Magnetic sensors installed on hydraulic cylinders are widely used to monitor piston position in injection molds. When a sensor fails repeatedly, however, replacing the sensor is not necessarily the correct solution.
A technical case handled by the Vega Team shows why repeated sensor failures should be investigated at system level, considering temperature, electrical supply, wiring, magnetic detection and possible voltage transients generated elsewhere in the machine.
The application was particularly demanding: the mold operated continuously, with approximately 32,000 cycles per month, and 12 switches had failed within one year.
The original technical discussion also included the use of a dedicated tester for checking magnetic sensors and the possibility of using an oscilloscope to investigate electrical disturbances.
When a Magnetic Sensor Keeps Failing, Do Not Automatically Blame the Sensor
A magnetic switch is only one component of a larger system:
Cylinder → Magnet → Sensor → Wiring → Power Supply → PLC
A failure detected at the sensor does not necessarily mean that the sensor itself is the original cause of the problem.
This distinction is particularly important when several sensors fail repeatedly under the same operating conditions.
In the application analyzed by the Vega Team, the Customer reported frequent sensor failures despite having already modified the sensor mounting arrangement to reduce heat transfer from the cylinder.
The application involved:
- continuous operation;
- approximately 24 hours per day;
- approximately 32,000 cycles per month;
- two mold bases;
- two hydraulic cylinders on each base;
- 12 switches burned during a 12-month period.
This type of failure pattern calls for a structured diagnostic procedure rather than simply installing another replacement sensor.
1. Check the Actual Sensor Temperature
Temperature was one of the first parameters considered in the technical investigation.
The Customer had already moved the sensors away from the cylinder body using a specially made bracket in an attempt to reduce heat transfer.
The important point is that the relevant temperature is not necessarily the temperature displayed by the injection machine.
What matters is the actual temperature experienced by the sensor in its installed position.
Measure the temperature where the sensor is installed
In the case analyzed, the Customer inserted a thermocouple into the cylinder sensor well and connected it to the press monitoring system.
The measured temperature was approximately:
164 °F / 73 °C
The system was monitored for one hour and the measured temperature did not exceed 164 °F, including during injection.
This was a useful diagnostic step because it replaced an assumption about temperature with an actual measurement.
2. Sensor Temperature Is Different from Mold Temperature
The temperature around a hydraulic cylinder can be influenced by several factors:
- mold temperature;
- oil temperature;
- heat conduction through the cylinder;
- sensor position;
- cycle frequency;
- duration of operation;
- distance between the sensor and the hot area.
For this reason, a temperature measured somewhere else on the mold does not necessarily represent the temperature experienced by the sensor.
The best diagnostic approach is to measure as close as possible to the actual sensor location.
3. Check the Electrical Supply
The Customer also reported an unusual electrical condition.
During some failures, approximately 16–18 VDC were measured at the contact instead of the expected 24 VDC.
This raised an important question:
Was the reduced voltage caused by the magnetic field, the magnet, the cylinder, the oil or the sensor itself?
The Vega Team’s technical response was that the voltage variation should not automatically be attributed to the magnetic field or oil.
The switches provide a signal to the PLC and do not directly drive the eventual load where a high current could explain the voltage reduction. Vega therefore recommended checking whether the voltage decrease was present only in the sensor contact circuit or elsewhere in the machine circuit as well.
4. A Voltage Drop Does Not Automatically Mean a Magnetic Problem
This is an important diagnostic distinction.
The magnetic circuit and the electrical circuit perform different functions.
The magnetic field is responsible for activating the sensor.
The electrical circuit is responsible for supplying and transmitting the sensor signal.
Therefore, if an abnormal voltage is measured, the investigation should establish where the voltage changes occur.
A useful sequence is:
24 V supply
→ sensor
→ sensor output
→ wiring
→ PLC input
If the voltage is already abnormal before reaching the sensor, replacing the sensor will not solve the underlying problem.
5. Check the Sensor with a Dedicated Tester
The documentation supplied with the case includes the Vega MRC2 controller, specifically designed to test magnetic sensors.
According to the MRC2 instructions, the device can be used to:
- check magnetic sensors with Reed contacts;
- test electronic sensors;
- test PNP and NPN sensors;
- recognize the sensor polarity;
- check the position of the magnetic ring inside the cylinder;
- activate magnetic sensors using its internal magnet.
The same manual also provides connection diagrams for different sensor types, including Reed switches and electronic sensors.
This makes a dedicated tester a useful first diagnostic tool when the objective is to determine whether the sensor itself is operating correctly.
6. How the MRC2 Tester Works
The MRC2 instructions describe a straightforward testing procedure.
The sensor is connected to the tester and the TEST ON button is pressed.
The red LED initially indicates the testing condition. When the sensor is activated, the corresponding LED indicates the detected polarity for the sensor type being tested.
The tester can also be used to investigate the magnetic detection area of a cylinder.
The manual explains that the magnetic area can be located by bringing the tester close to the cylinder and moving it along the cylinder body perpendicular to the tube.
This is particularly useful when the objective is to determine whether the sensor is responding correctly to the magnetic field.
7. The Tester Checks the Sensor, Not the Entire Machine
It is important to understand what this test can and cannot prove.
If the sensor works correctly with the MRC2, this demonstrates that the sensor can operate correctly under the test conditions.
It does not automatically prove that:
- the machine supplies the correct voltage;
- the wiring is correct;
- the PLC input is functioning correctly;
- there are no electrical transients;
- the sensor is correctly positioned in the mold.
Conversely, if the sensor does not operate correctly with the tester, the investigation can focus more directly on:
- the sensor;
- its connection;
- the magnetic field;
- its position relative to the cylinder.
This separation between sensor testing and machine-circuit testing is essential.
8. Check the Wiring and Connectors
The physical condition of the sensor wiring should also be inspected carefully.
In the documented application, the Customer reported discoloration and burning on the switch leads where they entered the cylinders.
This type of evidence should not be ignored.
When a sensor fails repeatedly and the wiring also shows signs of burning or discoloration, the investigation should include:
- cable condition;
- connector condition;
- terminal connections;
- supply voltage;
- possible overheating;
- possible electrical transients.
Replacing only the sensor could leave the original cause untouched.
9. Investigate Electrical Transients
Another possible cause considered by the Vega Team was electrical interference generated when solenoids are switched off.
This is particularly relevant in injection molding machines because hydraulic valves, relays and other inductive devices can be switched frequently.
The technical correspondence describes a previous application in which an oscilloscope revealed that the cause of the sensor problems was a reverse voltage generated during the printing cycle.
The technical explanation was that devices such as:
- solenoid valves;
- contactors;
- relays;
can generate reverse voltages when their coils are deactivated. The documented case notes that these transient voltages can reach approximately −60 to −100 V for a few milliseconds.
10. Why an Oscilloscope May Be Necessary
A conventional multimeter is useful for checking a stable DC voltage.
It may not, however, reveal a very short voltage transient.
A voltage pulse lasting only a few milliseconds can therefore remain unnoticed during a conventional measurement.
An oscilloscope allows the technician to observe the voltage as a function of time and identify:
- voltage spikes;
- reverse voltage;
- short-duration transients;
- abnormal switching behavior.
In the previous application described by the Vega Team, this type of measurement was decisive in identifying the electrical problem.
11. Check the Protection of Inductive Loads
The Customer in the documented case had already installed transient-voltage protection on the solenoid coils.
This is an important consideration.
Protecting the sensors alone is not necessarily enough.
The sources of electrical transients should also be considered.
The Vega technical correspondence describes the use of arc-quenching diodes installed in parallel on the power supply of the sensors and relay contacts as a possible solution in the previous application.
The exact protection method, however, should always be evaluated according to the electrical architecture of the machine.
12. Do Not Assume That the Magnet Is the Cause
A common troubleshooting mistake is to assume that an abnormal sensor signal must be caused by the magnet inside the cylinder.
The documented technical response specifically stated that the observed voltage decrease had no relationship with the magnetic field or oil.
This does not mean that the magnetic system should never be checked.
It means that the diagnosis should distinguish between:
magnetic detection problem
and
electrical signal problem.
They can produce similar symptoms but require completely different solutions.
13. Verify the Magnetic Detection Area
The MRC2 manual provides a practical method for checking the magnetic detection area.
The tester can be moved along the cylinder to identify the point at which the magnetic signal is detected.
This is useful for checking whether:
- the sensor is correctly positioned;
- the magnetic field reaches the expected area;
- the sensor switches consistently;
- the cylinder provides the expected magnetic detection.
Vega’s current documentation also confirms that magnetic sensors are an integral part of hydraulic-cylinder position monitoring and offers dedicated magnetic-sensor solutions for its cylinders.
14. Sensor Position Must Be Considered Together with the Cylinder
A sensor is not simply an electrical component attached to the outside of a cylinder.
Its position depends on:
- cylinder construction;
- magnetic system;
- piston position;
- available sensor groove;
- installation space;
- temperature;
- required switching position.
Vega’s current technical documentation emphasizes that sensor integration should be considered during cylinder selection and mold design rather than added as an afterthought.
This becomes especially important when a sensor is moved away from its original position to reduce temperature.
The new position may improve thermal conditions while simultaneously changing the relationship between the sensor and the magnetic field.
15. Short Effective Strokes Can Make Sensor Adjustment More Critical
Another aspect that should be considered in applications using magnetic sensors is the effective stroke.
If a cylinder has a relatively long nominal stroke but the mold uses only a very small portion of it, sensor adjustment can become much more demanding.
Vega’s current technical documentation explains that when only a small portion of the available cylinder stroke is used, the adjustment window for magnetic switches becomes narrower and installation tolerances become more critical.
This does not mean that a partial stroke automatically causes sensor failure.
It means that the sensing system should be evaluated according to the actual movement used by the mold, rather than only the nominal cylinder stroke.
16. A Practical Diagnostic Procedure
For repeated magnetic-sensor failures, a structured troubleshooting sequence can significantly reduce unnecessary component replacement.
Step 1 – Check the physical condition
Inspect:
- sensor body;
- cable;
- connector;
- terminals;
- signs of burning;
- discoloration;
- mechanical damage.
Step 2 – Measure the actual temperature
Measure the temperature close to the sensor, preferably under real production conditions.
Step 3 – Check the power supply
Verify the voltage supplied to the sensor.
If the nominal supply is 24 VDC but a significantly lower value is measured, establish where the voltage drop occurs.
Step 4 – Test the sensor independently
Use the MRC2 or an equivalent appropriate tester to determine whether the sensor itself switches correctly.
Step 5 – Check the magnetic field
Verify that the sensor is positioned correctly relative to the cylinder’s magnetic detection area.
Step 6 – Check the wiring
Inspect the complete signal path between the sensor and the PLC.
Step 7 – Check the PLC input
If the sensor works correctly but the PLC does not receive the expected signal, the fault may be downstream of the sensor.
Step 8 – Investigate electrical transients
If the previous checks do not explain the failure, investigate switching transients from solenoids, relays and other inductive loads.
Step 9 – Use an oscilloscope when necessary
If short-duration voltage spikes are suspected, an oscilloscope may be required to identify them.
17. Centralized Sensor Monitoring Can Simplify Diagnostics
When a mold contains several hydraulic cylinders, monitoring all the sensors individually can become increasingly complex.
Vega’s current SIM08 documentation describes a centralized interface for multiple sensor signals, including PNP magnetic switches, PNP inductive switches, Reed switches and mechanical switches.
This approach can make it easier for the operator to see the condition of the individual sensors and identify an abnormal signal.
For complex molds, centralized monitoring can therefore complement the individual diagnostic process.
18. Sensor Selection Depends on the Cylinder
Not every hydraulic cylinder can use the same sensor technology.
The sensor must be compatible with the cylinder construction.
For example, Vega’s current documentation explains that some all-steel cylinder configurations require mechanical switches rather than magnetic switches.
This is why sensor selection should be made together with cylinder selection.
A sensor should not be treated as a generic accessory that can simply be added to any cylinder.
19. The Main Lesson from the Application
The most important lesson from this technical case is that repeated sensor failures should be approached as a system diagnosis.
A useful troubleshooting sequence is:
Temperature
↓
Power supply
↓
Sensor
↓
Magnetic field
↓
Wiring
↓
PLC
↓
Electrical transients
↓
Machine protection
This approach helps distinguish between a genuinely defective sensor and a sensor that is being damaged by another condition in the machine.
Conclusion
Repeated magnetic-sensor failures on hydraulic cylinders should not automatically be solved by replacing the sensor.
In the documented application, the mold operated under demanding conditions, with approximately 32,000 cycles per month and 12 failed switches in one year. The Customer had already modified the sensor mounting arrangement to reduce heat transfer, yet the failures continued.
The investigation therefore had to consider several possible causes:
- actual sensor temperature;
- supply voltage;
- wiring;
- magnetic field;
- sensor operation;
- PLC signal;
- electrical transients generated by inductive loads.
The supplied MRC2 documentation shows that a dedicated tester can be used to check different magnetic and electronic sensor types, verify polarity and investigate the magnetic detection area of the cylinder.
The technical correspondence also demonstrates why an oscilloscope can become necessary when short electrical transients are suspected: in a previous application, it revealed reverse-voltage spikes generated during the deactivation of inductive devices.
The key principle is therefore:
A magnetic sensor that repeatedly fails should not automatically be considered the root cause. It may be the component where an underlying problem in the machine becomes visible.
A structured diagnosis—starting with temperature, supply voltage and direct sensor testing, and progressing to wiring, PLC signals and electrical transients when necessary—is a much more reliable way to identify the real cause.
Useful and Verified URLs
I checked the links against the official icvega.com and vegacylinders.com domains.
- Monitoring Hydraulic Cylinder Position Sensors on Injection Molds — Official Vega technical article explaining the role of hydraulic-cylinder position sensors and centralized monitoring in injection molds.
- How to Connect Multiple Hydraulic Cylinders to a PLC Using a Signal Interface Module — Official Vega article covering multiple sensor connections, SIM08 and PLC signal management.
- Magnetic Sensors for Hydraulic Cylinders — Official Vega product category for magnetic position sensors used with hydraulic cylinders.
- Why Using Only Part of a Hydraulic Cylinder Stroke Can Create Sensor Problems — Official Vega technical article explaining why short effective strokes can make magnetic-sensor adjustment more critical.
- How Minimum Stroke Affects Position Sensor Performance in Hydraulic Cylinders — Official Vega article about the relationship between cylinder stroke and reliable position detection.
- Why Some Injection Molds Require Custom Hydraulic Cylinders — Official Vega article explaining why sensor integration should be considered during cylinder selection and mold design.
The attached MRC2 controller instructions are also directly relevant: the manual includes the sensor-testing procedure and connection diagrams for Reed, electronic, PNP and NPN sensors.
