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PCM Power Fuse blowin

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Old 10-29-2005, 02:34 PM
REDONE97 REDONE97 is offline
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PCM Power Fuse blowin

I have a 97 4.2 F-150 and I have a short somewhere & it keeps blowin the PCM Power Fuse. At the suggestion of someone we unhooked the inertia switch and the fuse didnt blow, so then we hooked that back up and unhooked the fuel pump and the fuse didn't blow, so then we hooked the fuel pump back up and unhooked the wiring under the intake and still it did not blow, but we hooked that back up then the truck started. My question is has anyone had this problem before and if so can they say which wire was it specifically? Also was this a good method to find the Short?
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Old 10-29-2005, 03:15 PM
Bob Ayers Bob Ayers is offline
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I would trace the 12V (switched and unswitched) wiring from the fuse box to the PCM connector. The PCM does not SOURCE any current into the fuel pump circuit. The PCM SINKS current from the Fuel pump relay to pick it (close contacts).
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Old 10-29-2005, 09:39 PM
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Did it blow a fuse in the fuse box under the hood? I don't have a 97 diagram, but the 99 diagram for a 4.2 shows several different fuses that power the PCM circuits, but there is only one that feed the computer itself. Which fuse is blowing?

And it's obvious from your experimentation, you have moved the wiring around and temporarily fixed the problem. I would go over the engine wiring with a fine tooth comb and see if you can find any bare spots.
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Old 10-30-2005, 04:29 PM
REDONE97 REDONE97 is offline
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it is the pcm power fuse (30A) under the hood.
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Old 10-30-2005, 09:55 PM
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In the 99 diagram, there is a 30 amp fuse that feeds the contacts of the PCM relay. After it leaves the relay, it feeds several other fuses in the fuse box under the hood.

One of them is fuse #23, 15 amp and it feeds a red/yellow wire.

Another is fuse #18, 15 amp that feeds a red wire.

The last is fuse #24, 15 amp that feeds a violet/white wire.

If your truck is like this, and none of these fuses are blowing, then the problem must be on the yellow black that feeds the PCM power relay or the violet wire that leaves the PCM power relay.
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