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12.4 volts at battery, 11.7 volts on MFD signal test

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Thane, Mar 25, 2018.

  1. Thane

    Thane Junior Member

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    2004 Prius
    Model:
    Two
    Main question below in red:

    I just had my car damaged in a hit and run while parked, totally innocent and minding its own business btw, barely any drug use. Anyway, i had to let the car sit for about 3 months after it was hit and when i got to fixing it, i noticed the battery was completely dead. I did charge it up but i suspect i sulfated it at least a little bit, and probably didn't help the hybrid cells either. It does test okay at three different auto parts stores, but twice now, i got all kinds of codes about the 12v battery, coolant sensor, banks 10 and 11 of the hybrid battery low, replace the hybrid bat, replace the car, get therapy and more.

    But heres the thing. As many of us know, when the 12v battery gets low, the Pruis can do all sorts of crazy things (like not let the fully charged hybrid bat start your car.............to charge the 12v bat..........so everything would be fine and we'd live happily ever after.............ugh). So i cleared the codes once and the errors did come back when just starting to drive the car two days later. But i had not driven on the freeway, only around town for brief outings just to give the electrical system some work.

    So today i drove on the freeway for about 45 minutes which should charge the system pretty good (unless automated controls limit it), and then cleared the codes and it hasn't come back yet. Im hoping the system gets some good charging to condition the battery cells a bit. The battery is a 2 year old dealership bought battery.

    I noticed something weird besides the above and since my Prius has always been so seemingly vulnerable to problems with only a slight drain on the 12v battery, i wonder if the following inconsistency is a problem. My current test is after the car sat overnight without the key fob proximity detector on. The voltages are:

    Directly at the battery terminals
    12.3................and under headlight load: 11.7
    MFD "signal test" screen
    11.7................and under headlight load: 11.2

    I also get mostly the same readings at the jumping terminal up front under the hood as the battery terminals. So, why would the the MFD signal test way under the voltage reading at the terminals? has anyone ever heard of the MFD signal being wrong?

    Here is the list of errors i will get when the red triangle of death has occurred:

    C1241: Low + battery voltage or abnormally high battery
    C1378: Capacitor communication circuit Malfunction
    P0A80: Replace hybrid bat pack
    P0A7F: Hybrid bat pack deterioration
    P1116: Coolant Temp sensor circuit stack for coolant heat storage system
    P1121: Coolant flow control valve position sensor circuit stack

    The second time it happened:

    All the same ones plus:
    C1259: HV system regenerative malfunction
    C1310: Malfunction in HV system
    P3020: Battery block 10 becomes weak
    P3021: Battery block 11 becomes weak

    Now today i've driven it on the freeway for a long time giving the hybrid battery a good charging up i hope, but i have my doubts. So for now, just wondering about the MFD signal test and what it means if it is reading right.
     
    #1 Thane, Mar 25, 2018
  2. ChapmanF

    ChapmanF Senior Member

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    Well, it isn't wrong, it's just telling you the voltage it's seeing at its own plug. It's a dozen feet of skinny wire and numerous fuse links, fuses and junctions away from the battery terminals where you measure higher, and when the car is on, there's current flowing over those paths, and current flowing over resistance drops voltage.

    You're seeing about a 0.6 volt delta on a 12 volt (nominal) system; that's no more than a 5 % drop, which isn't far from what engineers would shoot for in designing such a wiring system.

    It's been covered before in other posts. I usually just describe the difference as "a significant fraction of a volt" to allow for possible variations between cars, but your 0.6 is pretty close to what I'm used to and what others have posted.

    Once you learn what the difference is between those points on your particular car, it will stay pretty consistent, letting you estimate from one reading what the other one probably is. (You'll generally notice the drop is larger in IG-ON and smaller in ACC.)

    You will see, now and then on PriusChat, a thread where somebody reports a voltage without saying where measured, and another poster (who usually believes that the automotive-grade computers in the car will fall over because of mere tenths-of-volt differences) will start running with that figure without asking where it was measured ... you can see the problem. :)

    -Chap
     
    #2 ChapmanF, Mar 25, 2018
    SFO, scona and jerrymildred like this.
  3. Thane

    Thane Junior Member

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    Vehicle:
    2004 Prius
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    Two
    The thing is i've seen tests of the MFD signal test on youtube with readings on 12.4 volts and many reports of 12.4 in a big thread here on Prius chat. Thats a big difference if all Prius's exhibit this differential between the battery and the MFD voltage sensor reporting unit. So if we assume that some models do this and some don't, that leaves some possibilities IMO:

    1. My 2004 model is wired differently than some others and 11.7 is the correct voltage detected, after some voltage drops.
    2. My 2004 model is wired differently than some others and 11.7 is incorrect, but the system is operating normally.
    3. 11.7 is correct and the system has a problem
    4. 11.7 is incorrect and something is wrong with the MFD sensor and the system has a problem

    ...but how would i test this...
     
    #3 Thane, Mar 25, 2018
  4. dolj

    dolj Senior Member

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    Nothing in the data you have provided adds credence to your theory, The 12 V battery would have to be pulled down well south of 10.4 V before there would be concerns.

    Further, there is no correlation between the 12 V and HV batteries such that one affects the other in the way you imply.

    You have two separate problems. 1) the 12 V is low and needs charging. That's it nothing more. 2) you have HV battery problems clearly indicated by the codes below:
     
    #4 dolj, Mar 25, 2018
  5. jerrymildred

    jerrymildred Senior Member

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    I agree with @ChapmanF and @dolj. Your 12V is not low enough to cause those codes. My '05 started fine with only 8.5 volts in the battery. No codes. My only symptom (afaik) was that it took several seconds for the brakes to build pressure.

    I think if you charge your 12V you'll still have those codes and then can move on to the more serious problems your car has.
     
    #5 jerrymildred, Mar 25, 2018
  6. exstudent

    exstudent Senior Member

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    Actually 3 issues. Issues 1 & 2 listed in post #4 above.

    3rd issue is the 3-Way Coolant Control Valve. That valve will fail over time, being stuck in the wrong position. Thus the cause of P1121. It may become unstuck, and the CEL will go away. But, it will eventually fail, and be stuck permanently in whatever position it was last in, which will trigger the CEL, b/c the ECUs will want the valve to be in a certain position, to direct coolant, but the valve won't move.

    P1116 may be related to the P1121, because this valve directs coolant to the engine, CHRS, and heater core. Replace the valve is first. If P1116 comes back, than you will have to address that error. But majority of times, P1116 goes away when the valve is replaced.

    Do nothing and you will soon become stranded due to the HV battery. It is on the decline, has been failing, and the car is telling you to STOP driving. No 12V conspiracy theory here.

    You just need to repair what is wrong.
     
    #6 exstudent, Mar 25, 2018
  7. ChapmanF

    ChapmanF Senior Member

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    I may not have seen the youtube vid in question, but I can say that if you were to show me any Prius that was in ACC or IG-ON and reading 12.4 volts at the MFD and at the battery terminals at the same time, I would buy Bob Cratchit that giant turkey. (Or I would look for the extra 10 ga. wire you had snuck from the hatch to the dash.)

    -Chap
     
    #7 ChapmanF, Mar 25, 2018
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