As the cooling season begins to wind down, I’ll have a few more posts on air conditioning service and diagnostics before we get into the heating season and discuss topics relating to heating and service.
This post will address how to properly diagnose scroll compressors. (Information provided in this post is directly from Copeland )
So, continuing on with compressor diagnostics, scroll compressors need to have the following functional tests performed to determine if a compressor is bad.
A functional compressor test with the suction service valve closed to check how low the compressor will pull suction pressure is not a good indication of how well a compressor is performing. Such a test will damage a scroll compressor. Also keep in mind, that you CANNOT pump down a unit with microchannel coils. The holding capacity of the coil will not allow pump down of refrigerant into the coil. this can also lead to damaging the compressor.
The following diagnostic procedure should be used to evaluate whether a Copeland Scroll compressor is working properly:
1. Proper voltage to the unit should be verified.
2. The normal checks of motor winding continuity and short to ground should be made to determine if an internal motor short or ground fault has developed. If the protector has opened, the compressor must be allowed to cool sufficiently to allow it to reset.
3. Proper indoor and outdoor blower/fan operation should be verified.
4. With service gauges connected to suction and discharge pressure fittings, turn on the compressor. If the suction pressure falls below normal levels, the system is either low on charge or there is a flow blockage in the system.
5. If suction pressure does not drop and discharge pressure does not rise to normal levels, on 3-phase scroll compressors, reverse any two of the compressor power leads and reapply power to make sure compressor was not wired to run in reverse direction. If pressures still do not move to normal values, either the reversing valve (if so equipped) or the compressor is faulty. Reconnect the compressor leads as originally configured and use normal diagnostic procedures to check operation of the reversing valve.
6. To test if the compressor is pumping properly, the compressor current draw must be compared to published compressor performance curves using the operating pressures and voltage of the system. If the average measured current deviates more than ±15% from published values, a faulty compressor may be indicated. A current imbalance exceeding 15% of the average on the three phases may indicate a voltage imbalance and should be investigated further.
7. Before replacing or returning a compressor: Be certain that the compressor is actually defective.
And Finally, if you do find a “bad” compressor, be sure to determine the cause of the original failure and correct it. Make sure all wiring is correct and connections are “tight”. Make sure the run (and start gear) capacitors are the correct size and the µf rating is within the tolerance rating on the capacitor and the voltage rating of the capacitor matches the new compressor (on single phase units). Verify correct air flow both across the evaporator and the condenser coil. Just replacing a compressor without correcting what caused the original compressor to fail will only lead to another compressor failure in the future.
Believe it or not, more than one-third of compressors returned to Copeland for warranty analysis are determined to have nothing found wrong. They were misdiagnosed in the field as being defective. Replacing working compressors unnecessarily costs everyone.