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Johnson Controls P66 Troubleshooting - PartsAPS

johnson controls p66 troubleshooting

Johnson P66 Electronic Fan Speed Controls are designed primarily to control condenser fan motors in many Heating, Ventilating, Air Conditioning and Refrigeration (HVACR) applications. These are pressure-actuated electronic motor speed controllers. P66 controls are used as an operating control. A failure or malfunction in the control could lead to personal injury or property damage to the controlled equipment. One can follow the below Johnson Controls P66 troubleshooting tips and precautions to avoid problems.

Johnson Controls P66 Troubleshooting

The P66 series controls will vary the supply voltage to the fan motor in response to the sensed refrigerant pressure. It provides a direct response to make changes in condenser pressure regardless of the variations in air delivery curves.

Johnson Controls P66 and speed controlled motor can replace multiple speed motors, on/ off fan cycling controls, condenser flood back systems, and temperature fan speed controls.

Make sure P66 series controls must be applied to single-phase motors that are made for use with a solid-state motor speed control only. An applicable motor should be a single-phase permanent split capacitor (PSC) motor. And, it must be designed to dissipate the motor heat which is generated at reduced speed operation.

If you are failed to use a single-phase permanent split capacitor motor, it would lead to damage to the motor and other components.

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Fan is Not Operating

  • Check if the input pressure is below the operating range. If it is, no problem with it, consider it as a normal operation.
  • Check for 24 VAC at control. If there is no 24-volt control voltage, then the fan will not operate.
  • There might be no input pressure to P66 control. Make an alignment. The Schrader valve depressor should depress the Schrader valve enough to allow pressure into the capillary.
  • Inspect the fan motor. If it is bad, you have to disconnect the power. You need to place a jumper between L1 and M1 and connect the power. Now, if the fan doesn't start, the problem is with motor. So, it should be replaced.
  • Check for the pressure transducer problem. If there is an issue with the pressure transducer, disconnect a six-pin connector from the right side of the electronic fan speed control. Now, place a jumper wire from the third pin (from the top) to the bottom pin on the control, do not place cable. If the fan is running at full speed, you need to check for input pressure. The pressure is adequate when the transducer is bad. If it is bad, you must replace the control.

Fan stops when pressure reaches the high end of operating range

Is fan not running when the pressure reaches the high end of the operating range? Check whether Johnson control is wired properly or not. Make sure, 24 VAC power supply is connected in-phase with the motor power supply. Moreover, once check the wiring diagrams in your manual and follow accordingly.

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No Fan Modulation

If there is an on-off operation, any of the following might be the causes.

  • Control is not wired correctly.
  • Problem with a pressure transducer.

You can check the possible solutions for these causes in the above section. Therefore, try them once to fix the issue.

Fan Starts at Full Speed

Examine that your P66 fan speed control is wired properly. Check to see if the control voltage, i.e., 24 VAC is on the same phase as motor.

Also, check for the pressure transducer problem. Refer to the above section in this post to get the pressure transducer troubleshooting process.

Erratic Fan Operation

If you are getting erratic fan operation,

  • Check if the condenser coil is blocked or dirty. If so, you should clean the condenser coil.
  • Inspect the wiring of electronic fan speed control. The 24 VAC power supply should be connected in-phase with the motor power supply.
  • Check the pressure transducer. If you notice that problem is with the pressure transducer, fix the problem with the above-given troubleshooting tips.

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Fan Motor is Cycling Thermal Overload

A blocked or dirty condenser coil could be the cause for fan motor is cycling thermal overload. Hence, you need to clean the coil.

You might be used the wrong motor for Johnson fan speed control application. If yes, you should replace it with the motor that is approved for fan speed control application.

P66 Fan Speed Control Operation

Condensing unit installation, operation, and maintenance will determine the overall effectiveness of maintaining a satisfactory pressure. The control provides air delivery in direct proportion to heat rejection requirements within the operating range. This allows the refrigeration system to perform efficiently in very low ambient temperatures.

Regardless of the variations in fan delivery curves, Johnson Controls P66 pressure transducer gives a direct response to changes in condenser pressure


You should mount the control away from heat sources. And, mount the P66 control with the cooling fins in a vertical position with no obstruction preventing airflow through the fins.


As Johnson Controls P66 electronic fan speed control provides an alternative to fan cycling control in many HVAC applications, keep it in an effective condition. We hope our Johnson Controls P66 troubleshooting steps helped you to operate correctly. Bookmark to get more HVAC equipment/ components troubleshooting procedures.

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