1. Introduction
This manual provides detailed instructions for the installation, operation, and maintenance of the Johnson Controls A421ABG-02C Electronic Temperature Control. The A421 Series controls are single-stage, electronic temperature controls designed for precise temperature management in various applications. Please read this manual thoroughly before installation and operation to ensure proper use and to prevent damage to the unit or associated equipment.
2. Safety Information
Always observe the following safety precautions to reduce the risk of electric shock, fire, or injury:
- Disconnect power before installing or servicing the control.
- Ensure all wiring complies with local and national electrical codes.
- Do not operate the control in environments exceeding its specified operating conditions.
- This device must be installed by a qualified technician.
- Do not attempt to repair or modify the control. Refer servicing to qualified personnel.
3. Product Overview and Features
The Johnson Controls A421ABG-02C is an electronic temperature control unit featuring a single-pole, double-throw (SPDT) output relay. It is designed for both heating and cooling applications.
Figure 1: Johnson Controls A421 Electronic Temperature Control unit. The main unit is a gray rectangular box with a digital display showing '35°F', 'A421', and 'MENU' button, along with up and down arrow buttons. A pre-wired power cord extends from the unit, coiled next to a metallic temperature sensor probe.
Key Features:
- Adjustable backlit LCD for temperature and function status.
- Three-button touchpad for setup and adjustment.
- LED indicator for output relay On/Off status.
- Heating or cooling control capability.
- Sensor offset adjustment.
- Temperature setback function.
- Adjustable anti-short cycle delay.
- Restricted user adjustment mode.
- Temperature display in °F or °C.
- Pre-wired power cord for convenience.
4. Installation and Wiring (Setup)
The A421ABG-02C control is designed for easy installation with its pre-wired power cord. Ensure the power source matches the control's voltage rating (120 VAC).
Mounting:
- Mount the control in a location free from excessive vibration, moisture, and corrosive fumes.
- Ensure adequate ventilation around the unit.
- Use appropriate fasteners to secure the control to a stable surface.
Wiring:
The control comes with a pre-wired power cord. Connect the plug to a standard 120 VAC outlet. The output relay wires are also pre-wired for connection to the controlled device (e.g., heater, compressor). Refer to the internal wiring diagram (typically found on the unit's label or inside the cover) for specific connections to your load.
- Power Input: The pre-wired cord connects to a 120 VAC power source.
- Load Output: Connect the load (e.g., refrigeration unit, heating element) to the designated output terminals or wires. The A421 has an SPDT relay, allowing for both normally open (NO) and normally closed (NC) connections depending on whether heating or cooling is desired.
- Sensor Placement: Position the temperature sensor probe in the area where temperature is to be controlled. Ensure the sensor is not directly exposed to air currents or heat sources that do not represent the true ambient temperature of the controlled space.
5. Operating Instructions
The A421 control features a three-button touchpad for easy operation: MENU, Up Arrow (▲), and Down Arrow (▼).
Setting the Setpoint Temperature:
- Press the MENU button once. The current setpoint temperature will flash on the display.
- Use the ▲ (Up) or ▼ (Down) arrow buttons to adjust the setpoint to the desired temperature.
- Press the MENU button again to save the new setpoint and return to normal operation, or wait a few seconds for it to save automatically.
Viewing Current Temperature:
In normal operation, the display shows the current temperature measured by the sensor.
6. Programming Parameters
The A421 offers several programmable parameters for advanced control. Accessing these parameters typically involves a specific key sequence.
Accessing Advanced Menu:
Press and hold the MENU button for approximately 5 seconds until the first parameter code appears. Use the ▲ or ▼ buttons to scroll through parameters.
Common Parameters:
- Control Mode (C/H): Selects between Cooling (C) or Heating (H) operation.
- Differential (d): The temperature difference between the setpoint and the cut-in/cut-out temperature.
- Anti-Short Cycle Delay (AS): Prevents rapid cycling of the controlled equipment, protecting compressors. Adjustable from 0 to 12 minutes.
- Sensor Offset (OS): Allows calibration of the sensor reading to match a known accurate thermometer.
- Temperature Units (F/C): Toggles between Fahrenheit and Celsius display.
- Restricted User Adjustment Mode (rU): Limits user access to certain parameters.
To adjust a parameter, navigate to it using the arrow keys, press MENU to select, use arrow keys to change the value, and press MENU again to save. Exit the advanced menu by holding MENU or waiting for a timeout.
7. Maintenance
The Johnson Controls A421ABG-02C is designed for reliable operation with minimal maintenance.
- Cleaning: Periodically wipe the exterior of the control with a soft, damp cloth. Do not use abrasive cleaners or solvents.
- Sensor Inspection: Ensure the temperature sensor probe is clean and free from debris that could affect its accuracy.
- Connections: Occasionally check electrical connections for tightness and signs of corrosion. Disconnect power before performing any checks.
8. Troubleshooting
If the control is not functioning as expected, review the following common issues and solutions:
| Problem | Possible Cause | Solution |
|---|---|---|
| Display is blank | No power to the unit | Check power supply, circuit breaker, and connections. |
| Temperature reading is inaccurate | Sensor offset incorrect, faulty sensor, or improper sensor placement | Adjust sensor offset (OS parameter), check sensor wiring, reposition sensor. |
| Controlled equipment not turning On/Off | Incorrect setpoint, differential, control mode, or anti-short cycle delay | Verify setpoint, differential, and control mode (C/H). Check anti-short cycle delay (AS) setting. Ensure load wiring is correct. |
| Unit cycles too frequently | Differential set too low | Increase the differential (d) parameter. |
If problems persist after attempting these solutions, contact Johnson Controls technical support.
9. Specifications
| Specification | Value |
|---|---|
| Model Number | A421ABG-02C |
| Voltage | 120 VAC |
| Temperature Range | -40 to 212°F (-40 to 100°C) |
| Output Relay | SPDT, 12 Amp |
| Enclosure Rating | UL TYPE 1, IP20 |
| Product Dimensions | 7.5 x 6.75 x 5 inches |
| Item Weight | 2.25 Pounds |
10. Warranty and Support
Johnson Controls products are manufactured to high-quality standards. For specific warranty information, please refer to the documentation included with your purchase or visit the official Johnson Controls website. For technical support, troubleshooting assistance, or service inquiries, please contact Johnson Controls customer service or an authorized distributor.
Online Resources: For additional information, product updates, and detailed technical specifications, please visit the Johnson Controls official website.
