Pcm Short/bad Ground?

Brad Hauk

New Member
Aug 17, 2017
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#1
I bought this car a few months ago. Turbo car built. Who ever did the wiring obviously messed something up. I had a no start issue where the theft light was flashing. Initially I thought it was a pats issue or a bad key. Had a locksmith come out and he confirmed that was not the issue. Well last night I was out in the garage messing with the car trying to figure out the issue and turned the the key on... And poof my MSD tack driver for my shift light went up in smoke. I'm sure you all know that that it wires into the (coil wire)? coming from the main harness. Thinking that this was the issue and maybe I just had a bad tack driver i checked to make sure I had voltage still at the coil wire and then removed the tack driver and reconnected the coil wire to itself in the harness. CAR STARTED AND RAN! That's not the end of it though. Come home today, go to start the car and nothing... Dash lights worked but the fan and fuel pump did not come on. I go back to the coil wire and it's fried, luckily it blew the fuse for the PCM instead of burning the car to the ground. Where do I go from here? Where do I check for bad grounds? How do I check if maybe the PCM has a bad ground, or maybe the ccrm? I'm kind of lost as you can tell from this post LOL... HELP?
 
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wmburns

SN Certified Technician
Aug 14, 2009
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#2
I give up. What model year Mustang are we dealing with as there are differences in the electrical across the model years? It would also help to mention WHICH fuse blew. In case this is a 1999-2004 Model year, use this to identify the fuse.

1999-2004 MY fuse panel schedule:
http://www.allfordmustangs.com/foru...r-swap-wont-fire-please-help.html#post2669271

If looking for a WAG (well maybe an educated guess), there's a short in the +12 ignition VPWR red circuit (the one you modified). When this blows the fuse, the PCM looses power since on the same circuit. With no PCM power the fuel pump doesn't run, cooling fan runs all the time, and the Theft light blinks.

Since we are in the WAG mode, I would first look for the short to be down stream of the tach driver.

Or the tach driver is pulling down the engine VPWR circuit. The PCM is affected by the voltage on the ignition red VPWR circuit. Must be stable, solid 12 volts with a sold ground all the way back to battery negative. The motor itself also has to have a solid ground (see the alternator voltage drop test). After a motor swap the engine grounding strap is often over looked.

Bottom line. Double check your work. Be sure that solid wiring practices were used. No corners cut.

Or one of the coil pack signal return wires are shorted to ground causing excessive dwell which will burn up a coil pack.

But if looking for a place to start regardless of the model year, first perform a voltage drop test to the alternator.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-fo...perform-charging-system-voltage-drop-test.56/

The voltage drop test is also the best way to confirm grounds. See this post for more information.
https://www.stangnet.com/mustang-forums/threads/help-please.899405/page-2#post-9068227
 
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