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Discussion Starter · #1 · (Edited)
Had the truck scanned during a crank-no start and it read "can't communicate with scan tool". Truck started fine for 3 weeks after, then no start. (Copied from the mechanic's summary) 1999 F-350, 7.3, 280,00 miles, intermittent no start hot & cold. Tested powers & grounds to PCM connector & relay, tested good. Verified signal to PCM. PCM & relay both getting hot after truck runs. Tested for 5 volt reference across sensors, tested good. Has ignition signal. Swapped PCM relay & truck starts.

The shop says they suspect a faulty PCM, I'm wondering if just the faulty relay will cause this? another shop says the PCMs rarely fail. I did hear kind of brief zapping noise a few times when it wouldn't start. Truck did it intemittently for a few weeks, 3 weeks with no issues, then a no start for 4 days.
 

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I'd be more inclined to believe the ignition switch is at fault than the PCM but would need more information to confirm that.
There are 4 separate circuits in the ignition switch and if one has burnt contacts it may cause intermittent issues.

Does the WTS light come on every time the key is turned on? If not, it means the PCM isn't powering up.

I don't understand "PCM & relay both getting hot after truck runs." Buy "hot" does he mean powered up or temperature "hot?" I suppose it's possible the PCM power relay has bad contacts, which might cauise it to get warm.
 

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Discussion Starter · #3 · (Edited)
I'd be more inclined to believe the ignition switch is at fault than the PCM but would need more information to confirm that.
There are 4 separate circuits in the ignition switch and if one has burnt contacts it may cause intermittent issues.

Does the WTS light come on every time the key is turned on? If not, it means the PCM isn't powering up.

I don't understand "PCM & relay both getting hot after truck runs." Buy "hot" does he mean powered up or temperature "hot?" I suppose it's possible the PCM power relay has bad contacts, which might cauise it to get warm.

It means hot to the touch.

Yes, the WTS light comes on whenever the key is turned on.

I should try a new ignition swith, its cheap & looks like a DIY'er can do it.
 

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You might try replacing the PCM power relay also, they're cheap and readily available. Or you can swap out the heater blower relay with the PCM power relay. Even cheaper, and you can check if it gets hot still after swapping.
 

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Discussion Starter · #5 ·
Yes that's what the mechanic did, he swapped the PCM relay with the blower motor relay.

He showed me where they are and told me try swapping them if I have trouble.

Tomorrow I'm going to Napa and getting a new relay and ignition switch and put those in. And hopefully we'll take care of this gremlin that's causing the truck to not start now and then.
 
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