A/C Issue

[FlyingDutchman]

Today I decided to trouble shoot my air conditioning system in my Camaro. It hasn't blown cold air since I bought it late 2008. Without it I sweat in the summer and can't see to save my life in the colder months. I took it to a mechanic thinking it needed more Freon, but it wouldn't take any. Kinda pushed it to the side and forgot about it.

Lately, its been too hot to live. So I did some research and noticed the compressor did not turn on. Switching relays didn't make a difference. So I jumped it and found it kicked on which was indicative of low freon. Bought 2lbs of R134a and evacuated the system (some residual pressure). The system held 28-29psi vacuum so I am assuming no leaks.

After emptying a can, I started the car with A/C on MAX. Compressor kicked on like it should and I added the second can. Car now blows cold (around 60*F) and I was a happy guy until I drove down the street.

I left the car idling for a good 10 mins before driving (during that time the A/C was still nice and cool). Less than a mile away the air became really hot like it was outside.. because it was. Got all the way home and sat there playing with the dials but the compressor would not cycle back on for anything. Turned the car off, waited a minute, restarted, and the compressor ran fine and I had cold air.. but as soon as I drove around it would kick off and not come back on unless I restarted the car.

Hooked up a set of manifold gauges and found 250psig H / 43psig L which dropped to about 225psig H / 36psig L (which are good values for 90-100*F greater than 40% humidity based on A/C performance chart). Played with the throttle and engine rpm didn't make a difference either, compressor kept running. This was a while later so I did not lose pressure from when I filled it to when I checked it later.

I decided that maybe the really hot air was too much, so I waited until dark. Same problem though, compressor on at idle but after driving turns off and does not come back on without restarting. Really not sure what to do, looked all over but could not find an other problems.

I don't suspect any vacuum issues (recently resealed the intake) due to the vents all working properly. Im guessing electrical, like maybe a pressure switch? Obviously the car knows there's enough Freon now to start the compressor. High pressure switch next to the passenger strut on the high side line? Not looking to shotgun parts at it or disassemble something when the heart of the system works fine.

Any tips?

[sweetbmxrider]

Air Conditioning System

The Thermostatic Expansion Valve (TXV) refrigerant system, on 5.7L VIN P equipped vehicles, is designed to cycle the compressor on and off to maintain desired cooling and to prevent evaporator freeze. Passenger compartment comfort is maintained by the temperature lever on the control.

Control of the refrigeration cycle (on and off operation of the compressor is done with the evaporator temperature sensor The evaporator temperature sensor is a freeze protection device for the evaporator. When the air conditioning mode (max, norm, bi-level, defrost) is selected, voltage is supplied to the compressor clutch coil. When evaporator temperature approaches the freezing point (the low setting of the switch), the switch opens the compressor clutch circuit and disengages the clutch. The compressor remains disengaged until the temperature rises above a preset temperature. As the temperature rises and reaches a preset point, the switch closes, re-energizing the clutch coil. This cycling continues and attempts to maintain evaporator temperature at 1°C (33°F) with slight variation due to outside air temperature and humidity. Because of this cycling, some slight increases and decreases of engine speed/power may be noticed under certain conditions. This is normal as the system is designed to cycle to maintain desired cooling, thus preventing evaporator freeze-up. The thermostatic expansion valve (TXV) regulates the flow of liquid refrigerant into the evaporator. Its operation reduces the pressure and temperature of the refrigerant flowing to the evaporator.

The compressor, depending upon engine usage, is also cut off under certain other conditions such as wide-open throttle, low idle speed and low air temperature.

Air Conditioning Receiver and Dehydrator

The receiver and dehydrator is located on the right side wheelhouse panel, just downstream from the condenser. It functions as a moisture separator that receives refrigerant liquid and refrigerant oil from the condenser. It only allows refrigerant liquid and a small amount of oil to continue on to the thermostatic expansion valve.

At the bottom of the receiver and dehydrator is the desiccant, which acts as a drying agent for moisture that may have entered the system. A filter is also located near the bottom of the receiver and dehydrator outlet pipe to keep any particles from plugging the thermostatic expansion valve.

Air Conditioning Compressor

The air conditioning compressor is driven by a belt from the engine crankshaft through the compressor clutch pulley. The compressor pulley rotates freely, without turning the compressor shaft, until an electromagnetic clutch coil is energized. When voltage is applied to energize the clutch coil, a clutch plate and hub is drawn rearward toward the pulley. The magnetic force locks the clutch plate and pulley together as one unit to drive the compressor shaft.

Air Conditioning Condenser

The air conditioning condenser in front of the radiator is made up of coils, which carry the refrigerant and cooling fins to provide rapid transfer of heat. The air passing through the condenser cools the high-pressure refrigerant vapor, causing it to condense into a liquid.

Engine Electric Coolant Fans

Coolant fan operation is crucial to the proper operation of the air conditioning system. The coolant fans ensures the proper amount of air flow across the condenser throughout the vehicle operating range.

Fan operation should be checked during any air conditioning diagnosis procedure. Special attention should be given to the fan whenever excessive high side pressures are encountered.

Air Conditioning Thermostatic Expansion Valve

The purpose of the thermostatic expansion valve is to regulate the flow of refrigerant through the evaporator to optimize its cooling performance. This is accomplished by monitoring the refrigerant conditions at the evaporator outlet and adjusting the inlet flow accordingly. This assures that all the liquid is evaporated before it is returned to the compressor.

The refrigerant flow is controlled by the movement of a shaft that pushes a ball from its seat. As the ball is unseated, more flow is allowed through the evaporator.

Shaft movement is controlled by the power dome which has two separate sides. The thermal side of the power dome is sealed and charged with refrigerant. The system side of the power dome is exposed to the pressure of the evaporator outlet or low side. Refrigerant exiting the evaporator flows over part of the thermal side of the power dome. The sealed refrigerant in the thermal side responds to the temperature changes of the refrigerant flowing out of the evaporator. As the refrigerant temperature rises, the pressure of the thermal side increases.

If the pressure on the thermal side of the power dome is greater than the pressure on the system side (the evaporator outlet or low-side pressure), the shaft moves down and pushes the ball off the seat allowing more refrigerant flow through the evaporator. When the opposite is true, the shaft moves up and re-seats the ball reducing flow. A spring at the bottom of the thermostatic expansion valve is not adjustable.

Although the thermostatic expansion valve controls evaporator refrigerant flow, air conditioning capacity control (to prevent ice build-up) is managed by compressor cycling which is controlled by the powertrain control module (PCM).

Air Conditioning Evaporator

The evaporator is a device which cools and dehumidifies the air before it enters the vehicle. High-pressure refrigerant flows through the thermostatic expansion valve (5.7L VIN P) or the orifice tube (3800 VIN K) and becomes low-pressure refrigerant before entering the evaporator. The heat in the air passing through the evaporator core is lost to the cooler surface of the core, thereby cooling the air. As the process of heat loss from the air to the evaporator core surface is taking place, any moisture (humidity) in the air condenses on the outside of the evaporator core and is drained off as water.

Air Conditioning Compressor Pressure Relief Valve

The compressor is equipped with a pressure relief valve which is placed in the system as a safety factor. Under certain conditions, the refrigerant on the discharge side may exceed the designed operating pressure. To prevent system damage, the valve is designed to open automatically at approximately 3036 kpa (440 psi). Any condition that causes this valve to open should be corrected, and the refrigerant oil should be replaced as necessary. The air conditioning refrigerant pressure sensor should also be checked for proper calibration.

[FlyingDutchman]

Feel like I'm in thermodynamics all over again lol good information though!

Anything I can monitor with a scan tool? Gonna go stare at some wires and visually inspect for any potential issues...

[sweetbmxrider]

I'd wonder what the pcm is seeing the evap core temp at.