Compressors: to spin or not to spin

Jan. 1, 2020
As the weather warms, more and more of your customers will be switching over their HVAC control settings to Max A/C. And if the system doesn't respond by blowing that refreshing cool air, they will show up at your door to find out why.
A current test can verify a mechanical failure on a compressor clutch, and it’s easier than removing the compressor on most cars.

As the weather warms, more and more of your customers will be switching over their HVAC control settings to Max A/C. And if the system doesn't respond by blowing that refreshing cool air into their faces, they will show up at your door to find out why. Have you noticed how they seem to be more than ready to fix an air conditioning related problem but hesitate when it comes to performing needed maintenance on their cars? Oh well, ours is not to reason why.

When faced with a "not cold" air conditioning complaint, the first thing I do is verify the complaint. As I bring the car around to my service bay, I set the controls for maximum cooling; blower on high, temperature setting to full cold, recirculation mode door closed. I want to listen carefully to the sound of the blowing air as I close the recirc door to make sure it's working, and then I switch airflow through all ranges to make sure the interior modes are fully functional. I place a hand over the center ducts not only to gauge how cold the air is but to also gauge how hard it's blowing. With so many cars making use of cabin air filters, and so few of them being changed when they should be, I have solved more than one complaint by simply swapping out a clogged filter with a new one.

If the interior checks pass, but the air exiting the ducts is too warm, the next step is to pop the hood and see if the compressor is engaged. If you see that it's not, follow some of these tips to quickly find out why.

What's the Turn On?

Excessive air gap can prevent a clutch from engaging. But with so many compressors buried in the engine compartment it can be hard to check.

A conventional belt-driven compressor uses a magnetic clutch to link the belt pulley to the compressor shaft. It is made up of three parts: the clutch coil, the belt pulley and the pressure plate. The clutch coil provides the magnetism needed to draw the pressure plate inward and to keep it solidly in contact with the face of the belt pulley. Typically, the coil has a constant 12-volt power feed coming in and is operated by completing the ground side of the circuit.

The ground control on OBDII cars rests on the shoulders of the Engine Control Module (ECM). Why? Air conditioning uses power that could be directed to the drive wheels, impacting fuel economy and emissions. Under OBDII guidelines, the ECM is in charge of any system related to emissions.

However, the driver's request for cold air may make its way through other modules before it gets to the ECM. It's not unusual for the A/C request to originate in the Heating, Ventilation and Air Conditioning (HVAC) control module (if equipped), only to be forwarded to a Body Control Module (BCM). Like any module, there will be certain criteria that need to be met before the BCM will approve the request and send it down the line to the ECM.

Testing the current flow through the circuit can quickly isolate where the problem is.

Once the request does get to the ECM, it will have rules of it's own that will need to be met before it will allow the compressor to turn on. So even if all is well up to this point, if the ECM doesn't like what it sees it will deny the driver the cold air he desires.

If the ECM approves of what it sees, it will close an internal switch (or driver) to complete the ground path for the compressor clutch relay. A relay is used in many computer-controlled circuits to protect the module from excessive current loads. The relay then acts as the control device in the clutch coil circuit, closing the path to ground and energizing the coil.

The energized coil creates a magnetic field that draws the pressure plate into contact with the belt-driven pulley. There is a specified gap between the pressure plate and the pulley when it is not engaged to make sure the two don't come into contact when they are not supposed to. However, if this gap is too big, the magnetic field may not be strong enough to engage the clutch at all. For that matter, if there is any factor that prevents the magnetic field from reaching its designed strength, the clutch will not engage or slip under load.

Low air flow from the ducts? Don’t forget to check the cabin air filter.

Now what could go wrong with that?

One Test, Three Possibilities

The ECM relies on inputs like this pressure switch to decide if it will allow the compressor to turn on. A zero current reading could mean an input signal has failed.

How do you handle this scenario? It seems to me that one good place to start is at the relay. On one hand, the relay has to be turned on by the ECM; on the other, it turns on the compressor clutch coil. If I can determine which of the two is not happening, won't I get a quick idea of where the problem lies? If you're like me, one of the first things you do is swap the relay with another one.

But there are some problems with that. First, when a relay is turned off there could be a kickback voltage generated by the collapsing magnetic field in both the relay coil and the compressor clutch coil that could find its way back to the ECM. That's why some relays and coil circuits have a diode installed.

If you swap the suspect relay with another that isn't the same, you could be opening the door to a circuit board damaging spike that may cause a failure of its own. And if the compressor doesn't come on after the swap, what have you learned? You're still not sure if the problem is turning the relay on (the ECM side) or the turned on relay (the compressor clutch coil side).

While low refrigerant caused by a leaking system can result in no compressor engagement, why pull out the sniffer until you know for sure that’s the reason?

So I offer this idea, one that was shared with me a few years back. Take a moment to look at the wiring schematic for the car you're working on and identify the fuse(s) that power the compressor clutch coil and relay. Often, it's a duty handled by one fuse and that makes this technique even easier. Remove the fuse and install a fused jumper wire so you can use your low amp clamp to measure the current passing through the circuit. If you don't have a low amp clamp, you can use the amp function of your Digital Multimeter (DMM) instead. Just be sure to set the amp scale to the 10 amp or higher range. (Note: This technique can be used on any relay-controlled circuit but before you use your DMM be sure that the normal circuit current does not exceed the capability of your tool.)

Once you're ready to measure the current flow, start the car and turn the air conditioning on full blast. Now check your current reading. You'll have one of three scenarios to choose from.

Current Reads 0.0 Amp

Summertime brings new customers who want ice cold air. Funny how they’lI spend money on a/c and argue over an air filter.

Current won't flow if the circuit path is not closed. And what could cause the relay circuit to not close? However you answer that question is where you need to look first.

Verify there is power at the appropriate relay terminals in the junction box. If not, look for an open power supply between the relay and the battery. Yes, I know it's tempting to jumper the relay terminals to see if the compressor comes on, but that will only verify what you already know. You can, however, swap the relay with another to see if the internal coil of the original relay is the cause of the open circuit. Or more simply, measure the resistance between the two coil relay terminals with your DMM.

If the power side of the relay circuit is intact and the relay test passed, you can switch your attention to the ECM. The ECM is not completing the ground path. But before you condemn the ECM, hook up your scan tool to see if the inputs needed by the ECM are correct. Examples of potential problems are system pressures that are below specification due to a loss of refrigerant from a leaking component, a misreporting input sensor or even a lack of communication issue between the HVAC module, BCM and/or the ECM.

See if you have a bidirectional capability on your scan tool to command the compressor on. And don't forget to consider the wiring between components. In many parts of the country, our little rodent buddies love to snuggle in a nice, warm engine compartment and seem to have an affinity for wiring harnesses.

Current Reads About 0.5 Amps

Dye is great for pinpointing leak sources.

What do you think this tells you? Current is flowing, so something must be turned on.If you said the relay is, you're correct. It takes very little to turn on a relay. That's why it is used as a control device for so many components an electronic control module is in charge of. A reading of approximately 0.50 amps eliminates the ECM side of the relay equation entirely. The ECM has received the request for A/C from whatever string of modules is needed upstream, and it is satisfied that there is sufficient refrigerant in the system. All other required inputs have also gotten the A-OK, so there is no need to check scan data.

The problem is now limited to an open in the compressor clutch coil circuit. Now you may bypass the relay with another that is known good (and correct for the application) to see if it is a failed relay. You also can use a fused jumper lead across the junction block terminals. Just be sure that the lead ends are the correct size for the application and you've identified the correct connections.

If you remove the fuse that powers the coil, you then can use the compressor-side relay junction block terminals and your DMM to measure the resistance of the coil circuit. If it's open, disconnect the harness connector at the compressor and measure the resistance of both wires individually or install a test bulb in place of the clutch coil (using the correct terminal ends) to see if the bulb lights when the A/C is turned on. This will help narrow it down to a wiring issue or an open clutch coil winding.

Current Reads Between 3.0 and 5.0 Amps

If your first current measurement is reading in this range (and that number will vary), there is nothing wrong electrically with the compressor clutch coil. Current is flowing, the relay is turned on and working, the ECM is happy. You have to be dealing with a mechanical issue in the clutch assembly. Possible causes are excessive air gap, damaged clutch faces or a separated pulley. See all the time you would save now because you weren't swapping relays or hooking up pressure gauges right off the bat?

This can monitor intermittent engagement issues. If there's a problem with the coil heating up and dropping in resistance (partial internal short), you'll see the current rise. In turn, the magnetic field becomes weaker, maybe to the point the symptom becomes similar to those caused by excessive air gap.

To get an idea of what a normal current range should be, take the resistance specification of the coil and plug it into Ohm's Law along with the actual measured battery voltage with the engine running. You do remember Ohm's Law, don't you? Voltage equals resistance (in ohms) times current (in amps). Let's say, for example, the spec is 4 ohms for the coil's resistance and the running voltage measured is 14.2 volts. Using Ohm's Law, you'll get 14.2/4.0 = 3.55 amps as an expected measurement.

Now don't freak if it isn't exactly right. What you'll be looking for is a reading that is way off. I've already covered what a high reading would indicate. But what about a low reading? Low current indicates a high resistance issue according to the lessons of Ohm's Law. Now you need to perform a voltage drop test right at the compressor (or as close to it as you can get). If the extra resistance isn't in the circuit, it has to be in the coil.

For more examples of this technique, or to learn more about voltage drop testing, join our online community at MotorAge.com. And to stay in touch with all that we have to offer, including tips and news just for techs, sign up for the Certified Technician newsletter at MotorAge.com/ForTechs.

Pete Meier is an ASE certified Master Technician and sponsoring member of iATN. He has over 35 years practical experience as a technician and educator, covering a wide variety of makes and models. His primary goal is to bring working techs the information they need.

Email Pete at [email protected]

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