Troubleshooting Cooling – Old Sunline Units


The older Sunline units, before Simplicity boards were introduced, had a “lock-out” relay board in the units.  All of the safeties were wired in series and, if any one of them opened, it would engage the lock-out relay, shutting down that system.  There were no flash codes on the board to direct you as to why the unit was not running.

The following is a procedure that can be used to diagnose the older Sunline units with the lock-out relay boards to trouble shoot the cooling part of the unit.

When you walk up to a unit, before disconnecting power to the unit, remove the electrical access panel and check for 24 volts between B and X on the low voltage terminal strip. “B” is the common connection on these boards for the 24 VAC.  “X” is an output that could be connected to a thermostat to show a “lock-out” condition. If power is present, the refrigeration system is in a lockout on one of the safeties. This could be a high pressure switch, low pressure switch, or suction line freeze stat. On systems with multiple refrigerant circuits, each system is equipped with these safeties.  Check each safety individually. They are wired in series, so a point to point check can be performed. Where voltage is read, that is the open safety.  Be aware that if some time has passed since the unit shut down, the safeties may have reset and will not show open but the unit will still be locked out on the lock-out relay. That is why you need to perform the initial check of seeing if the lockout circuit is activated by checking for power from B to X.

The low pressure/loss of charge switch will only open in case of near total loss of charge. On some units, the switch is located in the liquid line of the unit. It opens at 7 PSIG and closes at 22 PSIG. If this safety is opened, check the refrigerant charge. You will also, probably, need to perform a leak check to determine why the charge was loss.

The high-pressure switch will open when the liquid pressure goes above 380 PSIG and will close once pressure drops to 300 PSIG. The primary cause of this switch tripping is condenser airflow. Check for obstructed or dirty condenser coils or a malfunctioning condenser fan motor and capacitors.Check the capacitors to make sure they are within the correct µf rating.  Amp out the condenser fan motor(s) and make sure they are within the specs on the motor. Keep in mind, that on some of the larger tonnage units, the condenser coil is a “split coil”.  The outer coil could be clean, but the inner coils could be loaded with dirt or cotton wood causing a lack of air flow through the coil. You need to see if the coils is a split coil and that is done by lifting the condenser fan deck and examining the coil.

The suction line freeze stat will open when the refrigerant temperature drops below 26 degrees F. The primary cause for this switch to open will be evaporator air flow.  However, a low refrigerant charge can cause it to open under certain conditions. Check to see if the filters are dirty or plugged. Do the same for the evaporator coil.  Check to see if the blower is operating properly. Check for proper CFM or if registers are closed or returns are blocked.

If voltage was not initially present at B and X, the system is NOT in a lockout due to a refrigerant safety tripping. Since the safeties are not the cause of the unit not running, check for 24 volts at the coil of the 1M and/or 2M contactors.  If 24 volts is present and the contactor does not pull in, replace the contactor.  If 24 volts was not present, check the wiring from the contactor to the board. Don’t forget to check the economizer, if the unit has one, since the Y1 & Y2 signals both went back through the economizer in these units. If there is no economizer, check to see if the J1 jumper on the lock-out relay board is in place on the single stage boards or the economizer by-pass plug is in place on multi-stage units.

INDOOR BLOWER MOTOR: if 2M on the smaller units or 3M on the larger units is pulled in and the blower is not running, shut off the power and carefully feel the blower motor.  If it is hot, the motor may be off on internal overload. Allow the motor to cool down and reset the overload.  Meanwhile, check the wiring to the motor.  Check the capacitor if  it is a single phase motor. Verify that power is present at all line side connections.  If the motor is cold, and power is present at the load side of the contactor, replace the motor.  If the contactor is not pulling in, check for 24 VAC at the coil.  If it is not present, check for 24 VAC out of the board.  On the lock-out relay board, there is a “pilot relay” that energizes the blower contactor.  If “G” is present at the thermostat connections, the pilot relay should be activated and 24 VAC should be present going to the contactor. If 24 VAC is not present, the lock-out relay board may be faulty.

COMPRESSOR: if the compressor is not running, check for power at the line and load sides of the 1M and/or 2M contactors. If power is present, if a single phase unit, check the capacitor.  Also, check the compressor for open, grounded, or shorted windings by removing the wires from the compressor and, using an Ohmmeter, check for resistance of the windings.

On 3 phase units equipped with SCROLL COMPRESSORS, verify that the compressor is running in the right direction. REMEMBER that Scroll compressors can only run in 1 direction.  Check the charge, if pressures are almost equal, or if the amp draw is low, the Scroll compressor may be running BACKWARDS.  As with any 3-phase motor, reverse any two leads to correct the rotation. This can also cause the Scroll Compressor to go off on overload.  Remember, that on the small Sunline 3-phase units, the only 3-phase component may be the compressor.

Hopefully, these checks and diagnostics will help you when working on pre-Simplicity board units.

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About yorkcentraltechtalk

I have been in the HVAC industry most of my life. I worked 25 years for contractors on anything from residential to large commercial boilers and power burners. For the past 23+ years I had been employed by York International UPG Division ( a division of Johnson Controls) as a Technical support/Service Manager but I am now retired. One of my goals has always been to "educate" dealers and contractors. The reason for starting this blog was to share some knowledge, thoughts, ideas, etc with anyone who takes the time to read it. The contents of this blog are my own opinions, thoughts, experiences and should not be construed as those of Johnson Controls York UPG in any way. I hope you find this a help. I always welcome comments and suggestions for postings and will do my best to address any thoughts, questions, or topics you may want to hear about. Thanks for taking the time to read my postings! Mike Bishop
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