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Pcm not getting power?

Tiny
MARKD1977
  • MEMBER
  • 1998 FORD F-150
  • 4.2L
  • V6
  • 2WD
  • AUTOMATIC
  • 168,308 MILES
PCM is not powering up when I turn on the ignition switch. I replaced it with a new one but it still not working. At first, I thought it was the coil pack, so I replaced it still wouldn't start. Then replaced the crankshaft sensor and the cam sensor. Tracked it down to the no power to the PCM. Need help tracking down the problem.
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Wednesday, December 25th, 2024 AT 6:24 PM

7 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 48,571 POSTS
The first thing I would do is check the fuse # 24 in the fuse panel under the hood and if that is okay let's swap out the PCM power relay to see if the PCM gets power. Here are the locations of the fuse and relay. Check out the images (below). Please let us know what happens.
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Thursday, December 26th, 2024 AT 1:31 PM
Tiny
MARKD1977
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  • 4 POSTS
I have already done that; the fuse and relay are getting power. I'm not getting power from the ignition switch to the under-dash fuses or the PCM. Can the ignition switch cause this problem?
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Thursday, December 26th, 2024 AT 5:50 PM
Tiny
KEN L
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  • 48,571 POSTS
Does the EEC relay click when you turn the key on?
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Friday, December 27th, 2024 AT 10:37 AM
Tiny
MARKD1977
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  • 4 POSTS
I don't know for certain, I don't hear it click, I replaced the old one. I tested the wires in the power distribution box and I'm getting 12 volts to everything there.
But the under-dash fuses box I'm not getting any power to the fuses for the PCM/ECU with key on or off. That's why I think the problem is in the ignition switch.
I have been trying to find a wiring diagram for the ignition switch but cannot find any info on Google for my specific model truck. I was driving and the truck just died.
I have replaced the crankshaft sensor, the cam sensor, and the PCM/ECU and still no spark signal on the ground pulses of the coil.
I hooked up my code reader and it says I cannot communicate or connect.
I'm at my wits end on this dang thing. It my only vehicle and it's been broke down since September.
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Saturday, December 28th, 2024 AT 1:49 AM
Tiny
KEN L
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  • 48,571 POSTS
There are 3 main fuses that run the ignition switch which you can see from the wiring diagrams below. Also, I have seen the GEM go out and cause problems like this as well. Check out the images (below). Let us know how it goes.
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Saturday, December 28th, 2024 AT 10:18 AM
Tiny
MARKD1977
  • MEMBER
  • 4 POSTS
How do I tell if it's a bad gem?
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Monday, December 30th, 2024 AT 12:46 PM
Tiny
KEN L
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  • 48,571 POSTS
You will need to test the inputs and outputs, here is a guide to help and I have included the wiring diagrams as well. Did you test through the ignition switch? The GEM should be under the dash.

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

Check out the images (below). Let us know what you find.
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Tuesday, December 31st, 2024 AT 11:16 AM

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