Carel AL34

What does Carel AL34 mean?

AL34 on the Carel Standard Roof-top controller (program +030221501 on pCO1/pCO3/pCOxs hardware) indicates that the thermal overload protection device on Compressor 4 has tripped. The thermal overload relay or internal motor protector opens when the compressor motor draws current beyond its rated limit — typically from mechanical overload, single-phasing, high discharge pressure, or a winding fault. The controller stops Compressor 4 immediately and requires a manual reset.

Symptoms

• AL34 displayed on the pCO terminal with Compressor 4 output off.
• Manual reset is required — the alarm does not clear automatically.
• Overall system cooling capacity is reduced proportionally to the loss of Compressor 4's contribution.
• The thermal overload relay or motor protector in the compressor circuit is in a tripped or open state.
• The alarm relay activates and the buzzer sounds.

Common causes

• High discharge pressure forcing the compressor to work against excessive head, increasing motor current.
• Compressor motor winding failure or insulation degradation drawing elevated current.
• Single-phase supply condition on a three-phase compressor causing unbalanced and elevated phase currents.
• Thermal overload relay sized or calibrated incorrectly for the installed compressor motor rating.
• Compressor operating at low suction pressure (starved of refrigerant) and running unloaded for extended periods causing overheating.
• Ambient temperature around the control panel too high, causing the thermal overload bimetal to trip prematurely.

Diagnostic steps

1. 01

   Identify the protection device that tripped

   Locate the compressor 4 thermal overload relay or electronic motor protector in the control panel. It will be in a visibly tripped position. Allow the bimetal relay to cool for at least 15 minutes before attempting to reset it.

2. 02

   Measure phase voltages and current

   Before resetting, measure all three supply phase voltages at the compressor contactor. Low or absent voltage on any phase confirms single-phasing. If voltage is correct, reset the relay and immediately measure all three running currents with a clamp meter and compare to the compressor nameplate FLA.

3. 03

   Check condenser conditions

   Inspect condenser fans and coil cleanliness. Elevated discharge pressure (high head) is the most common driver of compressor overcurrent — resolve any condenser faults before restarting Compressor 4.

4. 04

   Verify thermal overload relay setting

   Confirm the overload relay trip current matches the compressor motor full-load amps (FLA) as stated on the compressor nameplate. An incorrectly sized overload set too low will trip without a true overload condition.

5. 05

   Reset the overload and clear AL34

   After cooling and investigating, manually reset the thermal overload relay. Then press the Alarm button on the pCO terminal to display AL34 and press again to reset the controller alarm. Monitor Compressor 4 current during the first few minutes of operation.

When to call a professional

Call a licensed refrigeration engineer or electrician if Compressor 4 draws current above the nameplate FLA after reset with no obvious cause, or if the thermal overload trips again within minutes. Repeated trips indicate a compressor winding fault or refrigerant circuit problem that requires refrigerant-side diagnosis, insulation resistance testing, and potentially compressor replacement. Do not bypass or upsize the thermal overload relay — it is a primary protection device.