Compressor Trip occurs if compressor proving input(s) on the SCM/MCM (1U49-IPAK 1 or 1U3-IPAK 2) does not close within three seconds of the call for a compressor to start or the input opens for 3 seconds during compressor operation. The input is connected to the normally open contacts of an auxiliary switch on the side of the lead compressor contactor on each circuit. This is a manual reset diagnostic.
Some units with later versions of the RTM/MCM typically built or ordered after 02/2009 if the compressor proving input proves closed for 3 seconds it will allow the unit the unit to retry after 15 minutes and on the 4th occurrence go into a Compressor Trip Diagnostic. In Status>Compressor Status Submenu, you will notice that Compressor Relay is Locked due to "Compressor Protection" on the first three occurrences.
Compressor Trip Troubleshooting Steps
Step #1 Safety Devices
If any of the safety devices, such as high-pressure cutout, winding temperature limit, Compressor Protection Module (CPM), or compressor circuit breakers, are open, the contactor or K42 (VSC) cannot be energized, and the diagnostic will occur. Verify all safety devices are closed, the 115-volt control fuses are good, and 115 VAC is present. If all the safeties check good, continue to the next step.
Step #2 Wiring Error
Verify the wiring between the compressor proving input's auxiliary switch and SCM (IPAK 1 20 – 30 ton units) or the MCM (40-162 ton units). Also verify the wiring of the compressor contactor coil circuit. Refer to unit schematics for wire numbers and connections. If the wiring checks good, continue to the next step. On IPAK 1 units there should always be 115VAC at 1TB6-6 (CKT#1) and 1TB6-9 (CKT#2) and on IPAK 2 units at 1TB17-6 (CKT#1) and 1TB17-7 (CKT#2).
Step #3 Switch/Contactor Checkout
Reset the unit diagnostic. Using the Service Mode at the Human Interface, turn on the compressor circuit causing the lockout. If the contactor energizes, verify that the aux. normally open switch closes. Replace switch if it fails to close when contactor pulls in. You should read 23 VDC when the auxiliary is OPEN and 0 VDC when it closes. If 23 VDC is not read check wiring between the SCM/MCM. If no voltage is present on the module with the auxiliary open, then replace SCM /MCM
If the contactor does not energize, check for 115 VAC across coil. If 115 VAC is present, replace contactor. If 115 VAC is not present at contactor coil, verify voltage is present at the SCM J12-15 and at the MCM at J12-11 (CKT#1) and J18-7 (CKT#2). If voltage is not present, check at terminal block locations again to determine where voltage is dropping out. Refer to unit schematics for wire numbers and connections.
If voltage is present, the SCM/MCM may have a defective onboard relay and may need to be replaced.
Step #4 Testing Onboard Relay for MCM CKT#1
Place meter between J12-11 and J12-9 on the MCM and w/o a call for stage 1 cooling you should read 115VAC across these two terminals. When the Stage 1 compressor calls and relay K11 shows “ON”, the voltage should drop from 115VAC to 0 VAC. If not, replace SCM/MCM module. Refer to unit’s schematic to perform same test for SCM and MCM’s Stage 2.
Variable Speed Compressor (VSC) units
The VSC is controlled through a TRV200 VFD and instead of a contactor giving the VFD a run command, its run command is handled by the K42 relay. Compressor Proving is through Relay 2 (terminals 4 and 5) on the VFD. Check for VFD diagnostics and if the VSC does not energize at all, check the K42 relay. There should be a RED LED illuminated when it has 115VAC at the coil. The contacts on these relays at times fail to change states and you may see the RED LED but the contacts may remain open. You can confirm this by reading VDC at terminal 18 on the TRV200 VFD.
If no problems are found, continue to the next step.
Step #4 Control Module
If no problems are found in the above steps, the proving input on the SCM/MCM may not be functioning. Reset the unit diagnostic and place a jumper on the Compressor Proving Input. If a Compressor Contactor Fail diagnostic appears, the input/board is good. If not, replace the SCM/MCM.
I have Attached several MCM/SCM Wiring schematic to reference, The one you are troubleshooting should be located on the door panel, or you can download one from E-library per schematic wiring numbers on unit’s nameplate.
Additional factors to consider when diagnosing intermittent compressor trips include:
- Low refrigerant charge, which may cause the compressor's thermal overload (winding temperature thermostat) to activate.
- For 460/575V units, verify the consistent operation of the Benshaw Motor Protection auxiliary switch.
- Check for consistent operation of the compressor contactor's Normally Open aux switch. This auxiliary switch is connected to the compressor proving input on the SCM/MCM module.
- Inaccurate readings from the saturated condensing temperature sensor. Ensure that the thermistor is properly mounted inside the copper tube brazed to the condenser coil U-bend.
- Confirm that sufficient thermal grease has been applied to the saturated condenser temperature sensor.
- The saturated condenser temperature reading can be accessed in the STATUS > COMPRESSOR STATUS submenu.
WARNING:
Information in this article is intended for use by individuals possessing adequate backgrounds of electrical and mechanical experience and who comply with all federal, state, and local laws, rules, orders, or regulations related to the installation, service, or repair of a heating or central air conditioning product. Any attempt to install, service, or repair a heating or central air conditioning product may result in personal injury and/or property damage. The manufacturer or seller cannot be responsible for the interpretation of the information contained herein, nor can it assume any liability in connection with its use.