Aircon Outdoor Unit Not Running

Case Details

What We Checked

• Indoor unit was sending the start command correctly — communication link tested normal.
• Mains voltage present at the outdoor terminal block — supply was not the issue.
• Contactor relay coil energised when the start command was received, but contacts were pitted and not closing cleanly.
• No voltage reaching the compressor terminals despite relay actuation — open circuit across the contact faces.
• Compressor windings tested with a megger — insulation resistance normal, no short to ground.

The Diagnosis

Every time the system calls for cooling, the contactor relay closes its contacts to connect mains power to the compressor. Over nine years and thousands of switching cycles, tiny electrical arcs formed across the contact surfaces each time the relay opened and closed. These arcs eroded the contact metal and left pitted, uneven surfaces. Eventually the pitting was severe enough that the contacts could no longer make a clean electrical connection when the relay coil energised. The relay coil pulled in mechanically, but the damaged contact faces could not pass current to the compressor. From the outside, the outdoor unit looked completely dead — no compressor hum, no fan spin, no sound at all. But the compressor motor windings, the run capacitor, the outdoor PCB, and the fan motor were all tested and found to be in working condition. The unit was not dead — the power path to start it was broken at a single relay.

What Fixed It

We replaced the contactor relay with a matching unit rated for the same coil voltage and contact current — 24V coil, 25A contacts on this Daikin model. After installation, we energised the relay and measured voltage at the compressor terminals to confirm a clean power path with no voltage drop across the new contacts. The outdoor unit started on the first command without hesitation. We ran a full cooling cycle, monitoring compressor current draw with a clamp meter — the reading held steady at 6.8A, within the manufacturer's specification. We also checked discharge temperature at the outdoor coil with a contact probe and measured the temperature differential at the indoor coil to confirm the refrigerant circuit was performing normally after the idle period.

Cooling returned to normal on the first cycle. The compressor and all other outdoor components were retained without further work.

Why This Happens

A silent outdoor unit does not always mean a failed compressor.

• The compressor cannot start if the contactor relay never closes. A completely silent outdoor unit may simply mean the start signal was never delivered to the motor, not that the motor has seized. The two conditions look identical from the outside — no hum, no fan, no sound at all.
• Contactors wear from electrical arcing across their contacts over thousands of switching cycles. Each time the relay opens under load, a small arc erodes the contact surface and deposits carbon on the metal. Over nine years and an estimated fifteen thousand cycles, the pitting becomes severe enough to prevent current flow even when the relay coil pulls in mechanically.
• Testing the contactor response and the start-path voltage takes a few minutes with a multimeter. Measuring voltage at the compressor terminals while the relay is energised immediately reveals whether power is reaching the motor. Full voltage at the relay coil but zero at the compressor terminals means the relay contacts are the bottleneck — the diagnosis is definitive.
• A contactor replacement is a fraction of the cost of a compressor swap or full outdoor unit replacement — typically under two hundred dollars versus several thousand. Ask your technician whether they tested the start path before recommending major parts. A silent outdoor unit deserves a relay check first.