Scroll Chillers Critical Interlock

Daikin McQuay AGZ Alarm E3: Compressor Motor Overload — Thermal Trip

Published: 2026-04-29

🚨 Critical Diagnostic Alert

Alarm E3 on the Daikin McQuay AGZ scroll chiller indicates that a compressor's motor overload relay has tripped — the motor current exceeded the relay's trip curve (typically Class 10: trips within 10 seconds at 600% FLA, or within minutes at 125% FLA). The overload relay is a thermal device — it heats up proportionally to the motor current and trips a mechanical switch when the internal bimetallic strip reaches the trip temperature.

E3 is a manual-reset alarm by design — the technician must physically reset the overload relay at the chiller. This forces an on-site inspection, which is appropriate because an overload trip usually indicates a genuine problem (electrical supply, refrigerant operating conditions, or compressor mechanical condition) rather than a transient event.

🔍 Symptoms Checklist

⚠️ Specific compressor trips on motor overload
⚠️ Compressor contactor is energized but overload relay has tripped
⚠️ Compressor housing is too hot to touch (>180°F)
⚠️ Alarm does not auto-reset — requires manual reset at the overload relay

🛠️ OEM Replacement Parts

Part Name	OEM Part Number	Est. Price
Compressor Overload Relay (Manual Reset, Class 10)	`DKN-OLR-010`	$160
Compressor Contactor (3-Pole, 40A with 24V Coil)	`DKN-CONT-40`	$140

📋 Interactive Diagnostic Procedure

Click each step to expand detailed diagnostic instructions. Follow in sequence — each step builds on the previous one.

1 Measure Compressor Running Amps vs. RLA

After resetting the overload relay, measure the compressor's running amps on all three phases after 10 minutes of operation at full load. All three phases should read within 10% of each other and below the compressor's rated load amps (RLA) on the nameplate. Amps at 120%+ RLA indicate a mechanical overload — the compressor is working against abnormally high head pressure, or the scroll set has internal mechanical drag from wear or liquid slugging damage.

2 Check Voltage Balance at Compressor Terminals

A 2% voltage imbalance causes approximately a 10% current imbalance, which can push one phase above the overload relay trip point while the other two phases read normal. Measure all three phase-to-phase voltages at the compressor contactor input. Calculate imbalance: (max deviation from average) ÷ average × 100. If imbalance exceeds 2%, the problem is in the building's electrical supply — not the chiller. Correct at the distribution panel (check for loose lugs, corroded connections, unbalanced loading across phases).

3 Inspect Overload Relay Trip Setting

The overload relay's current dial must match the installed compressor's RLA. If a compressor was replaced previously and the new compressor has a different RLA than the original, the overload relay setting may not have been updated. Compare the dial setting against the compressor nameplate RLA. A relay set to 15A when the compressor RLA is 18A will nuisance-trip. A relay set to 22A with a 18A compressor will not protect the motor.

🔗 Related Error Codes — Scroll Chillers

Other diagnostic guides in the same equipment category. Common failure patterns often share root causes — cross-referencing speeds up diagnosis.

Scroll Chillers Critical Interlock

⚡ Fault-Induced Energy Waste Estimator

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References & Industry Standards

ASHRAE 15 — Safety Standard for Refrigeration Systems
ASHRAE 34 — Refrigerant Designation & Safety Classification
AHRI 550/590 — Water-Chilling & Heat Pump Packages
SMACNA — HVAC Duct Construction Standards
Manufacturer Service Manuals — Carrier, Trane, York, Daikin, Lennox

Daikin McQuay AGZ Alarm E3: Compressor Motor Overload — Thermal Trip

🔍 Symptoms Checklist

🛠️ OEM Replacement Parts

📋 Interactive Diagnostic Procedure

🔗 Related Error Codes — Scroll Chillers

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⚡ Fault-Induced Energy Waste Estimator

References & Industry Standards