AC Not Starting or Fan Not Spinning? Dual Run Capacitor 36-505R3744 — Fix Starting Failure Fast
If your air conditioner hums but won’t start, or the condenser fan spins slowly or only when manually pushed, a failed dual run capacitor is the most common cause. In most HVAC systems, the dual run capacitor supports both the compressor and the condenser fan motor from a single component. When it fails, one or both circuits lose the electrical boost needed to start and run. ⚡ Common failure point in residential central air and heat pump systems. Part 36-505R3744 is the Rotom OEM replacement that restores proper starting torque and run efficiency to both motor circuits.
🇺🇸 Matching U.S. Part Number: 36-505R3744
❓ Is This the Right Part?
This is the correct fix if ALL of the following are true:
- ✔ Unit powers on and the contactor is pulling in
- ✔ Electrical supply to the unit is confirmed OK
- ✔ Compressor or condenser fan motor is not seized
- ✔ ONLY problem is the compressor or fan failing to start or running weakly
Not sure? Send us your model number — we’ll confirm before you order.
🔧 Common Symptoms
- AC hums but compressor won’t start
- Condenser fan spins slowly or only when manually pushed
- Unit short-cycles or shuts off shortly after starting
- Higher than normal energy bills with reduced cooling output
- Capacitor is visibly bulged, leaking, or measures out of spec
✔ If these symptoms match, this is the most likely failed component in the system.
✔ In most cases, replacing the dual run capacitor restores normal compressor and fan operation immediately.
📦 Part Details
- Part Number: 36-505R3744
- Type: Dual motor-run capacitor (compressor + condenser fan)
- Capacitance: 50/5 µF
- Voltage: 370/440 VAC
- Shape: Round
- Diameter: 2.36 in
- Height: 5.12 in
- Terminals: C / FAN / HERM
- Features: Built-in pressure interrupter; UL and cUL recognized
- Brand/OEM: Rotom / AllTemp
- Direct Round Equivalents: PRCFD505, PRCFD505A, TRCFD505, 4JR0550, CD50+5X440R, LCD50+5X440R, 12290, 12790, 14290, USA2237
Inspect the contactor at the same time. A worn or pitted contactor is a common companion failure in systems where the capacitor has failed — replacing both during the same service call prevents a repeat call-back.
⚙️ Installation
Skill Level: Moderate — 30 to 60 minutes. Disconnect and lock out power to the unit before opening the electrical compartment. Capacitors retain a hazardous charge after power is removed — discharge safely before handling. Match the terminal labels (C, FAN, HERM) exactly to the original wiring. Confirm mounting strap fit and cabinet clearance before installing — dimensions vary between equivalent brands. Diagnosis and replacement should be completed by a qualified HVAC technician.
✓ Compatible Models
Compatibility for this capacitor is specification-based. Both capacitance values must remain exactly 50/5MFD, and the replacement voltage must equal or exceed the original equipment requirement. Confirm the round shape and dimensions (2.36” × 5.12”) fit the mounting strap and cabinet clearance before ordering. Contact us to verify your model number before ordering.
💡 Pro Tip
Dual run capacitors carry two capacitance ratings — one for the compressor (HERM) and one for the fan (FAN). Both must match exactly. A 45/5 or 50/7.5 capacitor is not a substitute for a 50/5MFD unit even if the voltage and shape match. When in doubt, read the label on the old capacitor directly rather than relying on the unit’s model number alone.
✅Compatibility & Cross Reference
Not sure about your model number? Call 1-877-899-7278 and we’ll help you confirm. All brand names are the property of their respective owners and are used for compatibility reference only.
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