Auto Service World
Feature   July 1, 2008   by Peter F. Meier

“My A/C Doesn’t Work!”

A few tips to cut down diagnostic time.

Ah, summertime! That means air conditioning season, and customers with complaints ranging from “No cold air,” to “It blows fine for a while, then starts blowing hot!”, and everything in between.

Today’s systems are more complex than ever, with most compressor circuits controlled by the Engine Control Module (ECM). Strategies for allowing compressor operation vary from manufacturer to manufacturer, with some even disabling the a/c in the event of a detected misfire. I am sure you have a diagnostic strategy for engine complaints. It’s time to apply the same logic to HVACrelated issues.

No compressor operation

Let’s start with an example of a “no cold air” complaint.

As you bring the car into the bay, verify that all the controls are operational: mode doors switch as they are designed to, temperature controls appear to be working just fine, and the blower is operational on all speeds. Once inside, you set the HVAC controls for maximum cooling, and open the hood. As you suspected, the compressor is not engaged. Where do you go from here?

Most compressor clutch circuits use a relay to complete the path from power to the clutch coil itself. On later-model cars, the ECM operates the relay. A number of techs

will start their diagnosis by checking the fuses, then bypassing the relay to see if the clutch coil engages. If it doesn’t, the test won’t tell you anything about where the fault lies. If it does, you know that the path from power to the clutch is okay, but you’re still left with the problem of whether the fault is the relay or the relay control side of the circuit.

Other techs swap out the relay with a “known good” one, usually robbed from the same fuse panel. But be careful! If you decide to use this tactic, it’s important to use the same style relay, to prevent damage to the ECM driver. The relay has a diode inside to prevent voltage surges from reaching the ECM. Use the wrong type of relay, and you can cause a “hot spot” in the ECM that may not cause it to fail immediately, but cause it to show its ugly head weeks or months down the road.

The compressor clutch coil circuit may be considered as one circuit, but for diagnostic purposes it helps to consider it as two, with the relay being the common link. One circuit is the path from the power source to the clutch coil; in this circuit, the relay acts as the switch to make or break the current path. The other circuit claims the relay as its load, and the path supplying power and ground to the relay’s internal windings is turned on and off by the ECM. In most applications, the power supply to both circuits is handled through one fuse. And that’s the best place to check both circuits at the same time.

Current makes it work

Volts, ohms, and amps: the three basics of electricity. Voltage supplies the “push” needed for electron movement; ohms is a measure of the resistance to the flow of electrons; and amps are the measure of electron flow. It is this current flow that actually makes the circuit work, and is easily measured with a low-amp current clamp. Low-amp clamps are available as standalone units, or as attachments for your Digital Volt-Ohm Meter (DVOM) or scope. By measuring the current going to the clutch coil, you can quickly isolate the cause of no engagement to one of three things.

Here’s how:

Remove the fuse supplying power to the clutch coil and relay and install a fused jumper lead. That is where you’ll read current flow with your clamp. Adjust your DVOM to read to three decimal places and zero out the clamp. Place your clamp jaws around the jumper lead, start the engine, and turn on the AC. Now read the amperage measurement on your DVOM. Don’t forget to correct it by the factor specified by your amp clamp’s manufacturer. In my example, the DVOM reading is multiplied by 10 to convert it to actual amperage.

A reading of 0.0 amps indicates that there is no current flow through either the internal relay coil or the compressor clutch coil. Your next diagnostic step would be to make sure that all the needed inputs are correct to allow the ECM to turn the compressor on. Time to hook up the scan tool and start checking Parameter Identifications for the inputs/outputs required by the particular make and model you are working on. It may be something as simple as a low refrigerant charge, to something more involved, like a failed ECM driver.

A reading of less than 1.0 amps means that the relay coil itself is working just fine. Typical measurements actually run in the range of 0.30 to 0.80 amps, depending on design. This means that all inputs and out- puts the ECM needs are there and working and the relay is being activated to close the path to the compressor clutch, but that path, however, is not complete. This is where you start your pinpoint testing.

A reading of 2.5 to 5.0 amps is typical of a compressor clutch coil that is passing current. If you are reading this much current flow, but the clutch is not engaged, start looking for a mechanical issue with the clutch itself. Excessive air gap or plate separation are likely candidates. If the amperage is on the low side, consider a voltage drop in the supply side of the circuit as an additional culprit. Pinpoint testing at the clutch coil connector will quickly narrow this down.

A reading much higher than 5.0 amps on most cars indicates a shorted clutch coil winding, and a lower magnetic field that is incapable of pulling the clutch face in and keeping it there.

Current flow and intermittent operation

You can apply this same technique when looking for an intermittent loss of compressor operation. When the clutch fails, you can quickly check current to see what section of the circuit has quit, and narrow down your diagnostic direction from there. One additional note here: if current starts to increase above normal, suspect a clutch coil that is shorting out internally as it heats up. The lower resistance caused by this type of failure will cause current flow to increase.

This technique will help you narrow down your diagnostic direction, and can also be used on any relay-controlled circuit. Next time you have a “no compressor engagement” issue, dust off that low-amp clamp and incorporate it into your electrical diagnostic strategy.


Pete Meier is an ASE CMAT, member of iATN, and full-time tech in Tampa, Fla. He is also an advanced automotive instructor, and regular contributor to industry trade publications. Pete’s 30+ years of experience cover almost every vehicle nameplate. Visit Pete at his Internet home at

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