Me and my 13 year ford master tech can't figure it out it has a 5.4l and every time we put a new pcm in it and program it to the pats with no problem we try to crank it up and it fryes the pcm every time (3 times now) when it is attempted to crank.... please please help me I don't even know where to start I've looked at all of the wires that I can see and no chafing or bare wires... I naturally think that the pcm is short to ground because it takes out the main or the largest processor in the pcm but have no clue what would cause this? Please help me I have a week to get this running or I can't finish college because my wife needs a car to take my children to school and I've already burned up 900 bucks in pcm's I can't do this again...

 

Something expen$ive or at least sensitive is getting fried during engine crank?

This points towards not a "short to ground" but lack thereof - a loose , missing, or corroded ground and/or cables related to the starter and system.

Don't just look at them put a wrench on them, remove and grind down to bright shiny bare metal, chase threads out, install new cables, etc. you may be able to observe the voltage spike during cranking (using an already smoked component) by placing your DVM in parallel from the negative battery terminal to PCM case, etc. Let us know what you find.

 

I will do tomorrow after work and respond with findings thank you...

 

But just out of ignorant question do you think a lack of ground would fry the pcm over and over?

 

Yes, Every. Time. If this is the problem. Not necessarily saying for certain this IS the problem, but it needs to be ruled out.

What I mean by that, a loose or missing or corroded ground or cable etc. will cause all kinds of mischief. In this case, the starter draws a couple hundred amps. Big heavy cables.

The tell here, is that this only happens during engine cranking. Just for that split second or so, current is zapping things it shouldn't. Once the connections start getting marginal or something breaks the circuit, the current being drawn will always seek out other paths. In this case, it sounds like excess voltage or current is frying the PCM.

Depending on the vehicle and layout - Wheel bearings, gearboxes, brake lines, throttle cables, radiators - whatever. The energy stored in an auto battery is considerable, and should be respected.

I'm surprised this doesn't happen more often. Has any major maintenance happened recently - engine swap,, transmission work, radiator, maybe some painting in the engine bay?

Has the engine otherwise started OK in the past? No marathon cranking sessions and a no start condition?

 

Honestly yes I believe there was an engine swap or at least a harness swap I've looked over most of the obvious things but haven't checked the grounds... and I can't find any other occurrences of this on the net so you know how that is with self diagnosis... I would have thought my buddy would have had more brains than that though to shoot me in the right direction... I know that I also have gas in the crank case also after a fresh oil change from the pcm frying and causing the fuel pump to steadily pump with koeo... but yet again the fp functions normally during the whole reprogram and there after until my buddies attempt to crank it... and initially it tried to start like 3 times until the pcm fryed again....

 

Almost sounds like the main ground to the engine/starter is not good, and you're pulling the starter current through another wire that sends excessive currents through the PCM. Does it have a good or slow crank?

 

Slow and almost seems like it drags but seems like the starter is fine/strong... but I could possibly be wrong...

 

If that was the case what other wire would go straight to the pcm like that..?.. I'm just boggled that it doesn't pop fuses/relays so it I would definitely assume that it is on the unfused side of the pcm... what has the ability to push that much current to the pcm...? could a starter be bad in the sense that it could be passing power through the reference wire or what? This is boggling my mind...

 

Have you checked for a short from the primary to secondary on the ignition coils?

 

Please elaborate on how to do that chris...

 

Does that have coil on plug or are there plug wires on it?

 

Plug wires...

 

2 coil packs one on each side front of motor...

 

The ground is unfused, so if the positive wire is fine, but the ground wire is bad, like bad ground to engine/starter, it will pull ground current through whatever it can. This might include the PCM in certain cases. Verify that all heavy gauge wiring, specially ground, is in good shape to the engine. The fact that you said it's slow on crank and seems like it drags does sound like a wiring issue and that the starter can't get enough juice to do its thing. Also check everything from coil to plugs, and make sure it can't arc over and send high voltage into any wires that feeds it back to the PCM.

If all this checks out, you need to start measuring things. Does it crank when there's a fried PCM in there? You could then use a fried PCM to test and then start measuring supply voltage and inputs and outputs.