No spark will crank well but not fire?

2005 FORD F-150
250,000 MILES • 5.4L • V8 • 4WD • AUTOMATIC
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WRENCHINGTHEDREAM
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Had my AC compressor go bad, seized up. Replaced it and the other engine accessories. The truck sat a while in the process of the repair.
Now it will crank well but not fire. I tried starting fluid but no hint of sparking even with that so I figure there is no spark.
Not sure what my process should be for checking the various things related to ignition spark.
Any help is appreciated. I can tear things apart and put them back together but not great at diagnosing.
Oct 1, 2025 at 1:52 PM
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AL514
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Hello, looking at the AC compressor service info, the crankshaft position sensor is right next to it, if it was left unplugged or the connector was damaged during the compressor install. Ill post a diagram showing where the sensor is, but do you see the tachometer jump a little bit when cranking the engine?
Oct 1, 2025 at 2:11 PM
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AL514
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Heres the location of the crankshaft position sensor connector, thats where I would check first since the wiring harness has been moved around in that area.
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Oct 1, 2025 at 2:16 PM
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WRENCHINGTHEDREAM
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AL514
The sensor is plugged in and wires appear to be good.
I did not see the tach jump at all when cranking.
Oct 2, 2025 at 11:51 AM
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AL514
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If you can get access to a basic scan tool that can read live engine data along with any potential trouble codes stored, that would help. For example if you see no rpm signal on a scan tool while cranking, then you know theres an issue with the wiring or crank sensor.
And have you checked that the coils and injectors have power with the key On and cranking? If it sat for awhile, rodents love to chew on wiring, due to the taste of a chemical in the insulation so check the wiring harness for broken wires, or ground wires that are not making good contact. Did you check more than one coil for spark?
Im not sure how much equipment you have for testing, but also check for the Check engine light come on when you turn the key On? If it does not, then the PCM is not powering up, that can happen for multiple reasons.
Here are some guides to help and we can pull up any wiring diagrams needed.

https://www.2carpros.com/articles/no-ignition-spark
https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-use-a-voltmeter
Oct 2, 2025 at 1:48 PM