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Trouble diagnosing 1st gen p0705 and p0850

Discussion in '1st Gen. Tacomas (1995-2004)' started by ToyotaSam, Apr 27, 2020.

  1. Apr 27, 2020 at 12:40 PM
    #1
    ToyotaSam

    ToyotaSam [OP] Member

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    This is my first post in Tacomaworld!

    I'm hoping I can find some help. I recently bought a used 2004 automatic extended cab 4x4 tacoma. The check engine light was on and the codes were p0705 and p0850. The truck ran well in all gears and the 4x4 worked. I have been trying to diagnose this code and have learned a lot about it. I have also checked and replaced some things. here is what I have found.

    1. Replaced P/N switch. Didn't solve the problem (used)
    2. Replaced ECM. Didn't solve the problem (used)
    3. Tested wires from switch to ECM for continuity, all good.
    4. replaced junction block 3 under driver dash to the right of steering column. Didn't solve the problem (used)
    5. Serviced the transmission with new fluid and filter. Didn't solve the problem.

    I see that a lot of people have water/corrosion inside the left kick panel junction on the 2nd gens. I don't know if there is anything there to look for on the 1st gens.

    Each time I've done something to attempt to fix it, I have cleared the codes. It is a two trip light, and every single time the code has come on immediately when I shift out of park on the second start.

    Again, there are no noticeable symptoms. The truck drives, shifts, lights, blinkers, all work.

    Any advice or help would be appreciated.
     
    OneWheelPeel likes this.
  2. Apr 27, 2020 at 6:12 PM
    #2
    Glamisman

    Glamisman Well-Known Member

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    sorry if this is redundant information.
     

    Attached Files:

  3. Apr 27, 2020 at 6:45 PM
    #3
    ToyotaSam

    ToyotaSam [OP] Member

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    That is helpful. I've seen the wiring diagram, but not the checklist on troubleshooting. Thank you.
     
    cruiserguy likes this.
  4. Apr 27, 2020 at 8:37 PM
    #4
    Glamisman

    Glamisman Well-Known Member

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    in reading the fault that causes these codes... it seems too simple to be giving that many issues. There is a difference between the 3.4 V6 and the 2.7 4 wiring... which motor do you have?
     
  5. Apr 27, 2020 at 8:41 PM
    #5
    ToyotaSam

    ToyotaSam [OP] Member

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    It does seem simple, the fact is it isn't causing a single issue I know of, other than the code tripping the light. I can't let it go though. I have to figure this out. I have the 3.4 v6.
     
  6. Apr 27, 2020 at 9:32 PM
    #6
    frizzman

    frizzman Well-Known Member

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    OME 2.5,Tundra 17s,Falken Wildpeak AT3W hitch w/ 7-pin, ARE cap, JVC HU w/BT, HID/LED lights
    for mine, I had a relay connected to the reverse light that threw the code and would not allow me to "shift" up when using throttle but would "drift" fine since it was already in 3rd gear

    check for an errant wiring harness or trailer wire harness that might be causing back-feed into the circuit

    https://www.tacomaworld.com/threads/p0705-error.593307/
     
  7. Apr 27, 2020 at 10:28 PM
    #7
    ToyotaSam

    ToyotaSam [OP] Member

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    I have tried unplugging the wiring harness for the trailer which didn't change anything. Do you think it could be further up the wiring? I'll put that on the next place to look list. Thanks
     
  8. Apr 27, 2020 at 10:33 PM
    #8
    frizzman

    frizzman Well-Known Member

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    always could be, wiring is never fun to track down a bad wire :\
     
  9. Apr 28, 2020 at 11:19 AM
    #9
    ToyotaSam

    ToyotaSam [OP] Member

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    So I investigated the trailer wiring connector. I used a multimeter and couldn't detect any current. I went behind the driver's seat and looked at the wiring for the trailer.

    IMG_20200428_125710992.jpg
    There was a black box the large wiring connector plugged into and four separate 5 amp fuses labeled: left turn, right turn, park, reverse...something like that. One of the fuses was burnt out.
    IMG_20200428_125750688.jpg
    I didn't test the box.
    IMG_20200428_125728085.jpg

    I then used the multimeter to check this connector for power to the trailer wiring.IMG_20200428_125805276_HDR.jpg

    It had power to a couple spots, and "blinking" power to two spots (I had the caution lights blinking and lights turned on). This makes me think that the power to the trailer wiring harness is good. I don't have any 5 amp fuses on hand so I'll grab some of those.

    The real discouraging part is that with the wiring completely unplugged I reset the codes. Started and put the truck in each gear. Shut it off and on the 2nd start as soon as I put it in gear the code p0705 came on. This makes me think that the problem with the trailer wiring harness can't be the cause.
     
    cruiserguy likes this.
  10. May 7, 2020 at 2:58 PM
    #10
    ToyotaSam

    ToyotaSam [OP] Member

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    It is fixed! There were two issues. One I probably caused while trying to fix the other. As part of trying to remove the codes, I replace the p/n safety switch. When I did so, I did not properly align the switch. There's a mark on the transmission and the switch that must be aligned. It was reading correctly in park, but not neutral. This was verified by trying to start it in each position. Before adjustment it would only start in Park, after adjustment it started in neutral as well.

    After clearing the code, the p850 was gone, but still had p705. We removed the driver's side dash and disconnected the cruise control. It looked aftermarket, and low and behold the code did not come back. We knew through testing connections in the wiring that there was some voltage feeding into the system. With the cruise removed it worked like a charm. IMG_20200503_202733156.jpg
    This cruise control never worked since I bought it.

    IMG_20200504_112606107.jpg
    These are two connectors that plugged into the cruise control wiring that I removed.IMG_20200503_205214888.jpg
    These are wires that were left after I removed the cruise control switch from the column. The red connectors plugged into splices to the ignition and something else...
     
    cruiserguy likes this.
  11. May 7, 2020 at 4:50 PM
    #11
    cruiserguy

    cruiserguy Well-Known Member

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    You are correct, that is aftermarket cruise control. It was good of you to look at anything aftermarket installed with wires/circuits, or even dealer installed when truck originally sold, highest probability of the issue would be with those things. So good job on the sleuthing hermano :cool:
     
  12. May 7, 2020 at 9:30 PM
    #12
    spinyard

    spinyard Well-Known Member

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    Yep when you take your truck in to add a feature like cruise control, etc. They never add it cleanly and usually mess up the reliability Toyota created.
     

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