I'm not sure if this has been answered yet, but I have a 2003 Prius and about 2 years ago I started noticing a problem with the AC. The AC works fine and blows cold air without issue unless the outside temp is pretty hot (at least 80F). When it gets hot outside, the AC will work fine for about 5-10 minutes, then the green AC light on the AC button will start to flash and it starts to blow warm air. I will keep driving without AC for a few minutes and then turn the AC back on and it will run fine for another 5 or so minutes, then start flashing and turn itself off again. This cycle just keeps repeating for as long as the drive takes. It's also interesting because the amount of time from when the AC light starts flashing to when I turn the AC back on seems to influence how long the AC will run (ie, if I turn the AC back on a few seconds after it turns itself off, the AC will work fine for only about 10 seconds. However if I leave it off for a few minutes then turn the AC back on, it will work fine for maybe 10 minutes). I'm not sure what the issue could be since the AC works fine when its not very hot outside. Thanks for the help.
Welcome! I've been having the same symptoms and I'm planning to check for wear on the compressor clutch. There's a very thorough write-up on this procedure here, and I know others on here have done this fix as well. It's possible this isn't the problem, but I think it's a good first thing to check at least.
When you read that thread, be sure to notice that it starts out with a bigger job—actually replacing the clutch (plate, pulley, bearing, and coil!)—but it ends up with the much simpler, cheaper job that is all most people typically need: readjusting the clutch clearance, using the shim kit you buy for a buck. Takes about ten minutes to do. Pretty much any Gen 1 Prius is likely to need shims switched by now, if it hasn't been done already. If you take the belt off (this could be a good time to replace the belt too!) you can give the clutch pulley a spin and see if there is noise or roughness in the bearing. If there is, that could be a reason to escalate into an actual clutch replacement. But if it spins ok and the only issue is the clutch clearance having worn beyond spec, well, that's what the $1 shim kit is for.
The above messages have good advice, and may help you diagnose and fix your problem. I found another solution to the same issue in my '02. I replaced the AC relay that's in one of the fuse boxes under the hood. I used a generic part from AutoZone that cost a lot less than an OE replacement part. So far this has worked for me, and is a simpler fix than an actual mechanical repair. hope this helps
It's a fix for a different problem. If your A/C is malfunctioning because of the relay, replacing the relay will fix it, and adjusting the clutch will not. If your A/C is malfunctioning because of the clutch, adjusting the clutch will fix it, and replacing the relay will not. This illustrates why it is worthwhile to diagnose issues and then fix them. However, from the which-way-would-you-bet angle: clutch wear is an absolutely guaranteed, constant process over tens of thousands of miles of A/C use. Relays do occasionally fail, but it's not guaranteed to happen in the same way clutch wear is. Lots of cars finish out their useful lives with all of their original relays unchanged. To adjust the clutch, you need $1 for the shim kit, ten minutes of time, and some basic tools (and ideally a dial indicator, maybe borrowed from a buddy). Relay replacement is even easier to do, but likely more than $1 for the part. So you could try either approach first, without breaking the bank either way.
The OP said if they left the A/C off for longer then it would run longer once they switched it back on, could that indicate a heat overload shut-off then cool-down? I am curious if you set the AUTO A/C to 85 degrees (right under the "HI" setting) if it will run longer since it is a variable speed compressor and the compressor runs at minimum speed if it is that hot outside and you set the AUTO temp higher (still blows cold, just not high volume of air).
You're describing a Gen 2 or later Prius correctly, but this thread is about a Gen 1. That didn't have the variable-speed electric compressor, just a conventional one driven by a belt and a clutch. What I suspect is happening is that the clutch is worn, and it slips more as the high-side pressure increases, after the A/C has been running for a while, until the ECU detects excessive slippage and gives up. Mechanical wear of an old-school A/C clutch is normal and expected, and is adjusted for using a $1 set of shims.
The A/C light blinking is an indication that the computer has a stored error code. It's pretty easy to find. With the car off, turn the fan knob to off. Turn the car on - not starting it/ready mode. Just to where the dash lights all come on. Press the A/C button 3 times. Turn fan switch to Auto. The A/C & Max buttons should blink together a few times. Then start counting flashes on the A/C button. The codes will be two digits, so it might go "blink:blink - brief pause - blink:blink" That would be "22" - and can be triggered by by a slipping clutch/belt. (That the crank shaft and A/C compressor speeds are out of whack). The clutch issue is what ChapmanF has been talking about. I'm not leaving out a simple case of low refrigerant though. For that you need manifold gauges and to know what you're doing.
Thanks for the responses. I used the diagnostic mode and seem to be getting code 22. I've ordered the shims so once they come in I'll make sure the gap is correct and hopefully that fixes things.
If you see about 80 lb on your low and high side 80 to 100 just sitting at rest you know when the compressor is not engaged there is no high and low side both of those ports will have the same pressure high and low sides are only created when the compressors running so with the system off and the car sitting there even a car in a junkyard if you put a gauge on the low side fitting and you get around 80 to 100 lb that's your system at rest when you turn on your compressor it'll suck 40 of that 80 or 100 lb on the low side right down to about 40 and start to make cold if all is generally well and then your high side will go up to 280 whatever it is when the system is active
On a day like today (where I am anyway), it would be sitting at about 50 psig. 80 psig would mean the temperature is about 76 ℉, and 100 psig would make it about 88 ℉. When the system is at rest, the pressure gauge is really just a thermometer marked off in weird units. https://refrigeranthq.com/r-134a-refrigerant-pt-chart/
Yeah. Didn't sound to me like the OP was one to tell to go messing around with A/C pressure gauges so I don't see the point in discussing that. The place to start IMHO is by pulling those codes as it can't hurt anything. It takes about 5 mins, doesn't require buying anything, and can't hurt anything. If there's a 22 code I'd say to follow Chap's advice if one felt up to it (as it is also reasonably simple and harmless). But otherwise, it's likely beyond an "internet-enabled" someone with a flashing A/C light who doesn't fairly quickly learn that this means error codes are present. Apologies to the OP if I underestimate their techno savvy, curiosities, and willingness to learn of it all. If the error code is not a 22, then I'd say it's time to take it to a locally own specialty AC shop. Cough up the dough for a diagnostic and then decide what to do.
