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A/C Compressor Short-Cycling

Discussion in 'Gen 2 Prius Main Forum' started by Ed69, Apr 24, 2023.

  1. Ed69

    Ed69 Junior Member

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    The issue has been going on the same way for many years now so it isn't a serious problem. My A/C blows cold enough on MAX Cold and generally not a big problem. But I do notice other Gen 2 Prius compressors tend to spin pretty consistently while mine tends to cycle on an off every 5-10 seconds. I put a gauge on the H & L side before and the freon level seems fine, not low or anything. The only problem I experienced in the past was on very, very hot days where I set the A/C blower to blow High and noticed my compressor stopped spinning altogether, the air got progressively warmer, and I had to turn the system off for a while as if something was iced over and had to thaw? It would work again afterward if I kept the fan speed below medium.
    My question is whether there is a thermistor that can fail on these Gen 2 Prius? I ran into this problem on another Toyota (GX470) where an old thermistor in front of the evaporator in the dash was prematurely having high resistance and short-cycling the compressor, not allowing it to spin and build up enough pressure to really help get the cabin air cold enough. Or maybe its something else with the Prius?
    Any thoughts, suggestions where I should look? Thanks in advance.
     
    #1 Ed69, Apr 24, 2023
    Last edited: Apr 24, 2023
  2. TMR-JWAP

    TMR-JWAP Senior Member

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    Look at the sight glass installed on the refrigerant line. Put the AC in max with doors/windows open. When charged correctly, the siteglass is typically completely clear and you can't even tell that refrigerant is flowing through it. If you see bubbles or you can see obvious flow, then the charge is likely low. There's more info for the sight glass inspection listed in the repair manual.

    Unfortunately, if the charge is very very low, it can also look very clear.
     
    #2 TMR-JWAP, Apr 24, 2023
  3. Tombukt2

    Tombukt2 Senior Member

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    What kind of gauge readings did you see when you noticed that the system wasn't low by looking at the gauges which theoretically is impossible You see some pressures and that's what I would be interested in a steady 30 PSI on the low side and upwards of 200 on the high on a relatively warm day needing air conditioning like in the '80s and the '90s and 100s the high will go above the 200 more in the 250 or so range probably and this is before you start winding out into the super high speeds If you're inverter compressor is running like it's running a NASCAR race it's trying to pump a lot of nothing You're low on gas and your compressor is trying to make up for that running itself into oblivion. What I did when I got my generation to and didn't know where I was was I added 134A until the compressor almost began to lock up and then I backed a little of that gas out all in one shot so I run it up from the blue Arctic can till I can hear the compressor start to labor and then I let some out of the yellow fill hose on my gauge set until it comes back to running without trying to stop then I observe that for a few notice I have cold like I'm supposed to and go for a drive.
     
    #3 Tombukt2, Apr 25, 2023
  4. mr_guy_mann

    mr_guy_mann Senior Member

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    It might be useful to have a capable scantool connected to the car and watch A/C system data. Then you might see WHY the car is turning off the compressor.

    Does the high pressure switch go open? (Do the condenser fans work?) What does the evaporator temperature sensor show? Any problems with inverter temperatures?

    Posted via the PriusChat mobile app.
     
    #4 mr_guy_mann, Apr 25, 2023
  5. Ed69

    Ed69 Junior Member

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    Ah yes, finally, a reason to get Techstream on an old computer…

    Reporting back with my observations.
    On startup with A/C on Max cold the compressor spins for 70-80 seconds, bubbles in the sight glass come and disappear, then the compressor starts to slow down and labor before it shuts off. It waits a few seconds then comes back spinning fast initially thing gets bogged down, turning for about 20 seconds and goes off. This cycle continues as long as the A/C is on.

    Turning off the A/C and keeping the blower on allowed me to watch the sight glass for changes. When I turned the A/C back on the compressor spins up fast, bubbles disappear in the sight glass, and the compressor runs for about 30 seconds before shutting off as if laboring. It waits for about 8 seconds then turns on again for 12 seconds or so before shutting off for another 8 seconds more or less. Each time the compressor comes back on it is spinning at a different rate, sometimes fast, sometimes slower, not much consistency. But when it shuts off it often seems slower.

    the cold air blows cold consistently on this cool day.
     
    #5 Ed69, Apr 25, 2023
  6. TMR-JWAP

    TMR-JWAP Senior Member

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    Another thing to keep in mind is that intermittent AC is sometimes caused by failure of the inverter water cooling pump. Normally resulting in cool air/warm air/cool air/warm air cycles at the vents.
    I haven't heard of it causing that rapid of a cycle like what you're seeing on the compressor, but I've never actually dug deep into it. May be worth verifying before you get too deep into AC stuff.
     
    #6 TMR-JWAP, Apr 25, 2023
  7. ChapmanF

    ChapmanF Senior Member

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    If the cold air is blowing consistently, the system may just be doing what it is built to do. The compressor is made to be variable speed, so when it slows down, that doesn't have to mean it is 'laboring', it can just mean the A/C amplifier has decided it can ramp the speed down and still have enough cooling. In my Gen 3, it can usually ramp the compressor down to around 300 or 250 watts (as reported by the PID for A/C Watts using a scan tool). If that's still more cooling than is needed, it will also cycle the compressor off and on.
     
    #7 ChapmanF, Apr 25, 2023
  8. Tombukt2

    Tombukt2 Senior Member

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    As long as you're not hitting over pressure and that's what's cutting you out sometimes in the sight glass clears that fast you can be overcharged which could activate the system in different ways .
     
    #8 Tombukt2, Apr 25, 2023
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