So I have a 99 F-350 XL, 6.8 2WD, got 268K on it and it's been a great truck, hasn't ever given problem until last June it killed out a PCM, hydrolocked the engine and sent a connecting rod out the oil pan, so I swapped the engine and replaced the PCM and everything has been great, everything that it's equipped with works great, it's a base model truck, used to be a uHaul box truck, and we got it with 239K on the clock. Well fast forward from last June to this month and it killed out another PCM, so I got one out the JY with the same numbers and all on it, and it ran twice and killed that one too, and it's got me at wits end, since there's literally nothing that blows fuses or DOESN'T work, everything is perfect but it won't stop it's PCM habit. It's only killing them when it's switched on, to narrow things down.

 

What part of the PCM is fried? Have you opened up the case to look for any burned components? It might help you track down the issue.

 

The older trucks have been known to have leaking capacitors inside them. Ive open a few and soldered in new capacitors and they all work great. Maybe this one is having a similar issue?

 

Well I haven't personally, since I'm not that great with vehicle electronics, however it's holding the fuel pump running and giving off a no start condition, also won't connect to either of my OBD II readers.

 

most likely surges or over voltage condition.

get one of these and install across that battery. take a drive and if the diode is hot you have a voltage regulation problem

https://www.summitracing.com/parts/p...iABEgJDPPD_BwE

 

If the truck is rusty, or just hasn't been maintained well, it could also be bad grounds causing the PCM to blow. Check the terminals at the PCM connector for corrosion, check your battery and Alternator for corroded wires and check your body grounds and engine to frame grounds also, especially the body to frame ground underneath the passenger door area.

 

So I got busy a few days but got back on this, pulled the PCM and opened it up, no visible damage and it didn't smell burnt, nor was there any damage whatsoever in the connector, so I got curious and since the alternator was repaired last April (bearing failed) I took it off, and where the wire connects to it was melted, and the nut was tight, exceptionally so, so now the plan is to trace down the length of the battery cables and inspect/replace them since a clean used set off a southern truck is easy to come by.

 

I have to agree with speakerfritz that there is a glitch somewhere in the wiring that is killing your PCM. The PCM;s can take a fair bit of abuse but surges in voltage are straight out killers.

If you cant find your issue would bite the bullet and take it to a shop that does auto electric as a specialty and have them trace it down I wouldnt take it to a shop that doesnt do auto electric a lot because some guys just dont have the technical ability to trace down hard to find electrical issue in a vehicle

 

Measure output of alternator while running, should be around 14 volts
As mentioned above checking grounds is a good idea.

 

So, checked the grounds and they were all good EXCEPT the silver flat one going from the body to the back of the engine, replaced it and checked the harness through and through and everything was good and passed testing, truck also had one of those crappy lead replacement cable ends, replaced that with a brass one, and basically put it back together. After that, I pulled the last PCM that was sticking the fuel pump running and giving off a no start, and took it apart, and there was nothing wrong looking or burnt smelling, so I put it back together and reinstalled it, and then decided that I was gonna drain the gas tank, which in theory would involve removing the output line on the fuel filter and using a rubber hose to drain it into cans, then I had a no power to the PCM situation. So, new problem, ignition switch was first thing changed, since I had one on the shelf, and still no power, so I swapped the relay, still no power, so I replaced the PCM relay fuse, the 40A one, and reseated all the rest and it seemed to fix all the issues and it runs fine now? No idea what that did but it seems to have worked.

 

So, no dice, went out this morning, switch it on, pump running non stop with a no start condition, tried it half a dozen times and nothing, back in the house, went out this afternoon and it cranked first turn over? That's a curve I didn't see coming I'd have bet on it being fixed.

 

Alright, need some legit input, I'm at a total loss on this, ended up with a no power situation on the PCM again, so I replaced the underhood harness, I happened across one that was out an identical truck for dirt cheap, so I replaced it, and still nothing, so I got somewhat annoyed so I went and got the PCM out the donor truck, still nothing, checked the pins on the connector for the PCM and the relay, and everything is in order and functional I thought, but no CEL, no fuel pump, no start, so I got out the code reader and plugged it in, and did it with it switched on, and it went right to work, but only works with the reader plugged in? Fuses all look in order in the box under the steering wheel.

 

Check over all your grounds really well. It kind of sounds like the reader is completing a circuit that should already be there. If youve got no power to the computer, I would go to the pcm power relay. Make sure it has voltage to the relay.

 

The relay underneath the hood is functional and has been swapped around, and this problem is present on both of the underhood harnesses, so I'm thinking it's a problem somewhere in the grounds, considering they all look good visibly, I'll just begin replacement of them with ones off the donor that I can check before installation.

 

Update, grounds are either in amazing condition or replaced, same issue, the way I tested them was just take a test light, run a test wire from the battery close to where I'm working and test them with the test light, it's a low power draw but all of them lit up perfectly.