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.
Wednesday, December 25th, 2024 AT 6:24 PM

27 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
  • 51,506 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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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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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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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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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
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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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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
Tiny
MARKD1977
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Okay, I haven't had a chance to test the ignition switch yet, but a friend told me to try hard lights and turn signals and wipers if they all worked good the gem was probably okay. I did that and everything works fine. Right now it's freezing rain here so I can not get out and test the ignition switch, but as soon as I do get a chance I will let you know.
Is there a place you would recommend that I start?
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Sunday, January 5th, 2025 AT 2:54 PM
Tiny
KEN L
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Yes, I would start there.
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Monday, January 6th, 2025 AT 9:51 AM
Tiny
MARKD1977
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  • 16 POSTS
Sorry it took so long to get back to you.
I tested and replaced the ignition switch, but it still doesn't have power to the PCM fuses in the interior fuse panel.
What should I check next?
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Thursday, January 30th, 2025 AT 12:50 PM
Tiny
KEN L
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There are two main fuses and the PCM power relay that supply power to the computer, Fuse #f24 in the under hood fuse panel which should be hot at all times, and the fuse #f30 in the fuse panel inside the truck which should only have power when the key is on. I would swap out the PCM relay as well which is under the hood fuse panel as well. Please use this guide to check for power.

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

Check out the images (below). Please let us know what happens.
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Friday, January 31st, 2025 AT 10:06 AM
Tiny
MARKD1977
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Can I use a horn relay to check the PCM power relay? Or is it different amps?
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Saturday, February 1st, 2025 AT 3:19 AM
Tiny
KEN L
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Yep, you can use the horn relay.
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Saturday, February 1st, 2025 AT 10:01 AM
Tiny
MARKD1977
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Sorry it took so long to get back to you. Okay, tried switching the relays still not working, but I found this wire and don't know what it goes to, it had black tape around it.
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Wednesday, February 26th, 2025 AT 12:59 PM
Tiny
MARKD1977
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  • 16 POSTS
It's black with an orange stripe.
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Wednesday, February 26th, 2025 AT 1:00 PM
Tiny
KEN L
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I don't know where that goes. I would go back over this post and the wiring diagrams to see where you are losing power to the PCM.
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Thursday, February 27th, 2025 AT 10:56 AM
Tiny
MARKD1977
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I've gone over and over it, I guess I'm just going to have to take it to a shop and get it fixed.
I have already put too much in it just to scrap it.
Thank for your help and time.
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Thursday, February 27th, 2025 AT 12:53 PM
Tiny
KEN L
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Sounds good, please let me know what they say.
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Friday, February 28th, 2025 AT 5:21 PM
Tiny
MARKD1977
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Sorry it's been so long.
But I couldn't find anyone willing to work on it.
I tested the ignition switch wires, I. Getting voltage. But when I turn the key on to run I'm not getting any voltage to the PCM fuses in the interior fuse panel.
What could be causing this?
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Friday, April 25th, 2025 AT 9:24 AM
Tiny
KEN L
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Is it fuse #f30 you are talking about? If so, this fuse gets its power from the ignition switch.
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Friday, April 25th, 2025 AT 9:43 AM

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