Outside AC Unit Not Running but Inside Is: Follow the 24 Volts

When the blower inside runs on a cooling call and the outdoor unit sits there doing nothing, the indoor side is getting its call. The outdoor unit isn't. Silence (no hum, no click) usually means the contactor never pulled in, so start there and work in whichever direction the meter sends you.

If the outdoor unit hums but doesn't run, that's a different problem. See AC outdoor unit humming but not running.

Safety: Contactor line terminals are live with the disconnect on. Use a meter rated for the circuit and keep hands clear of the fan.

Start at the contactor coil

With the thermostat calling for cooling, measure across the two low-voltage coil terminals on the contactor.

The ACHR News puts it simply: if 24 V isn't reaching the condensing unit, the problem is outside the condensing unit.

If the contactor is pulled in: line voltage

1. Measure L1 to L2 on the line side of the contactor. Compare to the nameplate voltage.
2. No voltage: check the disconnect (pull-out or fused), its fuses, and the breaker. A fused disconnect with one blown fuse gives you 120 V to ground on one leg and nothing across the two.
3. Voltage on the line side but not across T1–T2 with the contactor closed: burned contacts. Replace the contactor.
4. Voltage across T1–T2 but nothing runs: follow the wiring to the compressor, fan and capacitor. Both stopping at once points at a common connection.

If there's no 24 V at the coil: follow Y and C

Work back from the outdoor unit one point at a time.

1. At the outdoor unit's low-voltage pigtails: measure Y to C where the thermostat wire enters the unit. Voltage here but not at the coil means something inside the outdoor unit is open (a pressure switch, a time delay or a board).
2. At the air handler or furnace terminal board: measure Y to C. Voltage here but not outdoors means the wire between them is broken, or the C wire is open. Rodents, string trimmers and corroded splices are common.
3. No Y at the indoor board: the thermostat isn't calling, or something between it and the board opened the Y circuit (often a float switch).

The usual causes, by where they sit

Condensate float switch

A float switch in the drain pan or drain line often opens Y (or R) to stop the outdoor unit when the drain backs up. The blower keeps running, so the customer gets warm air and no water on the floor. Check the pan and the drain before you go any further. See condensate float switches.

High- or low-pressure switch

Many condensing units run the Y wire through a high-pressure switch, a low-pressure switch, or both before it reaches the contactor coil. An open switch looks exactly like a missing call. A Nordyne R-410A installation manual notes its high-pressure switch interrupts the thermostat inputs to the unit, and the unit then waits through a 5-minute short-cycle delay after the switch closes.

• High-pressure switch open: dirty condenser coil, failed fan, overcharge or non-condensables. Check the condenser fan and the coil.
• Low-pressure switch open: low charge, a restriction, or a frozen indoor coil. See how to find a refrigerant leak and frozen evaporator coil causes.

With power off, ohm across each switch. An open switch with normal standing pressure in the system is a failed switch. An open switch with abnormal pressure is doing its job.

Short-cycle time delay

Many outdoor units and thermostats delay the compressor after it stops or after power is restored. Carrier's literature describes its cycle protector as an approximate 5-minute delay after compressor power is interrupted for any reason. If you just reset the breaker or flipped the thermostat, wait it out before you condemn anything.

Transformer, fuse and thermostat

If the indoor blower runs on a fan call but nothing happens on a cooling call, the transformer is probably fine. Check:

• The 3- or 5-amp low-voltage fuse on the furnace or air handler board. A shorted Y or C wire outdoors will blow it. See furnace blown 3-amp fuse.
• The thermostat itself: jump R to Y at the indoor board for a minute. If the outdoor unit starts, the problem is the thermostat or its wiring. See thermostat wiring terminals and colors.
• Heat pumps: a defrost or control board in the outdoor unit can hold the contactor off. Check its LED codes.

Common mistakes

• Replacing the contactor without measuring the coil voltage. If the coil has no 24 V, a new contactor won't help.
• Jumping out a pressure switch and walking away. It opened for a reason. Find the reason.
• Missing the float switch. It's often tucked on the drain line, not in the pan.
• Not waiting out a time delay right after a power reset.
• Using the house's R to Y jumper as a fix. It's a test, not a repair.

FAQ

Sources

• ACHR News, "Troubleshooting Single-Phase Residential Compressors"
• Nordyne R-410A split system installation instructions (high-pressure switch and short-cycle delay)
• Carrier Puron 38TXA manual (cycle protector, about 5 minutes)
• Service Magazine, "Common HVAC Electrical Failures: Contactors, Capacitors, Relays, and More"