Working the ins and outs of mobile A/C

Jan. 1, 2020
Refrigerant leaks are less common than they used to be. Fewer connecting points, lower refrigerant charges and better components are resulting in systems that leak less (as a total annual figure) than older systems used to from the compressor shaft s
The driver only can ask for cold air. It’s up to the ECM to decide whether or not the compressor will be allowed to engage.

The shop where I spend my days typically takes in later model cars, up to and including current model year vehicles. I’m sure you’re noticing the same thing I am: Refrigerant leaks are less common than they used to be. Fewer connecting points, lower refrigerant charges and better components are resulting in systems that leak less (as a total annual figure) than older systems used to from the compressor shaft seal alone.

What I am seeing are more electrical/electronic problems, many resulting in stored diagnostic trouble codes. These systems are more electronically integrated into the vehicle, and a simple request for cold air often requires the interaction of several electronic control units before that request can be fulfilled.

Oh, the complaints are the same. “It doesn’t blow cold air,” or “The fan isn’t blowing air right inside the car,” are just a few of the ones we normally get as the weather finally turns warm. But instead of grabbing your pressure gauges as the first tool of choice, you might want to reach for your scan tool.

Pressure sensor or multiple pressure switch? A check of the schematic will provide the answer.

HVAC Codes?
My routine for repairing A/C complaints of any kind starts by checking the operation of the system as I bring the car inside. Are all blower speeds working? Do the mode doors work properly? Once I have the car inside, I set the controls for maximum cooling; blower on high speed, full cold on the temperature setting, airflow from the center registers and the recirculation mode door closed. It shouldn’t take too long for cold air to start flowing. If not, a quick peek under the hood to see if the compressor is engaged. (Be careful with this one; some compressors are a variable displacement design and are always engaged.)

If the compressor is not engaging, I check to see if there is sufficient refrigerant pressure in the system to permit compressor engagement. If there is, or if I’ve determined there are mode door or blower issues, I’ll grab the scan tool and hook up.

Codes set in the HVAC system are usually one of three types: P, or powertrain. B, or body; and U, or communications. P codes will turn on the Malfunction Indicator Lamp (MIL), but HVAC B and/or U codes typically will not. While some models may flash the A/C Request switch or some other control head switch as a

A pressure sensor is like any other engine sensor using a reference voltage and can be tested the same way.

visual notification of stored codes, many models have no such feature. The only way to tell is to check and that might mean testing more than one module. That’s one reason I now make a habit of polling all the modules on any car I connect to, no matter the complaint.

P HVAC codes usually are associated with the compressor and/or the components that the Engine Control Module (ECM) needs to determine whether the compressor should be allowed to engage. The reason? The compressor takes a lot of energy to run and that affects emissions and fuel economy. Emissions control is the responsibility of the ECM. B codes typically will be related to the HVAC control modules, the components linked to these modules or the wiring between them. U codes usually mean that one or more modules are having a hard time being heard by the others. By polling all the modules, I can see if there is any relationship I need to be concerned with before diving into my diagnosis.

Some manufacturers will list a voltage specification versus pressure that you can use to check the accuracy of the sensor.

After retrieving any stored codes, the next step is to check my service information for any related Technical Service Bulletins (TSBs). All too often, the problem I’m facing is a matter of updated software or hardware that I’d never figure out on my own. According to the Mobile Air Conditioning Society (MACS), between 30 and 50 percent of HVAC performance issues requires reprogramming of a related control module to correct. As I’ve shared here in the past, don’t try to reinvent the wheel — check those TSBs first.

If I still don’t have an answer, I may dig deeper by using additional resources like iATN or Identifix to see if anyone has run into a similar problem. Let me stress that I’m not looking for a silver bullet here. This is all part of my diagnostic process, and that starts with fully understanding the system and problem I’m troubleshooting.

With that bit of homework done, I pull up the specific code information to see exactly what conditions are required to set the fault. The code definition often hints at whether it’s an electrical circuit issue or a component one, but never assume you know for sure until you’ve studied these conditions. Any testing I perform from here on will be related to these parameters.

It isn’t fancy, but any thermistor, like this evaporator temperature sensor, can be tested by heating or cooling the sensor and comparing its resistance to specs.

The Ins…
Control modules are relatively the same in how they function. They need information, and based on that information, they carry out their programming by operating other components. HVAC systems, however, often use more than one control module, with the driver’s request for cold air being divided among them.

As an example, on many GM cars, the A/C button on the dash simply sends a request to the Body Control Module (BCM). The BCM is in charge of blower speed and mode door positioning to accommodate the request, but it is not in charge of the compressor itself. That function is the sole responsibility of the ECM, so the BCM passes that request along on the bus network. The ECM then checks its inputs before honoring the BCM’s request and engaging the compressor.

As with any other electrical device, the individual control modules need good power and ground to work properly.

The evaporator temperature sensor probe fits in the evaporator core between the coils. Be careful not to damage them when installing a replacement.

You can verify this by performing a voltage drop test directly at the connectors for the control unit. You also might be able to verify by checking the control module data list with your scan tool for the related Parameter Identifiers (PIDs) and comparing those values to actual system voltage at the battery.

Input devices are any sensor or switch that reports on some condition of the system. These include pressure switches/sensors, evaporator and other temperature sensors, even feedback position sensors of various design that report mode door position. There is no single uniform list I can give you.

To be successful, you’ll need a solid foundation in electrical fundamentals, a good schematic and the ability to read it, and a Digital Multimeter (DMM). You’ll also need to spend some time reading up on the theory of operation of the system you’re troubleshooting.

Control modules are protected from flyback voltage by diodes. Usually they are incorporated in the controlling relay, but not always. Check the schematic to be sure.PAGE 3

More and more, I’m seeing single pressure sensors in the high side of the system designed to control both compressor engagement and fan operation. These sensors are similar to other engine sensors you’re used to, in that they use a reference voltage supplied by the ECM as a power source and are grounded through the control module. The third wire, usually centered in the connector, is the varying voltage signal back to the ECM.

Because other engine sensors share the reference voltage source, verifying the reference is one of the first troubleshooting steps I suggest you take when dealing with codes related to these sensors or compressor engagement. A loss of this voltage usually is caused by a shorted component that shares the reference, but also can be caused by a fault in the ECM itself or an open circuit. In fact, shorted A/C pressure sensors have been the cause of no start complaints on some GM models by pulling down the reference voltage needed by the Throttle Position Sensor (TPS) and Manifold Absolute Pressure (MAP) sensors.

Just because the pressure sensor has three wires going to it, don’t automatically assume it’s a sensor. It may be a multiple pressure switch instead. A quick check of the wiring diagram will tell you for sure.

Ambient air temperature sensors, evaporator temperature sensors and engine coolant temperature sensors all are resistive sensors that change resistance as their temperature changes, and are called thermistors. There are two types of thermistors: positive coefficient and negative coefficient. In the first, resistance increases as temperature increases, and in the second, resistance decreases as temperature increases. Your service information system often will list resistance specifications as a function of a given ambient temperature. You can easily test them using a little cold/hot water, an ohmmeter and a thermometer.

This blower resistor relay connector was burnt by heat as a result of a loose fit between it and the resistor.

And the Outs
Output devices are any component that performs a function. These are components like the compressor clutch relay (it engages the compressor), the blower motor and the mode door actuators. A little digging in the Motor Age archives will pull up a lot of information on diagnosing compressor clutch circuits and troubleshooting blower issues, so allow me to focus a bit on an increasingly common fault: mode door actuators.

Mode door actuation varies from make to make and symptoms can range from a clicking noise to no airflow from the selected vent. Some actuators are basic electric motors with no provision for feedback, while others include built-in sensors that let the control module know how much they have moved. It’s best to test them like any other electrical component; make sure they are receiving power, have a good ground and move when they are told to using the bidirectional capability of your scan tool.

If you suspect the door they are in control of isn’t moving as it should, remove the actuator and make sure the door itself is fully functional before condemning the actuator. By the way, be careful about operating mode door actuators that are not actually in place — they can be damaged if their travel is allowed to go too far.

(TOP) Most mode door actuators have some type of feedback circuit that lets the control module in charge know where the door is.
(BOTTOM) This actuator uses a potentiometer for feedback.

Blower motor circuits have changed, with GM using blower logic modules to control the blower rather than the multi-stage resistors of days gone by. And if not a logic module, many models are using MOSFETs (metal oxide semiconductor field effect transistors) to vary blower speeds. No matter the specific device used, the end result is a designed in voltage drop that reduces current to the blower motor itself, effectively controlling its speed.

The advantage, however, is more room for comfort engineering on the part of the designers. Blower speed can be varied infinitely rather than limited to just three or four speeds and controlled based on any additional inputs the engineers feel critical to cabin comfort.

Yes, the times they are a’ changing, but if you stay current in your training and take the time to study up on the system you’re working on, handling A/C electrical problems will pose no problem for you.