Car AC stars cooling in high rpm

Hi, I observed a staring behavior with my car AC system. When I start the car for the first time or after few hours of parking and switch on AC after driving for a mile or so, AC does not blow cool air. The AC is kept at high settings. When I rev the engine near to 4K RPM cooling starts and it continues to blow cool air even in low rpm, even In idling it blows cool air for a 10 mins of observation. I measured the temp continuously it shows at 40F.
I measured the pressures when it starts cooling and the readings are 28 psi low side and 100 psi high side, out side temp at 68F. (The chart says high side is quite low) but cooling is good when it starts. What could be the issue here in this case. Can some of you please share your experience. Thank you

To properly to check the refrigerant charge of an AC system for an automobile, the engine has to be at operating temperature, the idle at 2,000m RPM’s, with a large fan blowing at the condenser/radiator.

The high side pressure is too low.


If you have a passenger compartment air filter, make sure that is clean and in good shape.

I think you need a recharge but it should be checked and done by a qualified technician.

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The high side is way too low. It will never cool properly like that. The pressure will vary but the high side should be up around 225 or so.

Sounds like it needs to be charged and leak checked. I’m actually surprised it even puts out 40 degree air.

Why do uhave an a/c pressure set of gauges with r-134a fittings? Most appliances repair pros have r-12 stuff. Do u service building hvac stuff?

What is your pressure with system off? I’ll guess that equalized pressure at 68 degrees should be in the 60-70 psi range on a properly charged system? (It’s been a couple years since I’ve had to work on my cars A/C’s, so my memory might be off).

On average, the static system pressure (engine and A/C off) is usually around 120 give or take.

Thanks for all your replies. Here are the metrics I captured today.

  1. Engine and AC off: low side 95, high side 102
  2. Reading did not change even when I star the car and switch on AC. Kept the AC settings to high.
  3. Rev to above 4000, I observed that the readings did not change until the rev is above 4000.
  4. Once above 4000 pressures changed and I dropped the rev to idling rpm around 700rpm, Low side 39, high 175 outside temp 75F are constant. Started blowing cool air and temperature gauge dropping slowing and reaching 60…this is when car is parked no fan blowing in front.
  5. AC turned off, low side 70 high side 150 very slowing increasing and decreasing respectively.

The readings are captured with R-134a tool.

To me, and this is just my opinion. This sounds like a classic case of system low on refrigerant. Quit wasting time, find the leak, repair it, fill the system, and move on already. If I am wrong I can accept that, if I am right this could have been fixed days ago.


Does the compressor try to engage before 4000? Quick cycling on and back off?

If the system is low on refrigerant the low and high pressures won’t be in normal range when it starts cooling right. I see that clutch engages only at high rpm and it continues to work… so I’m trying to find what’s making this behavior.

No I tried multiple times… it engages only at or above 4000 rpm and then Ac works normal. It really blows constant at 40f. Today I went to a mechanic and told him th behavior, he says it could be clutch issue, and no way to just repair the clutch but to replace the compressor. I really want to check if that is required. I suspect some issue with the electronics may be clutch relay. Trying to find the clutch relay in the fuse box.

I’d start by checking for 12 volts right at the compressor; if yes, maybe bad clutch. If no, work your way back to the low pressure switch, then to AC relay etc. you may need a service/shop manual to do the best diagnosis and wire tracing.
It is an odd problem.

Try adding some refrigerant. The signs point to it being low.

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You shouldn’t indiscriminately add refrigerant, if 2 to 3 ounces overcharged in a small capacity system and your problems will become worse. If this is a Camry you may have a sight glass, after the A/C runs for a few minutes if the sight glass is cloudy (Gas) the system is low. If the sight glass is clear (Liquid) the system has sufficient refrigerant.

The need to rev the engine to 4000 RPMs suggests there is a problem related to the compressor lock sensor or A/C amplifier but each model car is different so who can speculate?

I’m not saying add refrigerant indiscriminately. The low static pressure alone means the system is shortchanged on refrigerant.

What year model and engine option is this? I took a look at several late model Camry A/C schematics and didn’t see much of anything there as far as complexity; other than some models being fitted with a solar sensor.

The static pressure indicates the under hood temperature is between 114F and 128F. The static pressure does not indicate how much refrigerant is in the system. A 14 ounce can will have the same pressure with 1 ounce as a can with 14 ounces. A full can of R-134A in the freezer can have less than 10 PSI of pressure.

It’s a 2011 Toyota Camry LE 4 cylinder. I will check the volts as suggested. Can you pl. tell me where is the compressor and compressor clutch relays inf the fuse box. I could not find the mcr marked on the fuse box diagram.

Well, an ac tech would check pressures, than determine why clutch is not being commanded on when requested. He might evac system and measure freon level.