2006->2008 4.6 swap, no crank with P0685

Discussion in '2005+ Mustang GT 4.6L Tech' started by lphill01, Jun 9, 2019.

  1. lphill01

    lphill01 Junior Member

    I had an engine bay fire 4 years ago, installed a “ new”06 body harness and just installed a supercharged 2008 4.6 from a Roush 427R. I’ve got a no crank with a P0685 code. Problem is I can’t find the PCM relay wire on the C172B harness (no wire on pin 37, unless I’m counting wrong), and no leads under the relay on C1035 in the BEC. Am I looking in the wrong spot, or was there a change on the 08 I’m not aware of?


  2. 08MustangDude

    08MustangDude forum member

    PCM Power Relay is top left relay, it's only in one spot. There is a PCM Power Diode too.

    Potential causes for the code to set are:
    A defective PCM power relay
    A blown fuse or fusible link
    Corroded or damaged wiring or wiring connectors (especially near the PCM relay)
    Faulty ignition switch
    Partially or totally disconnected electrical terminal at the ignition switch
    Loose or corroded battery cable ends
  3. lphill01

    lphill01 Junior Member

    All the fuses are good, and the relay clicks when the key is turned to on, but the starter relay doesn’t click when I bump the starter. I compared my original 06 c1035b connector to the 08 and the 08 is missing D5 (vt-yl) PCM relay power#2 (it ohms out at 0 with C5 which has 2 red leads and is also PCM-R #2. I also compared my original 06 C172B with the current one, and both have pin 37 blocked and unused. I’m at a loss since I don’t know where the PCM-R pin actually is on C172B since it’s differing from all the diagrams I’ve come across.
  4. Derf08

    Derf08 Junior Member

    If you go to Ford Parts Giant and look up Engine Harness for both a 2006 and 2008, the part numbers are slightly different so, yes there is a difference in the Engine harness. What exactly, don't know but, the fact the PN is different tells you something. Also, if the wire harness is slightly different the PCM may also be slightly different so, you have two things to chase down.

    When you swap engines on modern vehicles it's best to swap the engine, engine harness and PCM all from the same donor car. This helps avoid some of the problems in year-to-year differences.

    Edit, just checked the PCM, those are different as well. So, the problem, may be no problem at all if get the right PCM in there.
  5. lphill01

    lphill01 Junior Member

    The engine, engine harness, transmission and PCM all came together from the same donor. It looks like the disconnect is between the 08 PCM and the 06 body harness. I found the early S197 pinouts and compared them to iihs.net. Sometime in 07 it would appear the PCM pinouts on 172B changed considerably, (hence the missing pin). I'm going to try a 2006 PCM and see if that works.
  6. 06monera96

    06monera96 LeShawno

    05, 06 , 07-08, 09, 10 Were all different brother
    05 to 06 had slight changes
    07-08 had less sensor to the engine (oil pan specifically) including the routing of the coolant passageway.
    Fuel system were different between all ears, 05 to 06 then 07 to 09, 10

    They were a lot o changes in the harness across the entire generation of 3v..
    You got a lot of back wiring to do