Proper operation of the A/C, cooling systems becoming more critical

Knowing your vehicle A/C service basics
An A/C compressor is not just a compressor

When you think of drivability issues, the proper function of the A/C and cooling systems is probably the last thing on your mind. In addition, global warming is one of the “hot” issues of our day, and refrigerant emissions — both direct and indirect — are coming under increased scrutiny by a variety of regulatory agencies. This month, we’ll take a look at just a few of the concerns you may find showing up in your shop.

The Cool Side
A/C compressors can put a large load on the engine when engaged. That robs energy that otherwise could be used to drive the wheels, and impacts fuel economy and emissions. The focus on newer cars is more efficient compressors, controlled more precisely by the Engine Control Module (ECM). The design goal is to maintain more consistent temperatures at the evaporator.

Pressure switches on many models are now pressure sensors, providing more precise feedback to the ECM. Because any control module needs accurate feedback to do its job, making sure these sensors are functioning properly should be included as part of your system diagnosis.

Tech Tip: It has been reported that shorted pressure sensors, sharing a common reference voltage with other engine management devices like the Throttle Position Sensor (TPS), can result in a “no start” condition on some GM models.

Tech Tip: Some hybrid owners may complain of lower fuel economy during the dog days of summer. They may be running their A/C systems on MAX. Doing so can override the “idle stop” feature and keep the Internal Combustion Engine (ICE) running, resulting in higher fuel use.

Refrigerant emissions also are under fire. The European Union has banned the use of R-134a in any new model platform offered for sale starting with the 2011 model year. Here in the U.S., Minnesota is the first state to require OEMs to provide information on system leak rates starting with the 2009 model year. That information is posted on a state Web site for consumers concerned with global warming for comparison (www.pca.state.mn.us/climatechange/mobileair.html).

There was even talk of an A/C system leak test becoming part of California’s emissions inspection, with leaking systems failing that inspection. Modern A/C systems are using smaller refrigerant charges and fewer connections to reduce these leak rates. The old days when a compressor shaft seal could leak ½ ounce a year and be considered normal is now the total annual leak rate from the entire system on some models!

Tech Tip: There are models currently in production using charges of less than 1 pound of refrigerant. That means less room for error when recovering and recharging these systems, and the need to make sure the system is “tight.” New Society of Automotive Engineers (SAE) standards are in place for both RRR (recovery-recycling-recharging) machines and leak detectors.

Tech Tip: Due to smaller charges, finding leaks is more important. Even a 10 percent loss of system charge can have a tremendous impact on system lubrication and lead to possible compressor damage. Small leaks that had little impact on older systems can now cause system performance to be lost in the course of one season. For tips on leak detection techniques, see Handling the Pressure, Motor Age, April 2008.

The Hot Side
There are many factors that impact fuel economy and emissions, one of which is the ability to maintain consistent engine operating temperatures. You may not see it on your scan tool, but many ECMs are now equipped with thermostat monitors. The most common code I deal with related to this monitor is P0128, generically described as Engine Stays Cold Too Long.

In testing for this code, the ECM monitors the Engine Coolant Temperature (ECT) sensor and based on the starting temperature, looks for the signal to rise to a set point in a predetermined length of time. In other words, does the engine reach normal operating temperature as it should? Usually, the cause is a failed thermostat — stuck open, leaking or opening too soon. Occasionally, the fault lies in the ECT sensor itself, with the sensor sending inaccurate information back to the ECM or responding slowly during warm-up.

Tech Tip: Use the temperature function of your Digital Multimeter (DMM) to monitor the temperature at the radiator inlet. A rapid rise indicates thermostat opening, and if this occurs immediately after a cold start the thermostat is not sealing as it should.

Tech Tip: An infrared temperature gun can be used to ballpark the ECT reading. Take your measurement on the engine side of the thermostat housing and allow a variance of 15 to 20 degrees less than the ECT reading on your scan tool. A graphing multimeter or Digital Storage Oscilloscope (DSO) can be used to monitor the ECT signal if you suspect a problem with the sensor itself.

Cooling fan strategies and designs also are used to control temperature loads. GM, for example, uses an Electro-Viscous fan clutch design that allows the ECM to alter fan speed in response to high heat loads. In this design, the ECM monitors ECT, A/C refrigerant pressure, Vehicle Speed Sensor (VSS), Intake Air Temperature (IAT) sensor, transmission fluid temperature and ambient air temperature when deciding on how fast the fan should spin.

It regulates the on time of the fan clutch relay via a 12-volt Pulse Width Modulated (PWM) signal. This, in turn, controls the amount of opening of the fluid solenoid internal to the fan clutch. When the solenoid is open, fluid from a storage chamber flows to the fluid coupling of the fan clutch assembly, increasing fan speed. A Hall effect sensor in the clutch assembly provides the feedback to the ECM.

Tech Tip: On these GM models, some customers may complain of excessive fan noise on starting. If the cooling fan was engaged on shut down, there may still be fluid in the coupling. This is considered normal operation and should be an intermittent complaint. If the condition is continuous, suspect a leaking solenoid allowing fluid to bleed into the coupling from the storage chamber.

Tech Tip: When testing these fans, you can command the fan on with an appropriate scan tool. Just keep in mind that fan speed change will not be instantaneous. It can take a few minutes for fan speed to catch up to a 100 percent “On” command, especially at lower engine speeds and high ambient temperatures.

So Much More
Modern A/C and cooling systems aren’t what they used to be. The need to meet ever-tighter standards for emissions and fuel economy are affecting every system on the car. These are just a few examples of the changes and challenges you will face. The answer? Keep up your training and always refer to your service information for the specifics on the system you are repairing.