PLC Reading Wrong After Replacing a 4-20 mA Transmitter Malaysia
PLC Reading Wrong After Replacing a 4-20 mA Transmitter Malaysia
You replaced an old transmitter with a new one.
The new transmitter powers up correctly.
The wiring appears complete.
But now the PLC or HMI reading is wrong.
Does this mean the new transmitter is faulty?
Not necessarily.
After replacing a 4-20 mA transmitter, incorrect readings can be caused by differences in the measurement range, output configuration, wiring, loop power, PLC scaling, engineering units or transmitter setup.
For suitable troubleshooting applications, the FNIRSI SG-003A Signal Generator can provide known test signals to help determine whether the PLC receiving side is responding correctly.
For factory maintenance and instrumentation teams in Malaysia, this can be useful before returning or replacing a new transmitter.
First Question: Is the New Transmitter Really the Same?
Two transmitters may look similar and both provide:
4-20 mA output
but they may not have the same measurement range.
For example:
Old transmitter:
0-10 bar = 4-20 mA
New transmitter:
0-16 bar = 4-20 mA
Both use 4-20 mA.
But they do not represent the same pressure at the same current.
This difference alone can make the PLC reading incorrect.
Example: Old 0-10 Bar vs New 0-16 Bar
For the old transmitter:
0-10 bar = 4-20 mA
Therefore:
4 mA = 0 bar
8 mA = 2.5 bar
12 mA = 5 bar
16 mA = 7.5 bar
20 mA = 10 bar
Now consider the replacement:
0-16 bar = 4-20 mA
Its relationship is:
4 mA = 0 bar
8 mA = 4 bar
12 mA = 8 bar
16 mA = 12 bar
20 mA = 16 bar
If the PLC is still configured for the old 0-10 bar range, the system will not display the actual pressure correctly.
Problem 1: Wrong Measurement Range
This is one of the first things to check after transmitter replacement.
Compare:
Old transmitter range
with
New transmitter range
Examples include:
0-6 bar vs 0-10 bar
0-10 bar vs 0-16 bar
0-100掳C vs 0-200掳C
0-5 m vs 0-10 m
The signal standard may still be 4-20 mA, but the engineering values are different.
Problem 2: New Transmitter Is Configurable
Some transmitters can be configured for different ranges or output settings.
Do not assume that the factory configuration matches the old transmitter.
Check the actual setup according to the manufacturer's instructions.
Important parameters may include:
Lower range value
Upper range value
Output mode
Engineering unit
Damping
Sensor configuration
The available parameters depend on the transmitter model.
Problem 3: Wiring Is Different
The old and new transmitters may not use identical terminal arrangements.
For example, terminal locations or power arrangements may differ.
Never copy the old wiring based only on physical appearance.
Check the new transmitter's wiring diagram.
Confirm:
Power terminals
Signal terminals
Polarity
Loop configuration
Grounding requirements
A transmitter can power up while the output loop is still wired incorrectly.
Problem 4: 2-Wire vs Other Wiring Arrangements
Industrial transmitters can use different wiring arrangements.
The replacement instrument must be compatible with the existing control system.
Before installation, check:
How is the transmitter powered?
How is the output signal connected?
Does the PLC provide or expect a particular loop arrangement?
Does the replacement require a different connection method?
Follow the manufacturer's documentation rather than assuming all 4-20 mA transmitters connect the same way.
Problem 5: PLC Scaling Still Uses the Old Range
This is very common.
Suppose the old transmitter was:
0-100掳C = 4-20 mA
The replacement is:
0-200掳C = 4-20 mA
At 12 mA:
The new transmitter represents:
100掳C
But if the PLC still uses the old scaling, it interprets 12 mA as:
50掳C
The transmitter may be working exactly as configured.
The PLC simply has the wrong engineering range.
Problem 6: Engineering Units Are Different
Check whether the old and new instruments use the same engineering units.
Examples may include:
bar
kPa
MPa
or
掳C
and other configured units depending on the application.
A numerical value can appear wrong simply because the transmitter and control system use different units.
Problem 7: Transmitter Output Is Reversed or Configured Differently
Some applications may use special output configurations.
Do not automatically assume:
4 mA = minimum
and
20 mA = maximum
without checking the actual transmitter configuration.
For most standard linear applications this relationship is common, but configurable equipment may allow different behaviour.
Check the transmitter setup.
Problem 8: PLC Input Configuration Changed
If maintenance work involved more than replacing the transmitter, verify that the PLC analog input configuration has not changed.
Check:
Current input mode
Signal range
Channel assignment
Raw input configuration
Scaling
HMI tag
A configuration change can coincide with transmitter replacement and make diagnosis confusing.
How Can the FNIRSI SG-003A Help?
A useful troubleshooting question is:
Does the PLC still respond correctly to known 4-20 mA signals?
Suppose the PLC should display:
0-10 bar = 4-20 mA
A suitable functional check may include:
4 mA = 0 bar
8 mA = 2.5 bar
12 mA = 5 bar
16 mA = 7.5 bar
20 mA = 10 bar
If the PLC displays these values correctly with known signals, the receiving side is responding according to that scaling.
Attention can then shift towards the new transmitter and field-side configuration.
If the PLC Responds Correctly
Suppose the new transmitter produces a wrong HMI reading.
But known signal testing shows:
4 mA = 0%
12 mA = 50%
20 mA = 100%
This suggests the PLC input and scaling are responding correctly under the test condition.
Check:
New transmitter range
Output configuration
Field wiring
Loop power
Sensor setup
Engineering units
Do not immediately assume the PLC is faulty.
If the PLC Also Responds Incorrectly
If known signals also produce incorrect readings, investigate the receiving side.
Possible areas include:
PLC scaling
Analog input configuration
Wrong PLC channel
HMI scaling
Wrong engineering range
Software changes
This indicates that replacing the transmitter alone may not solve the problem.
Example: Replacement Pressure Transmitter Reads Too Low
The old transmitter was:
0-10 bar = 4-20 mA
The new transmitter is:
0-16 bar = 4-20 mA
Actual pressure is:
8 bar
The new transmitter therefore produces approximately the midpoint signal:
12 mA
But the old PLC scaling interprets 12 mA as:
5 bar
The operator says:
"The new transmitter is reading too low."
In reality, the transmitter may be correct.
The transmitter range and PLC scaling do not match.
Example: New Temperature Transmitter Shows Half the Temperature
Old configuration:
0-100掳C = 4-20 mA
New configuration:
0-200掳C = 4-20 mA
Actual temperature:
100掳C
New transmitter output:
approximately 12 mA
Old PLC interpretation:
50掳C
The displayed temperature is therefore half the expected value.
Before returning the transmitter, compare its configured range with the PLC range.
Example: Replacement Level Transmitter
Old transmitter:
0-5 m = 4-20 mA
Replacement:
0-10 m = 4-20 mA
At an actual level of:
5 m
the new transmitter may output approximately:
12 mA
If the PLC still assumes the old 0-5 m range, it displays approximately:
2.5 m
Again, the problem is the range mismatch.
What Should You Record Before Removing the Old Transmitter?
When possible, record the old transmitter information before replacement.
Useful information includes:
Manufacturer
Model
Measurement range
Output signal
Power requirement
Engineering unit
Terminal wiring
Configuration
PLC channel
Taking clear photos before disconnecting the old transmitter can also help during installation of the replacement.
Do Not Buy a Replacement Based Only on 4-20 mA
This is an important purchasing point.
A customer may say:
"My old transmitter is 4-20 mA. Give me another 4-20 mA transmitter."
That is not enough information.
You should also confirm:
What does it measure?
What is the measurement range?
What is the process connection?
What is the power requirement?
What are the engineering units?
What is the wiring arrangement?
What environmental or process conditions apply?
4-20 mA describes the electrical signal, not the complete transmitter specification.
A Better Troubleshooting Sequence
After replacing a transmitter, check in this order:
1. Compare old and new transmitter specifications
2. Confirm the new transmitter configuration
3. Check wiring and polarity
4. Check loop power
5. Determine the actual output signal
6. Compare the transmitter range with PLC scaling
7. Check the PLC raw input
8. Apply a known test signal where appropriate
9. Check the HMI value
This is more systematic than repeatedly changing hardware.
FNIRSI SG-003A for Maintenance Teams
For suitable applications, the FNIRSI SG-003A can support:
4-20 mA troubleshooting
PLC analog input testing
Transmitter replacement troubleshooting
Control-panel testing
Loop checking
Commissioning
Factory maintenance
Industrial automation
The purpose of the known signal is to help determine which section of the system requires further investigation.
Signal Simulation Does Not Calibrate the Physical Transmitter
A signal generator can help check how the PLC responds to an electrical input.
It does not automatically verify that a pressure transmitter accurately measures pressure or that a temperature transmitter accurately measures temperature.
Full transmitter calibration may require appropriate physical reference equipment, traceability and documented procedures.
Important Safety Reminder
Before disconnecting a transmitter or applying a test signal:
Review the wiring diagram
Confirm the signal type
Check the permitted range
Identify the correct terminals
Confirm polarity
Understand the loop power arrangement
Determine what equipment may respond
Follow site isolation and safety procedures
A simulated process value may activate pumps, valves, alarms, interlocks or other automatic functions.
Looking for FNIRSI SG-003A in Malaysia?
MTM Precision Sdn Bhd supplies the FNIRSI SG-003A Signal Generator in Malaysia for 4-20 mA troubleshooting, PLC testing, transmitter replacement checks and industrial automation applications.
If you have replaced a transmitter but the PLC reading is now incorrect, send us a photo of the old transmitter, new transmitter, PLC analog input module or wiring diagram through WhatsApp.
The old and new transmitter nameplates are especially useful because we can compare their range, signal and basic specifications.
MTM Precision Sdn Bhd
Showroom & Service Centre
No 29-1 & 2, Jalan Bandar 18,
Pusat Bandar Puchong,
47160 Puchong, Selangor, Malaysia
Tel: 03-8080 7172
WhatsApp: +6016-660 7346
Email: mtmpre@yahoo.com
Website: www.mtmpre.com.my
Serving customers across Selangor, Kuala Lumpur, Johor, Penang, Melaka, Negeri Sembilan, Perak, Pahang, Kelantan, Terengganu, Kedah, Perlis, Sabah and Sarawak.
04 Oct 2026
