failing sensors: bad wiring harness or ??

Discussion in '2005+ Mustang GT 4.6L Tech' started by tabstang, Aug 15, 2022.

  1. tabstang

    tabstang Member

    After installing a whipple 2.3 kit in my 07 GT I've had several electrical gremlins; 1st a bad (aftermarket) intake air temp sensor, reverted back to MAF and car ran fine. Then a bad MAF sensor, replaced it and car ran fine. Then a bad fuel pressure sensor, replaced it and car ran fine - but for less than an hour. Now I'm getting no codes, car cranks but won't start.

    For background, I rerouted part of the engine bay harness and extended one pair of coil and injector wires. I got a couple cross-wired, but figured it out and the car ran fine. But now I'm wondering what else may have been affected.

    Any thoughts on what would be causing all this?

    I'm thinking of replacing the engine bay harness, just to remove any doubt regarding faulty wires.
  2. 07 Boss

    07 Boss Senior Member

    When you rerouted the harness did you mess with the wires for the frps? Did the car start/run with the old frps?

    Our cars are old, shit wears out or fails. I wouldn't read too much into it

    I would work backwards from what you last did. Being the fuel pressure sensor, verify it is getting power and check if the pumps are coming on when you turn the key to on. Pull a plug and see if it's actually getting fuel to the cylinders. Check your grounds for the injectors and cops. If I remember correctly when I redid my wire harness they all shared common grounds. I would thing if it was one or two cops or injectors the car would still start.
  3. redfirepearlgt

    redfirepearlgt forum member

    1.) No start - Can you hear the FP cycle at Key ON Engine Off (KOEO)? If no. Check Fuse F1.41 (15A) in the BEC under the hood. If fuse is good pull the Fuel pump relay and check for voltage at 85 with KOEO. if no power present check F2.19 (5A) in the SJB located in the passenger side kick panel. If the fuse is good but still no power, check functionality of PCM power diode (C1018) located next to F1.52. This verifies the front side of the fuel pump system on the relay control side.

    On the fuel pump side, next verify that the intertia switch has not tripped. Located in the driver side kick panel as memory serves. They have been known to trip. If good, locate the FPDM in the trunk. Pull connector C4033 and check for voltage on pin 5 (white wire). If good next check voltage at the fuel pump C433 across pins 1 and 3. When the key is turned on this will provide a short burst of power to cycle the pump and then stop. It is a prime function. So you may need someone to cycle key on/off (do not start) to see the short prime fucntion. If you get power here and still hear no pump activity when you hook back up you may have a bad fuel pump. If all checks then you can move on to the spark side of the startup sequence.

    Gotta start somewhere on the no start. Not getting fuel would be the first.

    2.) Failing sensors - The sensors items you mention share a common splice referred to S106 located on the back side of the engine in the wiring harness. This splice is a common return for 7 items total that use the sig return point in the PCM at connector C175e pin 58. If there is a high resistance/ loose connection there this could cause you some issues. The splice is wrapped inside the wiring harness. Where the harness splits and goes down the back of the engine, the splice is located in the harness just past the split where the harness wraps back around the backside of the engine going back to bank 2. Otherwise I would agree with 07 boss that items are getting old. I can post a photo of the diagram page if you need it. I am referencing an 05 tech manual.

    Good luck.
    Last edited: Aug 15, 2022
    Kev555, DieHarder and crjackson like this.
  4. tabstang

    tabstang Member

    BTW, does anyone know of a good mechanic in the Nashville TN area who works on aftermarket modified stuff? I've been taking it to Delk but that's over an hour drive from me, and about $350 for a tow out there.
  5. tabstang

    tabstang Member

    Wow, this is great info, REALLY appreciate your help. I'm out of pocket for a week or so, will post back when I get back to this. The S106 sounds very suspicious in my situation, but I'll run the other checks first.
  6. tabstang

    tabstang Member

    07 Boss, It's quite possible I messed up some wires other than the injector and coil - that's why I'm looking at replacing the harness. I'm leaning toward that - not cheap or easy but maybe worth the piece of mind
  7. DieHarder

    DieHarder Senior Member

    Go back to basic troubleshooting techniques. Air, fuel, spark.

    Of the likely suspects fuel, I'd check that the fuel pump is priming (the pump is quiet so take the back seat out and put your hand on top of the rubber cover while you turn the KO/EOff; if the pump is working you'll feel it and should hear it for a few seconds; just be sure it's priming). That'll tell you if the fuel circuit is good from end-to-end or not.

    If the fuel pump isn't priming Red Fire Pearl GT outlines the procedure to troubleshoot it so follow that first....

    I've modified my car similar to yours (FI DOB setup) and had to extend several wires (but without the harness modifications). That said, I had a similar situation; car would crank but not start. I determined my fuel pump wasn't priming and working back found that power wasn't getting to the FPDM because a relay I'd added to provide higher current to it had failed... So, moral of the story is understand the circuit; go after the basics first and half-step from there (i.e. determine a half-way point and check things there. If good, continue. If not, determine why). Good luck.
  8. tabstang

    tabstang Member

    Sorry for the long silence here, busy elsewhere. But now I'm back on this, with no success but more info. (I'll start a new post later but wanted to follow up here first.)

    The main thing I've learned is, this is an electrical problem related to heat. The car will start when cool, but stalls after heat-up, about 1/2 hour. This is consistent behavior, I've done it more than 5 times. The fuel pump does not cycle after it stalls, so it's in the fuel pump power circuit for sure. I thought it might be the fuel pump relay, swapped it out and checked temps everywhere, didn't see anything abnormal (about 170f on the fp relay, same as some of the others). I could find no wiring faults - at least so far. (Very helpful video on fuel pump circuit here: )

    Found this here on S197forum (THANKS FOR POSTING CJ!!!): - almost identical to my experience. So I'm going to the Ford stealership later today to get some diodes, I'll post results later.

    Thanks again everyone here for all your help. Man I hate electrical issues!
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