P0689 issues

Discussion in 'Mustang Chit Chat' started by 05stroker, Mar 6, 2019.

  1. AnotherS197GT

    AnotherS197GT Serial Valve Dropper

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    You're welcome?

    Anyways, I'm still waiting to hear back from a couple guys at work to see if they have some insight into this. My gut feeling is that this is all calibration based because your PCM is looking for things it shouldn't have (manual car switch inputs for the cruise control) and is setting a DTC that, as far as I can tell, didn't exist for 05 Mustangs. At this point, I would suggest load testing the power and ground circuits to the PCM to ensure they're all functioning correctly. Do you know how to do that?
     
  2. 05stroker

    05stroker Never enough power guy!

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    No I do not, but I learn fast, school me please.
     
  3. AnotherS197GT

    AnotherS197GT Serial Valve Dropper

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    You'll need a small light bulb, usually a 9004 or something similar will work. You'll supply power along the circuit that you want to test to one side of the bulb and ground to the other side of the bulb. If the bulb lights brightly, then you're probably getting 10V+. Bright bulbs are usually an indicator of good power, but modules behave oddly if they're not getting all the power they need. They may "work" but will do weird stuff. You can (and should!) test this by performing a voltage drop test, where you place one lead of your multimeter on the power side before the load (you want to get as close to the bulb as possible), then put the other lead on the ground side directly after the bulb. You will want to see most of the voltage dropping over the load.

    For example, if you're using a 12V battery to provide power, you will want to see about 11.5V on your meter when reading on either side of the load. If you're seeing 10V, that means you're dropping 2V across the power and ground circuits. You can figure out which one by placing your lead on the voltage source then placing the other right before the load. In a circuit with low resistance, less than 0.3V should be consumed by the circuit. So again, if you're using a 12V battery, you'd expect to get a reading of about 0.3V (or less) on your multimeter since a multimeter measures the difference in voltage between two points. The same test is completed on the ground side, with one lead directly after the load and one lead at the end of the ground.

    So, looking at the 05 wiring diagram, I think you'll want to focus mostly on the PCM Power Relay #2. You would want to disconnect the PCM connector C175B and connect your test bulb to pin 35, then attach a ground to the other side of the bulb. The best thing to do would be to use a PCM ground so that you can test the ground side of the harness as well. Then you would want to close the PCM power relay so that you can test the whole circuit. Then, you would repeat the same test on pins 36, 45 and another PCM ground. The grounds should be on pins 10, 47, 48, 49, and 50.
    Screen Shot 2019-03-13 at 8.17.29 PM.png

    I would also load test the circuits between the PCM key-off relay and C175B. The key-off relay should be supplied constant power and ground to the coil side, then the switched side receives hot in run or start power from the SJB. For whatever reason, this relay supplies voltage to pins 35 and 36 of C175B too.
    Screen Shot 2019-03-13 at 8.30.31 PM.png Screen Shot 2019-03-13 at 8.30.45 PM.png
     
    Last edited: Mar 13, 2019
  4. 05stroker

    05stroker Never enough power guy!

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    Thanks for the detailed response, I will check it this weekend.
     
  5. AnotherS197GT

    AnotherS197GT Serial Valve Dropper

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    No problem!

    FYI, I checked out HP Tuners and all these DTCs can be disabled in HPT. I’m not sure if Manuel can edit a file in HPT and port it over to SCT with the DTCs shut off, but the ability to disable them in some form or another does exist.