: replied 2 months ago.
30,85,87 have ground; 11.8 volts
Expert:  A. Penland replied 2 months ago.

Remove the fuel pump relay and see if you still have ground on pin #87.

Customer: replied 2 months ago.
Fuel Pump Relay removed. Still have 11.8 on 87
Expert:  A. Penland replied 2 months ago.

Does it change with the key turned off compared to being on?

Customer: replied 2 months ago.
Next?
Expert:  A. Penland replied 2 months ago.

Does it change with the key turned off compared to being on?

Customer: replied 2 months ago.
I'll check
Expert:  A. Penland replied 2 months ago.

Thank you.

Customer: replied 2 months ago.
With ignition off, I still get 11.8
Expert:  A. Penland replied 2 months ago.

You have something grounding out that circuit. Have you had any work done to the vehicle recently and also, what engine does your F150 have?

Customer: replied 2 months ago.
5.4 Liter. Recently added new radio, but took fuse out as part of testing before I contacted you. Added fog type lights.
Expert:  A. Penland replied 2 months ago.

Check the wiring harness on the passenger side of the engine that goes down to the transmission. We have seen these harnesses contact the exhaust manifold and melt to the metal.

Customer: replied 2 months ago.
Nothing making contact with exhaust manifold or showing bare wire
Expert:  A. Penland replied 2 months ago.

I am going to have to do a little digging through the wiring diagrams. Is it ok to continue this tomorrow evening?

Customer: replied 2 months ago.
I'll be home after 5 pm eastern
Expert:  A. Penland replied 2 months ago.

Sounds good. Good night.

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