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When a run capacitor fails, the outdoor fan motor or compressor may struggle to turn on. The fan may hum or not spin at all instead of running normally.
Specifications
- Part number: 12788
- Brand: Mars
- Part type: Capacitor
- Capacitance: 45+5uf +/-6%
- Frequency: 50/60Hz
- Voltage: 440/370VAC
- µF (MFD): must be the same, like 45/5
- Volts: same or higher (440 can replace 370)
- Shape: round or oval, to fit the bracket
From the maker's and parts sellers' listings for this number. Compare them with the label on your old part.
What This Capacitor Does
This part is a dual run capacitor. A dual run capacitor powers two motors from one can instead of using separate parts. It connects to both the outdoor fan motor and the compressor.
A dual run unit has three terminals on top labeled HERM, FAN, and C. The HERM terminal connects to the compressor. The FAN terminal connects to the outdoor fan. The C terminal is the common connection.
Understanding the Specs
Every capacitor has ratings printed on its outer case. These ratings tell you how much electrical energy it handles and what power supply it requires.
- Capacitance: 45+5uf +/-6%. This means the unit provides 45 uF for the compressor and 5 uF for the fan motor, within a 6 percent tolerance range.
- Voltage: 440/370VAC. This rating means the part works safely on both 370 V and 440 V circuits.
- Frequency: 50/60Hz. This shows the alternating current frequency range the part is built to handle.
Signs of Failure and Safe Replacement
A fan that hums but will not spin is often a bad capacitor, not a bad motor. When the capacitor loses its charge capability, the motor cannot start rotating on its own.
Follow these steps to replace the part safely once you confirm it has failed:
- Turn off power at the household breaker panel and pull the outdoor disconnect switch.
- Test for power at the unit to ensure all electrical current is completely off.
- Discharge each capacitor terminal using an insulated tool before touching any connections.
- Take a photo of every wire before removing the old part so you have a clear reference.
- Move wires one at a time to the matching terminals on the new part, connecting HERM to compressor wires, FAN to the fan wire, and C to common wires.
- Secure the part inside the bracket and restore power to test the system.
Read your old part
- Look at the printed label on your existing capacitor.
- Check the part number to see if it lists 12788.
- Confirm the capacitance rating matches 45+5uf +/-6%, as the uF rating must always be the same.
- Check the voltage rating to ensure your unit uses 440 V or 370 V, since a 440 V part can replace a 370 V part.
- Check the shape of the can to make sure a round or oval part fits securely inside the mounting bracket.
When to call a pro
Contactor and outdoor wiring carries 240 V, which can kill. If you do not have the proper meter to test for power, or if you feel unsure about discharging stored electrical charge safely, stop immediately and hire a licensed HVAC technician.
Questions
Can a 440 V capacitor replace a 370 V capacitor?
Yes. Capacitor voltage must be the same or higher. A 440 V capacitor can replace a 370 V one, but you should never put a 370 V capacitor on a 440 V circuit.
What does a dual run capacitor do?
A dual run capacitor powers two motors from one can. It features three terminals: HERM for the compressor, FAN for the outdoor fan motor, and C for common.
Does the shape of the capacitor matter?
Round and oval capacitors with the same ratings do the same job. You simply need to pick the physical shape that fits your unit's mounting bracket.
Why does my outdoor AC fan hum without spinning?
A fan that hums but will not spin is often caused by a bad capacitor rather than a bad motor. The capacitor provides the necessary push to start the motor spinning.
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- 89M83Capacitor, LennoxSame part type
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