Q: ECM won't actuate AC relay

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A/C recently ceased working. When it is cool out it works fine, when the temp is above 75 degrees or vehicle is driven for more than 15 min the A/C cuts out. Troubleshot the compressor and determined it was going bad and causing excessive drag on the engine as well as making a chirping sound. Replaced the Compressor, Condenser and expansion valve and all seals involved; ensured to add appropriate oil to the condenser and performed oil balancing on compressor prior to install and flushed the lines and evaporator; drew 30lbs vacuum, checked for leaks and charged the system. Problem still persists. If I ground the clutch relay control thereby bypassing the ECM the A/C clutch engages and system works. Static pressure is roughly 110lbs, when clutch is engaged low is around 60 high is around 240. Pressures are good and the system can cool if the ECM would let it. Fairly certain the pressure switch is serviceable as the 5v reference is present and the signal voltage is within tolerances. help

My car has 180000 miles.
My car has an automatic transmission.

A: Your low side seems a bit high. At 120 degr...

Your low side seems a bit high. At 120 degrees, the low side shouldn't be above 44 psi. This could be a sign there is a restriction in the low side. I have access to technician reports from around the world through IATN (International Automotive Technicians Network), and there are reports of a screen that is on the outlet of the evaporator. Apparently this screen can become clogged in the same way an orifice tube does. You might want to take a look at this because it would cause a higher than normal low side. A normal low side pressure, depending on ambient temperature, is from 23-44 psi.

A few other things to note. Your A/C system is controlled by a combination of module that communicate over computer data lines. You won't be able to know what is going on here without a scanner that can access all the PID's (parameter identification) of these modules. PID's are simply data about your automotive systems. In this case your A/C. The information here will be necessary to diagnose your system.

Also, your A/C is failing after driving for 15 minutes. You won't be able to recreate these conditions while the car is sitting. I would drive the car with gauges connected while watching scanner data in order to see what is going on. But before I did this, I would want to see the low side pressure lower.

The pressure switch should certainly be suspect, and may be the simplest diagnostic path by replacing it. Depending on its cost. But the switch will not affect the low side pressure which is too high.

If you need some help with these checks, consider YourMechanic, as a certified technician can help you perform these tests and accurately diagnose the issue with your A/C.

Good luck!

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