This article describes how to diagnose and repair a bad vehicle air conditioning system pressure control switch. The pressure control witch for a car or truck A/C system is a safety device that monitors referigerant high and low side pressures and controls the turning on-off of the air conditioning compressor motor. If the switch is bad your vehical air conditioning output may be weak or it may not work at all.

Page top photo: the wires and connector locate a car AC system pressure control switch that is typically found mounted atop the vehicle air conditioning system's filter/drier canister.

My Car Air AC Won't Run - bad pressure switch

Question: my car AC button blinks green and the air conditioner doesn't run

2019/05/14 Archi asked

My car condenser fan do not blow when it is turned on, as the result of this the AC button blinks green light when on.

However, when the dryer wire is disconnected the condenser fan will blow and the blinking stops.

Please what's the cause and solution?

Wiring diagram: a typical car air conditioner system wiring schematic. Be sure to find the wiring diagram for your specific vehicle as details will vary.

[Click to enlarge any image]

Reply:

Sounds like a bad fan motor wiring or connection or control relay. But the rest of your question, unfortunately, leaves me confused. "Dryer wire?"

Reader reply:

The wire I don't really know how it's called, but it is the wire that is plugged to the dryer head. I think that is where the dryer gets its current from.

Moderator reply:

Oh so we're talking about a car? Can I see a photo of the part about which you're asking? Indeed a car or truck AC system uses a combination filter/drier canister to act as a refrigerant receiver, and on many vehicles there is a vehicle air conditioner system pressure control switch that's mounted there.

A car air conditioner light might blink green or yellow if the compressor motor can not run. The problem can be any of several causes ranging from a bad pressure control or a broken fan belt or bad AC clutch to a failed car AC compressor motor. If this is what you're asking about we can help sort out those problems and cures.

Reader reply:

The wire I was referring to is the one that is plugged to the head of the dryer cylinder.

Moderator reply:

The photos show what's probably the pressure control switch that happens to mount conveniently atop a dryer that also acts as a refrigerant accumulator. If the switch has failed the A/C in the car won't work and probably the compressor motor won't run because the fan relay won't pull in to engage it.

If the pressure switch is working properly it will still shut down the AC system if abnormally high or abnormally low pressure is detected.

Reader reply:

Thanks so much Dan. but how will I know whether the pressure switch is bad or not? Also the technician here had used an instrument, inserted it inside the pressure switch and it produces current. So I don't know whether that is how to test it to know whether it's good or not.

Moderator reply:

Symptoms of a bad car A/C AC pressure switch.

The car AC compressor switch monitors both high side pressure and low side pressure and has the job of protecting the compressor by shutting it off if pressures are out of range.

• The vehicle air conditioning compressor won't turn on at all - the clutch won't pull in
• Car AC short cycling: the car AC the compressor turns on but at idle the compressor turns on and off rapidly - this is the MOST CLEAR DIAGNOSTIC of a bad switch
• The car's engine RPM will fluctuate significantly when the car is idling or at a stoplight. On my Toyota FJ cruiser when this symptom became extreme the engine would surge at idle and at a stoplight - ultimately in that case we found that the compressor itself was seized.
• Reduced car AC cooling output: before the pressure switch has failed solid the AC may run but total amount of cooling may be reduced - this is the same symptom as a low-refrigerant problem (common as leaks in car AC systems are common)
• Car AC does not produce any cool air: If the pressure switch itself has failed solid then instead of fluctuating as I cite above, the basic symptom is the compressor won't engage ever and you'll have no cooling in the vehicle.

How to Diagnose a bad car or truck Air Conditioner Pressure Control Switch

1. If the AC pressures are abnormal:
   
   Abnormally high or low pressure in the system can both damage the compressor (low) or cause a refrigerant leak (high) that then damages the compressor (as refrigerant is lost); as the pressure switch has the job of monitoring pressure and shutting down the compressor if pressures are out of range, a check with a gauge set to compare the car AC high pressure side and low pressure side against the manufacturers' specifications would be diagnostic.
   
   The exactly correct refrigerant pressure in a car's high side and low side depend on the specific refrigerant gas and the specific ambient temperature. (It's hotter of course under the hood). The AC repair manual for the specific AC system or car as well as AC specs for the specific refrigerant will give a table of these pressures and temperatures.
   
   Example: for a modern car with current refrigerant, if the outside ambient temperature is 80 degF the low side pressure will be around 56 psi.
   
   See details at REFRIGERANT PRESSURE READINGS & CHARTS
2. If the AC compressor is short cycling that is usually diagnostic of a bad pressure switch
3. If the car AC compressor does not run at all and if you can spin the compressor motor and clutch with the belt removed I suspect it's a bad pressure switch. If the motor won't spin at all then I suspect the motor itself has seized.
4. Watch out: Before replacing the pressure switch be sure you confirm that the problem is not in the system's electrical wiring. With the switch connector plug removed and testing in the plug's contacts, using a DMM or VOM, You should see 4-5 volts DC at the wires bringing electrical power to the pressure switch.

Illustration: An Innova OBDL, OBDLL Scan Tool with ABS - used to read the onboard diagnostic codes or OBD codes from the standard connector found on nearly all modern vehicles.

5. How to Use the Onboard Diagnostic System or OBD to diagnose a bad car AC pressure control switch:
   
   Find the OBD port II for your vehicle. It's usually on the lower left steering column or on the rear of the center console.
   

Connect the OBD scanner (a tool provided by car manufacturers, auto suppliers, such as AutoZone

Read the diagnostic codes given by the scanner

Translate the diagnostic codes by the book provided with the tool or by the OBD code tables available online at sites such as AutoZone. A faulty AC Pressure switch will generate a code indicating that condition.

You'll see one of these codes

• P0745 - Pressure Control Solenoid Malfunction - I would replace the control
• P0746 - Pressure Control Solenoid Performance or Stuck Off - I would replace the control
• P0747 - Pressure Control Solenoid Stuck On - I would replace the control
• P0748 - Pressure Control Solenoid Electrical - before replacing the control check wiring and connectors
• P0749 - Pressure Control Solenoid Intermittent - before replacing the control check wiring and connectors

For these codes you will want to replace the vehicle A/C pressure sensor control switch.

Related OBD Trouble Codes for Car Air Conditioner Systems

• P0530 - A/C Refrigerant Pressure Sensor Circuit Malfunction
• P0531 - A/C Refrigerant Pressure Sensor Circuit Range/Performance
• P0532 - A/C Refrigerant Pressure Sensor Circuit Low Input
• P0533 - A/C Refrigerant Pressure Sensor Circuit High Input

For these OBD codes you will want to check wiring connections, wires, refrigerant pressures and charge, and possibly the sensor switch itself.

Resources for Car AC Troubleshooting

• Al‐Rabghi, Omar M., and Abdulkareem A. Niyaz. "Retrofitting R‐12 car air conditioner with R‐134a refrigerant." International journal of energy research 24, no. 6 (2000): 467-474.
• AutoZone OBDII code scanners: https://www.autozone.com/test-scan-and-specialty-tools/code-reader
• AutoZone OBDII Trouble Code Table: https://www.autozone.com/repairguides/Trouble-Codes/Diagnostic-Trouble-Codes/_/P-0900c1528026a8b3
• Lorentzen, Gustav, and Jostein Pettersen. "A new, efficient and environmentally benign system for car air-conditioning." International Journal of Refrigeration 16, no. 1 (1993): 4-12.
• Tamainot-Telto, Z., S. J. Metcalf, and R. E. Critoph. "Novel compact sorption generators for car air conditioning." International Journal of Refrigeration 32, no. 4 (2009): 727-733.

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Continue reading at REFRIGERANT PRESSURE READINGS & CHARTS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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