Carel AL55

What does Carel AL55 mean?

AL55 is the Driver 2 High Evaporation Pressure (MOP) alarm on the Carel Standard Air-conditioner (030221426). The EVD driver on circuit 2 has detected that the evaporation temperature has remained above the MOP (Maximum Operating Pressure) threshold longer than the configured delay. The protector closes the expansion valve to reduce evaporation pressure and protect the compressor from overload.

Symptoms

• EVD driver 2 displays MOP protection active.
• AL55 on the controller display.
• Evaporation temperature on circuit 2 is above the MOP threshold.
• Compressor 2 may show elevated discharge temperature as the protector reduces refrigerant flow.

Common causes

• Refrigerant circuit 2 overcharged, causing abnormally high evaporation pressure.
• MOP protection threshold set below the actual rated evaporation temperature of the system.
• High heat load at start-up before circuit 2 reaches steady state.
• Expansion valve on circuit 2 not closing adequately, maintaining high evaporation pressure.
• Condenser on circuit 2 underperforming, raising system pressures.

Diagnostic steps

1. 01

   Read evaporation temperature from EVD driver 2

   Check the current evaporation temperature on the driver 2 display. Confirm it is genuinely above the MOP threshold, not a probe-related false reading.

2. 02

   Review MOP threshold on EVD driver 2

   In the EVD driver 2 service menu, verify the MOP threshold is set above the system's rated evaporation temperature. A threshold set too low causes the protector to activate at normal operating pressures.

3. 03

   Enable MOP protection if disabled

   Confirm the MOP integration time is greater than zero in the EVD driver 2 parameters. A zero value disables the protector, allowing high evaporation pressure to persist without correction.

4. 04

   Check refrigerant charge on circuit 2

   Connect service gauges to circuit 2. High suction pressure relative to ambient saturation temperature indicates refrigerant overcharge. Partial recovery by a certified technician is required.

5. 05

   Stagger start-up sequence for circuits

   If MOP alarms occur consistently at start-up, configure circuit 2 to start at least 30 seconds after circuit 1 to prevent combined transient refrigerant load from exceeding the MOP threshold.

When to call a professional

If MOP alarms persist at normal operating loads with correctly configured threshold and protection parameters, a licensed refrigeration technician should evaluate refrigerant charge, condenser performance, and expansion valve operation. Refrigerant recovery for charge adjustment and pressure switch calibration require EPA 608 or equivalent certification.