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Ford PWM J1850, hardware limits

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  • Ford PWM J1850, hardware limits

    I've been fighting our dead 2001 Windstar. It died on the road with no communication to the DLC connector.
    I sent the PCM in for repair, I got it back and still can't connect to my Actron scanner and the Ford is still dead. The scanner worked fine with the Ford before the problem and it works fine with the old Saab 9000 setting right next to the Ford. I'm beginning to wonder if the problem is on the SCP buss on the vehicle. I tested all populated circuits between the PCM connector and the vehicle with an ohmmeter, the only strange things I noticed are:
    SCP Data+ = 120 ohms to ground
    SCP Data- = 0.6 Megohms to ground
    ..and Data+ to Data- = 1250 K ohm

    I would have expected less resistance between Data+ and Data-.
    Is the 120 ohms to ground too much load?

  • #2
    I'm sorry I don't know the answer to your question but noticed it's your first post so I wanted to welcome you to! There are a few guys around here that are eyeball deep in OBD systems, unfortunately I'm not one of them. I have Autoenginuity to read my Ford, it does a great job and that's about all I can tell you. Not much help, like I said, but again welcome, hopefully one of the guys in the know will respond.
    My 2007 Ford F350 Work Log located HERE


    • #3
      The 2001 Windstar Shop manual says:
      " The SCP communications network will remain operational even with the severing of one of the bus wires. Communications will also continue if one of the bus wires is shorted to ground or voltage, or if some, but not all termination resistors are lost."

      Since I don't have the Ford $5500.00 J2534 scanner, I'm kind of in the dark. Additional info:
      Two days before the Windstar died, I checked the ABS with my Actron CP9580A and a C1185 was the only fault noted. I'm 99% sure the scanner is fine. I just don't know if I should be chasing the 120 ohms to ground, and/or the lack of termination on the SCP pair, or just wait for a second opinion on the PCM?

      ..also, I found some confusion in the Ford diagrams: The printed version of "2001 Wiring Diagrams/Windstar" shows circuit 914 (Data+) grounded in the ABS module. Which would conflict with the above 'limits' statement. However, the 1996-2002 DVD shows no such ground connection, and its marked "Revised". What I suspect is both circuit 914 (Data+) and 915 (Data-) should have some kind of termination. Is it possible Data+ should be tied to a 120 ohm pull-down resistor and Data- should be tied to a 120 ohm pull-up resistor, someplace?????

      There is tons of information on the net about the hardware configuration of CAN networks, but I can find nothing about Ford's J1850 PWM.
      I can't believe it! No one has disconnected the negative battery terminal and probed the DLC with an ohmmeter??
      Last edited by oldgearhead; 07-02-2014, 12:23 PM. Reason: quote accuracy


      • #4
        I got the old Windstar back on the road by installing a re-built PCM and a couple of new transponder keys because I have PATS.
        The mystery about how the data bus should be terminated continues. I exchanged a few emails with FoMoCo Customer Service Department. At first they swore the old "Wiring Diagrams" book was correct, but finally admitted they were not capable of answering my questions, and of course they weren't about to put me in contact with a tech or engineer. They suggested I contact my local Ford dealer. I did, and I'm still waiting for an answer from the dealer's technical expert. I am rather concerned because I don't want another "Chevy Event" (died at 50 MPH) with my Ford, just too dangerous...Do you think I'll have to borrow an o'scope to find out if both sides of my network are operational?
        Click image for larger version

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