Interesting dc/dc issue

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by Racso, Dec 19, 2025 at 12:30 AM.

  1. Racso

    Racso New Member

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    Hi ya'll,

    First, Im new here but not with G1, G2 Prii, I have 6 in the stable. I collect, repair, rebuild G1 and G2s. I pulling a G2 off the back burner with an interesting issue on the 12v side of things. First some history.... This car was in daily service, 150k ish miles, no issues, no faults, good batteries etc.... One day I noticed a faint high pitched singing from under the hood and at the same time my normal 14.x volts dropped to 11.x. No codes or faults. Open inverter tank and coolant is flowing nicely but finger test indicates way too hot. Hot coffee too hot. This could have been going on for months as Im not the primary driver of this car. Again keep in mind, no codes, faults or lights, good 12v and hv batteries. So having no time at the time and plenty of spares it was parked. 5 years later (last week), I connected a new 12v battery and it started right up. About 2 minutes later, the whine started and the 12v battery sagged to 11 volts. So the problem did not fix itself over time. One authority i trust mentioned a connection on the transaxle, but outside of the parking paul, i dont that small wire could handle the current. Something is loading the dc/dc to its max output but not burning or causing codes or faults.

    So what do you think?
     
    #1 Racso, Dec 19, 2025 at 12:30 AM
  2. Tombukt2

    Tombukt2 Senior Member

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    With inverter fluid hot you should notice HV charging is diminished or throttled back. So at immediately starting inverts throwing 14 plus to 12Volt??? And you watch it drop? Or it just measures 11 something ? See if you can see it at the 14 at power button push dvom on windscreen so you can see it directly . Sounds like DC to DC converter issue
     
    #2 Tombukt2, Dec 19, 2025 at 9:55 PM
  3. pasadena_commut

    pasadena_commut Senior Member

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    No codes, really? Fire up techstream (or equivalent) and see what sorts of currents it is handling. There is a small possibility that somehow the car is demanding a ridiculous amount of power somewhere, and the inverter is overheating trying to deliver it. I'm thinking something like a fault in the AC compressor. The transaxle would be another possibility, except it sounds like this happens when the car is just in Park, and the transaxle shouldn't be doing anything then.

    High pitch tone is like an oscillator - a single steady frequency? That is a pretty common symptom for broken power converter circuits. These days most of those work by sending a single frequency wave (square or sinusoidal) through transformers, coils, and/or capacitors. Those are usually "potted" to hold them in place. If the component or potting breaks the component can oscillate at that frequency and the pure tone results. (Way back in the day many CRT devices had a 19 kHz tone like that from the flyback transformer even when the device was working normally.) So that would be compatible with a broken part in the inverter. Not sure how that would generate all the heat though. It might get hot like that if the radiator section where the inverter coolant goes is damaged. Like all the fins are mashed flat, or maybe even the radiator is bypassed. Fluid would move, but the heat wouldn't be drawn out where it should, so it just gets hotter and hotter.

    If none of that pans out, swap the inverter from a parts car (the OP may have one) or pick one up at the junkyard.
     
    #3 pasadena_commut, Dec 20, 2025 at 2:31 PM
  4. BiomedO1

    BiomedO1 Senior Member

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    Sounds like you have a heat sensitive component that's opening up when it gets too warm. The voltage drop is the car running off of it's 12V battery alone - Your voltage sensor, battery lamp should be lit. The same thing would happen if the alternator in an ICE "crapped out" - heat sensitive voltage regulator or diode opening-up when it gets too hot.
    The other hint is the high frequency whine being produced when that circuit opens up.
    You should check the TSBs, that might be a free repair of the inverter.

    Good Luck...
     
    #4 BiomedO1, Dec 20, 2025 at 4:04 PM
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