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AC blows warm air on GM vehicle

AC blows warm air on GM vehicle

How to diagnose an AC blows warm air condition on a GM vehicle

I’m using a 2004 Chevrolet Malibu for this article. But other car makers use similar systems. Start by opening the PDF to the right. It’s a typical wiring diagram for a GM AC system. If you have an older vehicle or your vehicle is overheating, skip down to the EXCEPTIONS at the bottom of this article. After you’ve read this, get your hands on the diagram for your particular vehicle from either alldata or eautorepair at the bottom of the page.

Recharge A/C system, A/C blows warm, A/C not working, How to fix A/C

Click on this to open PDF

In this Malibu, the compressor cannot operate if the compressor clutch fuse is blown. So that’s where you start. If the fuse is good, the next step is to check for power at the compressor clutch electrical connector while the engine is running and A/C turned to MAX. To do that you’ll have to remove the connector from the compressor, install one lead of your digital meter onto the power pin and the other meter lead to ground. Do this with the engine off. Then start the engine and turn on the A/C.

If you see battery voltage at the clutch coil connector:

Then the refrigerant pressure sensor is reporting enough refrigerant charge and the PCM is trying to operate the compressor. If you were getting NO cool air before you conducted this test, these result prove that adding more refrigerant will NOT help you. In fact, you run the risk of overcharging the system. The cause is likely a bad compressor clutch coil. To test the clutch coil, turn your

AC not cold, AC blows warm

Test the compressor clutch coil to see if it’s good

digital meter to ohms and connect both leads to the compressor clutch coil. You should see continuity. If not, the coil has an open. Next, connect one lead to one pin of the clutch coil and the other to ground. If you get continuity, the clutch coil is shorted to ground. Repeat this test on the other clutch coil pin.

If you’re NOT getting battery voltage at the clutch coil connector:

You don’t need an A/C manifold gauge to check on refrigerant in the system. You can check it with a digital multimeter. Do that BEFORE you start ramming more refrigerant into the system. In this vehicle, you’d connect one lead of your meter to the center pin of the refrigerant pressure sensor. Then connect the other lead to ground. The reading will be between 0 and 5-volts. A 0 reading means refrigerant pressure is low (or completely empty), 5 means it’s high. Both a 0 and a 5-volt reading will SHUT DOWN THE A/C SYSTEM. That’s right, if you rush to ram refrigerant into the system, you’ll shut it DOWN. Getting the point here? So a reading of less than 2.5 volts, but more than 0 usually means the system is low on charge. At that point you can comfortable charge the system.


Connect the recharge kit to the low pressure port. Note the outside air temp and set the pointer on the gauge to that temperature. Start the vehicle with A/C set to MAX and blower speed to high. Then press the trigger. Rock the can slightly from 12:00 o’clock position to 2:00 o’clock. The can should get cold. That means it’s charging the system.
Do NOT turn the can upside down to speed up charging. That can damage the compressor. The can should get cold. That means it’s charging the system.


The system I just described uses a single A/C refrigerant pressure sensor that reports high and low pressures. And the system is controlled by the PCM. Older vehicles used two sensors, one for each pressure. Plus, these older vehicles didn’t rely on the PCM to switch the compressor clutch. If the system was low on charge, the low pressure switch would open, preventing power from flowing to the compressor clutch. Same with the high pressure sensor. Here’s where do-it-yourselfers go wrong. They think they can jumper across the terminals on the low pressure switch to start up the compressor so they can charge the system. If the system is just low on refrigerant, that might work. But if the system is empty due to a leak, you’ll be running the compressor DRY, and that will NUKE IT! So you could easily turn a $200 repair bill into a $1,500 repair bill. The other thing they don’t realize is that you don’t need the compressor running to add refrigerant. It’ll take much longer to add if the compressor isn’t running, but it will still take it. So if you’ve determined that the system is low on charge, add a half can of refrigerant with the engine off. Then try starting the system. If the compressor kicks in, finish charging according to the directions on the can. If the system won’t start after adding a full can, you’ve got a serious leak and it’s time to take it to a shop. Seriously, if you add a second can, you’ll be throwing your money out the window.


If your vehicle is overheating, there’s no way this Malibu is going to let you run the A/C. The PCM will detect high pressure in the A/C system and will command both cooling fans to run at high speed. If that doesn’t bring the pressure down to an acceptable level, the PCM will shut down the A/C. I’ve shown you how the cooling fans work in this GM vehicle. But all vehicles are different. If you’re sure the thermostat, water pump, and radiator are working properly, your next step is to order up a wiring diagram and system operating description from either of the two online sources below and check out the fans, relays and temperature sensors.

© 2012 Rick Muscoplat

Posted on by Rick Muscoplat

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