Rick's Free Auto Repair Advice

AC blows cold then warm Dodge Ram

AC blows cold then warm Dodge Ram

Dodge Ram 1500 intermittent AC, Compressor quits, blows cold, then warm, warm AC

There are basically three sections to the AC system; the sensors (high and low pressure sensors and the AC on/off switch), the command unit (the powertrain control module- PCM), and the “doing” components (the compressor clutch relay, compressor clutch, and the compressor).

AC not cold, AC blows warm

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The sensors inform the PCM if system pressures are too high or low. And the low pressure sensor provides the information on when to cycle the compressor on and off. Based on that information, the PCM toggles a ground connection to the control coil on the compressor clutch relay—the unit that actually powers the compressor clutch. Finally, there’s the compressor clutch and compressor.

Since you get cold air on occasion, we can assume the compressor clutch works at least some of the time. When the clutch stops cycling, the first thing I’d want to know is if it’s getting power from the compressor clutch relay. You can check that by removing the electrical connector from the clutch, starting the engine, turning the A/C to MAX, and testing for battery voltage on the dark blue/black wire. If you see voltage there, check the other wire for good ground. The ground for the compressor clutch runs on a black/white wire from the clutch connector to a splice on the top of the transmission. From the splice, the ground continues and terminates at the front of the engine. If you’re getting power and ground at the clutch connector but the clutch isn’t engaging, you most likely have a bad clutch. My guess is that it’s heating up and creating an open in the clutch coil winding or the connector. If you’re NOT getting power at the clutch connector, then I’d check the compressor relay to see if it’s actually getting a ground connection from the PCM.

Power flows to the compressor clutch relay contacts from fuse J (10A) in the power distribution center (PDC). Power flows to the relay control coil from fuse 11 (10A) in the junction block in the left kick panel. Remove the compressor clutch relay, start the engine, and turn the A/C to MAX. Check for battery voltage on terminals 86 and 30 in the relay socket. If you’re getting voltage, check for PCM ground on terminal 85 in the same socket. Good ground on terminal 85 means that the high and low pressure switches, the A/C control switch and the PCM are all working properly, the PCM is asking for compressor operation and providing the proper ground for the relay control coil. If you’re seeing voltage and ground, try swapping the relay with another one with the same part number. Then check for power at the compressor clutch connector again. If you still aren’t seeing power at the clutch, you may have a corrosion issue inside the PDC, or an open in the dark blue/black wire going to the clutch.

Now, if you’re NOT getting ground on terminal 85 in the relay socket, then the PCM isn’t seeing the proper input from the sensor switches. Here’s how those sensors work. The PCM supplies power to the A/C switch in the heater control head on the light green/white wire. When you turn the A/C switch to ON or MAX, the switch completes the path to ground on the black/orange wire terminating in the center of the dash. The PCM sees the voltage drop and that’s its signal that you want A/C. Next, it checks the high and low pressure switches. It sends power out to the low pressure switch (located on the top of the A/C accumulator) on a brown wire. If the refrigerant pressure is above the low limit, power flows through that switch and out to the high pressure switch (mounted on the back of the A/C compressor) on a dark blue wire. If the refrigerant pressure is below the high pressure limit, the power flows out of the high pressure switch on a light green/white wire and terminates at the A/C switch in the control head. So the path goes from the PCM, through both the low and high pressure switches, through the A/C switch and to ground. The PCM is basically looking for continuity through the sensor switches and then to ground. If everything is working properly, the PCM will see almost 0 voltage on that circuit. That’s its clue to provide ground to the A/C compressor clutch relay.

The system must have at least 43-psi at the accumulator to close the contacts in the low pressure switch. With the contacts closed in both the low and high pressure switches, the PCM provides ground to the A/C compressor clutch relay. The high pressure switch contacts open at 450-490-psi and close again at 270-330-psi.

So the most likely suspects are: intermittent compressor clutch coil, bad compressor clutch relay, corrosion in the PDC, open in the wire to the compressor clutch, intermittent ground on the sensor side.

Posted on by Rick Muscoplat


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