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Troubleshooting the TD100 4-20 mA Transmitter Module

Low or No Loop Output

If the loop output is low, unstable, or absent, several conditions should be investigated.

 Troubleshooting the TD100 4–20 mA Transmitter Module

Low or No Loop Output

If the loop output is low, unstable, or absent, several conditions should be investigated.

Output Does Not Track Properly Near Full Scale

Symptoms

The output becomes inaccurate near the upper end of the measurement range.

Cause

The voltage supplied to Pin 4 is too low. The transmitter electronics are not receiving sufficient operating voltage to meet both the transmitter and PLC input requirements.

Corrective Action

  • Verify the supply voltage.
  • Check the loop resistance.
  • Ensure adequate headroom is available.
  • Increase the supply voltage if required.

Output Measures Below 4 mA but Above 3 mA

Symptoms

The measured current is less than 4 mA but remains above 3 mA.

Cause

This condition typically indicates that the loop is reporting an error.

Corrective Action

  • Inspect the transmitter wiring.
  • Verify probe operation.
  • Check the transmitter programming and configuration.

Output Is Stuck at 22 mA When the Tank Is Empty

Symptoms

The loop output remains fixed at approximately 22 mA, even when the tank or vessel is empty.

Cause

The loop has entered an error state, usually because of a transmitter fault.

Corrective Action

  • Inspect the transmitter for faults.
  • Check the wiring connections.
  • Verify that the module is operating correctly.

Output Reading Is Incorrect

Symptoms

The loop output does not match the actual level being measured.

Cause

The accuracy of the 4–20 mA system depends on the strapping table programmed into the transmitter. Any difference between the programmed strapping table and the actual probe length will result in inaccurate measurements.

Corrective Action

  • Verify the probe length.
  • Verify the tank dimensions.
  • Confirm that the correct strapping table is loaded into the transmitter.
  • Reprogram the strapping table if necessary.

Calibration Cannot Be Adjusted Using CAL Mode

Symptoms

The desired calibration cannot be achieved using CAL mode.

Cause

The TD100 4–20 mA transmitter module cannot be adjusted through CAL mode.

Corrective Action

All calibration corrections must be made by modifying the strapping table loaded into the transmitter.

Summary

The 4–20 mA current loop is a reliable and proven method for transmitting measurement signals in industrial applications. Proper operation depends on providing adequate loop voltage, understanding the receiver’s input resistance, and ensuring that the transmitter has sufficient headroom to produce the full 4–20 mA output range.

When troubleshooting, checking the supply voltage, loop current, wiring, and strapping table configuration will resolve most system performance issues.