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NOT Normal P Lock Malfunction - ONLY Happens Over 80 Degrees. Never When Cooler

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Rock_Star, May 25, 2024.

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  1. Rock_Star

    Rock_Star Junior Member

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    Yes, pin to ground and both connectors disconnected.

    I think I get what you're saying but maybe not. The wiring chart on page 262 shows H17-9 going directly to T4-7 and I didn't see it doing anything else (but I could have missed it.) In that case, your theory makes sense. But the H17-10 seems to connect to a lot of different things, so it seems plausible that there's supposed to be a high resistance because of some device in that circuit. And since it's low, it seems that indicated that the current would be bleeding off to ground, hence the low resistance and the P lock error. That's just my shade tree mechanic logic. Maybe it's not true. Whatever the reason, I assume Toyota knows what it should be.

    I'll redo those tests today and see if I get the same results.
     
    #61 Rock_Star, Jun 18, 2024 at 5:07 PM
  2. Rock_Star

    Rock_Star Junior Member

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    OK thanks maybe I'll be able to find it. I know roughly where it should be and I see the shape of it on page 384.

    I just saw the drawing of a junction connector. It's feasible that one of the "short terminals" is the problem. Maybe I can open up the connector and see them and tell if any are bad.
     
    #62 Rock_Star, Jun 18, 2024 at 5:12 PM
  3. pasadena_commut

    pasadena_commut Senior Member

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    If PPOS is not just point A to point C then all of those other connectors must be released before the "check for short" will be a valid test. Because other paths to ground may exist normally.

    I think that none of the other connectors must be released for the "check for open" path. Because even though there are other connections, none of those result in an alternate current path through the ohmmeter. The path between A and C may be carried on more than one wire in the cables, but as long as there exists at least one complete path between them the test will pass and the cable should work.

    If you still get different resistances to ground from A and C then put the positive lead on ground and the negative on the pin and measure again. Because that is strange and changing the polarity may tell us something.

    Also measure resistance A to C. If the short to ground accompanies a break between A and C then different resistances to ground from those pins is possible. Imagine a circuit which is A-B1-R1-B2-C and B1-R2-GND and B2-R3-GND. (This is a triangle B1-R1-B2-R3-GND-R2-B2 with paths A-B1, B2-C coming off two of the vertices.) If R1 is zero then you should be measuring the resistance to ground of R2 and R3 in parallel from A or C, but if R1 is large or infinite (open) the A-GND and C-GND can have different values corresponding to R2 and R3, respectively for infinite (open) R1.
     
    #63 pasadena_commut, Jun 18, 2024 at 8:13 PM
  4. Rock_Star

    Rock_Star Junior Member

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    I couldn't find the J14 connector. It must be behind the AC vent box and I didn't want to take that all apart.

    I did the
    PCON (H17–9) or PCON (T4–7) – Body ground
    PPOS (H17–10) or PPOS (T4–8) – Body ground
    tests again and had zero ohms on all of them. (Supposed to be 10kOhms or higher.)

    I used a few different ground points just to be sure and it all came out the same as before. BUT I just learned that I've been misreading my meter. It read "OL" which I was interpreting as zero. The meter's manual says that means "the input is too large for the selected range." The meter reads MegaOhms so hard to believe that the resistance is beyond that. Maybe that's a sign of something.

    Then I plugged in one connector and did the test on the other one's pins, and vice versa. That changed things.Those actually did show 0.00 ohms. But since the connector is plugged in, that's against the diagnostic instructions. Maybe it means something. Also when doing that there were pulses of resistance apparently. I think it would go blank and then back to 0.00.

    So if those OL readings mean mega mega resistance, that's over the 10k it calls for so maybe it's not the cables after all.
     
    #64 Rock_Star, Jun 18, 2024 at 10:59 PM
  5. Rock_Star

    Rock_Star Junior Member

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    After the above testing...Today was 88 degrees and 90% humidity. I ran the car in the driveway with the AC cranked, planning to drive and see if I got the P lock error. After sitting there 5 minutes in the cold air and not even putting the car in gear, the P lock error came.

    Maybe it's a clue that the car doesn't even have to be in gear or driving to get the error?

    That still keeps me thinking that the problem is on the engine side of the firewall. It didn't happen instantly, so perhaps five minutes of engine heat is all that was needed to put it into error mode. But on the flip side, when it was 78 degrees I ran the 140 degree hair dryer on all of the wires and components I could get to and didn't get the error, so maybe not. Or maybe I just didn't get the hot air in the right place for long enough.

    I inspected all the wires along the firewall again and didn't see any damaged ones. There's plenty of places where the squirrels have eaten the insulation off the hood and firewall, but I didn't see or feel any damaged wires. Still could be though.

    The P3102 diagnostic says these are the trouble areas:
    Wire harness or connector
    Transmission control ECU
    HV control ECU
    Power source control ECU

    Power source control ECU seems to be above the steering wheel on the firewall. Potentially getting hot from the missing firewall insulation?
    HV control ECU is to the right of the glovebox close to the body. Potential radiant heat from outside getting to it?
    I did run the hair dryer on/near them for a couple of minutes when it was cooler and didn't get the P lock error so maybe not.

    I'm running out of ideas for what it could be. Maybe I just need to start replacing ECUs.
     
    #65 Rock_Star, Jun 18, 2024 at 11:02 PM
  6. ChapmanF

    ChapmanF Senior Member

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    Throwing ECUs at it isn't the way I'd proceed, but it can be hard to give ideas about diagnostic technique over the internet.

    You've already posted freeze frames showing collapse of the supply voltage reaching the transmission control ECU at certain moments. If it were me, I'd be pursuing that lead aggressively.

    In an older vehicle, I once wired an LED into a suspected intermittent circuit and taped it to the dash, so I could drive around and just see when the LED went dark.

    The same thing might be useful here. A question could be: is the voltage reaching the transmission control ECU really dropping, or is there some fault inside that ECU and it's imagining that the voltage drops? A light independently wired in there would give you an independent answer.

    There are electrical testers that make a buzzy noise instead of a light, so your eyes don't have to be on them. They can be hooked up to some wiring while you concentrate on shaking it and stuff.

    The first page of a DTC section in the manual will give a list of several different things as possible trouble areas. That doesn't mean to just assume any of those things might be causing the problem. The list will include crazy unlikely, but just conceivably possible, causes: the kind you don't start fussing with unless there's evidence you've picked up while troubleshooting that clearly points there.
     
    #66 ChapmanF, Jun 19, 2024 at 11:01 AM
  7. pasadena_commut

    pasadena_commut Senior Member

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    OL on meters means "out of range". That is, the value is too large to measure. In this case that means the resistance to ground is huge, as it should be if the insulation is intact. Be aware though that you would also see OL if the wire near the pin being tested was broken with a gap introduced, or even if the ohmmeter lead wasn't plugged in.

    Historical trivia. There is some disagreement if OL stands for "Open Loop", "Over Load", or "Over Limit". OL arose when displays used 9 segment LED numeric digits and the only symbols that could be made were composed of combinations of those segments. The maximum value in the significant digits was 1999 (for a four digit display). If the meter needed to show 2100, for instance, it could not, so all such values were shown as OL. Meters with more sophisticated displays today will often show a phrase like "over range" instead.
     
    #67 pasadena_commut, Jun 19, 2024 at 1:03 PM
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  8. pasadena_commut

    pasadena_commut Senior Member

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    Or use a cheap DMM (Harbor Freight sells one for $7, sometimes less) set in voltmeter mode and tapped into the questionable 12V wire. That way you can also see if the voltage starts to decrease slowly as it approaches the failure point, or if it just collapses all at once. If you can get access to at least a cm of bare wire perhaps use a posi-tap - it only makes a pin hole in the insulation and can be removed (and reused).

    Posi-Tap- No Crimp Tap

    If there are multiple wires you want to monitor, you could use a multiposition rotary switch, run one wire to each position, and the meter to the common. Something like this:

    Rotary Switch - 10 Position - COM-13253 - SparkFun Electronics

    Just be sure that whatever switch you use does not short neighboring pins as it rotates. It shouldn't, but check before using it.
     
    #68 pasadena_commut, Jun 19, 2024 at 1:22 PM
  9. ChapmanF

    ChapmanF Senior Member

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    Rotary switches are made both ways, shorting on purpose, or not shorting. So, the kind that shouldn't, shouldn't.
     
    #69 ChapmanF, Jun 19, 2024 at 2:47 PM
  10. Rock_Star

    Rock_Star Junior Member

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    Good idea. Not sure how to do that though. I assume those buzzing circuit testers would need to be connected to two points, but which two?

    Would I be looking for a drop in voltage or ohms?

    Assuming I find a drop in one of those, I assume I'd still have to root around and try to find a bad wire, but at least I might know which circuit had the problem.
     
    #70 Rock_Star, Jun 19, 2024 at 3:09 PM
  11. Rock_Star

    Rock_Star Junior Member

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    Those positaps would be great to have. I can't find any locally. Auto zone has these which should do the trick. But they don't look removable like the positaps are.
    https://www.autozone.com/miscellaneous-non-automotive/electrical-wire-connector/p/dorman-conduct-tite-blue-16-14-gauge-t-tap-terminal/297424_0_0?searchText=positap

    But, I'm not sure which wires to tap into, what I'd be looking for, or how to test. I assume I'd need to attach the multimeter to two wires, or maybe one wire and ground, but I have no idea which.
     
    #71 Rock_Star, Jun 19, 2024 at 3:15 PM
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