measuring resistance main relay and the ground

Discussion in 'Gen 2 Prius Technical Discussion' started by Cody26, Sep 11, 2018.

  1. Cody26

    Cody26 Junior Member

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    Hello prius community,

    I am trying to find out the exact problem with my P0aa6 526 with 612 subcode. There are several places I have to measure the resistance but I don't know how to really measure them practically on HV battery part of it. Of course, I do know how to use multimeter. In service manual, it says

    "Using a megohmmeter, measure the insulation
    resistance between the positive terminal on the HV
    battery side of the system main relay and the body
    ground."

    Here the confusion is what is meant by the body ground. Is it the chassis ground?

    In other places, "the shielded ground" is used? I also don't know what is the difference between that and the body ground.
     
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    Body Ground is Chassis ground which is 12v negative. The intent is to check the resistance between the HV system and the chassis of the car. The HV system is supposed to be isolated from the car body, so it should be extremely high. A low value measuring from the HV side of the + relay to chassis ground indicates some portion of the HV battery is no longer isolated from the case/chassis. Could be HV battery leakage. A high value here means the ground is very likely on the car side of the relays. like the cables, AC compressor, inverter, connections/joints that may be dirty or wet, transaxle etc.

    The shielded ground is probably referring to measuring the resistance between the chassis and the shield wires that are part of the main power cable. This shield is easy to see on the main cables that attach to the HV battery. That shield is grounded to the case of the HV battery. The case of the HV battery is grounded to the chassis through the mounting bolts. This check is probably verifying continuity between the shield and chassis ground, or verifying high resistance between the center conductor and the shield. The shield is there for a couple reasons. Noise suppression from high amperage, physical protection to keep the center conductor from being damaged, etc.
     
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  3. ChapmanF

    ChapmanF Senior Member

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    Yes, you can find a good electrically-conductive (not paint covered) nearby metal part of the car.

    Note that what you're doing here is testing insulation, which should have very high resistance, so high that you need a "megohmmeter" to measure it, and that's a pretty specialized instrument, not to be taken lightly. (The newer ones can look a little bit like an ordinary multimeter, but your ordinary multimeter doesn't put out 500 or 1000 volts at the test terminals).

    There are inexpensive Chinese ones now (mroberds found one, discussed here and here) ... but just being able to get one for cheap should not tempt you to use it carelessly.

    -Chap
     
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  4. Cody26

    Cody26 Junior Member

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    So, as I understood I can't use a multimeter for this. I bought this one from amazon



    So, I need a megohmmeter? Any links from online retailers?
     
  5. ChapmanF

    ChapmanF Senior Member

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    Two avenues for getting one affordably are, 1, go with one of the recent no-name inexpensive Chinese ones (ok, not so much no name as you look around and see what's obviously the same gadget with slight repackaging under a dozen different names), like the one mroberds describes in that thread, or, 2, watch fleabay for a good deal on a Megger or Fluke or AEMC, etc. (I've bought a couple eBay Meggers that both needed some minor repair, which is always possible when they're listed for an attractive price.)

    Either way, if you order one, the time between order and arrival would be well employed in studying how to use it safely.

    -Chap
     
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  6. TMR-JWAP

    TMR-JWAP Senior Member

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    Cody,

    What is your level of familiarity with test instruments like a multi-meter or megger? I'm assuming that since you ordered one from amazon, you don't have significant experience. Please be careful. It doesn't take much for an accidental slip or twitch to cause a major catastrophe in a high voltage system. I had a well experienced electrician friend several years ago who accidently slipped and jammed his hand into a 480v circuit. He doesn't have a thumb on that hand anymore and two fingers are seriously jacked. He was working on live equipment and since he needed good finger control on his right hand, he was wearing a rubber glove on his left. Rubber mats and sheets were covering everything except the small portion he was working on. Unfortunately, the current went through his fingers and thumb.

    The multi-meter will work for detecting GROSS issues with insulation like significant damage where the center conductor is shorted to the shield or car body, etc. It MAY give indication of minor damage/failure of the insulation by giving a reading in the high Kohm/low Mohm ranges, but it will not provide indication of insulation breakdown that only occurs when the insulation/etc is exposed to high voltage. The more experience you have with a multi-meter, the easier it is to recognize small things it tries to tell you. If you're lucky, your problem may be closer to a direct short in the system, which a regular multi-meter should be able to help you find. If you find no gross problems, then the megger may be needed.

    I agree with Chap...use the available time to review safe practices
     
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  7. Cody26

    Cody26 Junior Member

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    I was also thinking that there should a way to find out where the leakage is without doing any physical inspection. I will follow the instructions on service manual and will use the megohmmeter to find out which went bad.
     
  8. Krisstroll

    Krisstroll New Member

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    hello everybody , I m new on the forum, back in Europe (France) , and not much of people who knows something about working on a prius here, i am trying to solve a pbm :
    i had a HT battery failure, bill from toyota that i refused (2500 €), i change my self the 2 faulty elements (thanks you tube) and put back everything : 212 V on testing
    when i brought my prius 2 back home the thermic was working D & R but of course electric was not working.
    since then , and I checked and rechecked everything, the orange big security plug position (as mentioned by several personns) all the 3 connectors to the ECU, the connector to the Fan , even the polarity of the HT connectors ....
    same old song : THE READY MODE DOESN T COME ON, NO D OR R POSITION POSSIBILITY, THE THERMIC DOESN T FIRE UP , I noticed that i don't hear the Clac Clac of relays ans that the 12 V battery is drained after 1 day if stays connected. NO error CODE on ODB device
    i desinstalled one of the 2 main relay to try to test it, but i am stucked
    any ideas ? any suggestions, HEEEEEELP PLEAAASSSE
    thanks

    I figured out that it was the good spot to post (measuring resistance..............)
     
  9. edthefox5

    edthefox5 Senior Member

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    Your hybrid battery has failed.

    One of the many battery modules has died and not providing power or you have a battery ground fault failure. The computer is detecting high voltage leaking to ground usually caused by a cracked module leaking electrolyte causing a current path. A ground fault error can be easily confirmed by disconnecting the 12 volt battery ground side which kills the car and resets the computer and erases the logged code. Once that code is removed the car does not know it has a GF issue and will be able to drive....for a while.... the code will eventually come back.

    Either way pretty straight forward the battery needs to come out and disassembled for inspection and test. Take the battery out and you will see pretty quick why its unhappy.
     
  10. ChapmanF

    ChapmanF Senior Member

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    This nearly always means the device you're using is incapable of reading the code.

    Ed's diagnosis is plausible, but if you had access to Techstream and could read the actual trouble codes you have, it would give you more detail.
     
  11. TMR-JWAP

    TMR-JWAP Senior Member

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    i had a HT battery failure, bill from toyota that i refused (2500 €), i change my self the 2 faulty elements (thanks you tube) and put back everything : 212 V on testing

    I'm assuming this means you pulled the HV battery out and replaced 2 failed modules.
    Is this correct?

    If so...
    Here's a couple ideas for you to look into.............just my 2 cents,

    Obviously, we're all just kind of guessing based on our own experience, until you have some actual codes, but....

    It sounds to me like the car still thinks the safety disconnect is not installed.

    I know you mentioned it has been verified, but I'll mention again the safety plug has 3 steps: insert, rotate handle, push straight downward until handle clicks into the interlock receptacle located at the bottom..

    Of the 3 cables you connect from the car harness to the HV battery, the smallest one is for the safety disconnect. It plugs into a receptacle that is mounted just above the safety plug. That receptacle has a wire harness about 6 inches long with another plug on the end. That plug inserts into a receptacle at the bottom of the safety disconnect. If you forgot to insert this plug while assembling the battery, it's just like the disconnect not being installed.

    There is a wire harness plug mounted right by the HV battery cooling fan on the fan mounting bracket. It probably has like 10 wires. Sometimes this plug/receptacle gets moisture/corrosion in it. Once again, this can make the car think the safety disconnect is not installed.
     
  12. Krisstroll

    Krisstroll New Member

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    ah...........thanks,
    ""I'm assuming this means you pulled the HV battery out and replaced 2 failed modules.
    Is this correct?"" YES

    i forgot to mention that the screen display shows the HV voltage battery (purple) 8/10 charged, seems that it reeds the HV, so is it disconnected ?
    I will look for all theese, thanks
     
  13. Krisstroll

    Krisstroll New Member

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    thinking it twice, .....
    IF I can measure 212 V on the Hot spot (battery side) when i connect the safaty plug, that means i correctly insert that safety pluf, but still i don't read voltage on the over side, the motor side, after the relays, it seems to me strange they would both go away, and how can i measure them ? do i dare to trigger them with a 12 V battery ?
     
  14. TMR-JWAP

    TMR-JWAP Senior Member

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    IF I can measure 212 V on the Hot spot (battery side) when i connect the safaty plug, that means i correctly insert that safety plug.

    Absolutely UNTRUE.
    If you hold your meter leads on the battery side of the relays, you will notice that full voltage is at the relays even when the safety plug is inserted only a few millimeters into it's receptacle. That's well before it's even in far enough to rotate the handle upward.

    The three steps are:
    Insert the safety disconnect as far as it will go into the receptacle.
    Rotate the handle upward, this pulls the safety plug even further into the receptacle until full contact is achieved.
    Push straight downward. The handle will move about 1/2 inch (13 mm) and click into place, satisfying the interlock located at the bottom of the receptacle.

    Even if there voltage available at the battery side of the relays, the car WILL NOT allow the relays to close unless the interlock is satisfied.
     
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  15. Krisstroll

    Krisstroll New Member

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    ok thanks....
    I did this, still : i have my 212 V on HV battery, my fully charged 12 V battery and when everything is plugged, no clac clac, no ready mode
    so i am going nuts, and though about taking out the relays, but how to test them, or bying new ? in any case
    i am going to work at 50 miles away with a rug and times is running, i am thinking of remonting everything renting a trailor to bring it to toyota for diagnostic, but it pains me already........i am sure i am close to solution : hight voltage is here, blocks are balanced but what ?
    inverter ? relay ? it is not, NOT the safety plug (i did it and redid it so many times)
    any ideas ? suggestions from anybody ?? as we say in our corporation : many eyes search farer and far beter

    thanks anyway for your time
     
  16. TMR-JWAP

    TMR-JWAP Senior Member

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    So let's start with the basics. Is there a red triangle or check engine light? If so, what are the codes being provided?
     
  17. Lib

    Lib New Member

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