Automotive Air Conditioning Information Forum (Archives)

beeson76 on Tue June 21, 2011 11:29 AM

Year: 2005
Make: Toyota
Model: Corolla

At slow speeds the AC compresser works fine and pressure is about 38 pounds on the low side. The AC feels cold.

At high speeds the compressor shuts off completely after a short while. It will then come back on with about 28 pounds pressure (again on low side). Then it will kick off again after a short while.

Does anyone know what could be causing this.

Thanks for any help provided.

mk378 on Tue June 21, 2011 11:42 AM

Do the condenser fans work? If not the high side will go up as the condenser overheats, until the over-pressure switch trips and shuts it down. Measuring the high side pressure is essential for any diagnosis.

beeson76 on Tue June 21, 2011 12:22 PM

Thanks mk378.

I will check it and high side and get right back to you.

Thanks again for the reply.

I appreciate it.

tomw on Tue June 21, 2011 4:26 PM

You can check the fans in an instant. Most[yes, most...] vehicles will turn the condenser fans [radiator fans] to full blast when you turn on the A/C with the engine at idle. If the fans don't come on, they may have problems or the controller that runs them may have problems.
A lot of the manufacturers will turn the fans off when the vehicle is traveling over a set speed, such as 35mph. Going down the highway at 50 or 60, the fans will not likely be running.
That said, if you put the gauges on, the low should go down into the high 20's, depending on ambient, and if it is hot enough outside, the compressor will not kick off. If the pressure goes lower than that with the doors open, blower on high, etc, the compressor will kick off due to LPCO switch protection. The LPCO protects the evaporator from freezing up, and AND protects the compressor from running with minimal lubrication. The oil is carried around the system by the refrigerant, and gets slurped up out of the accumulator by the vapor rushing out to get pulled into the compressor. If there is no refrigerant due to low charge, there is no LUBRICANT either. Good for TRB's business, but hurts in the wallet. FWIW
tom

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bromodragonfly on Tue June 21, 2011 6:42 PM

Is your AC not blowing cold between compressor cycles, as you're driving? I would guess since his problem occurs at high speeds, and works when he's idle or slow, it is not due to insufficient fan cooling.

You really do need pressure readings to diagnose correctly. Hook your gauges up and try to emulate the problem condition by increasing idle RPM, and using a large fan in front of the condenser.

You will need to see if the compressor is cycling on low or high pressure.

My guess (which should definitely not be even considered as remotely true until you get your pressures), is that your system is low on refrigerant and your low side is starving.

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