Testing a Sending Unit: Understanding the Process and Common Issues

[MikePhua]

Introduction to Sending Units
A sending unit is a crucial component of many vehicles and heavy machinery, serving to relay data from mechanical systems to electronic gauges or displays. It is typically used in systems that monitor fluid levels, temperature, or pressure. Common examples include fuel sending units, temperature sending units, and oil pressure sending units. These devices help operators maintain optimal performance by providing real-time data on the machine's critical systems.
However, like all components, sending units can fail or provide inaccurate readings over time. In this article, we will discuss how to test a sending unit, common issues that can arise, and how to troubleshoot these issues effectively.

How a Sending Unit Works
A sending unit operates by converting a mechanical measurement into an electrical signal that can be interpreted by a gauge. For example, in a fuel tank, the sending unit uses a float connected to a variable resistor. As the fuel level changes, the float moves, adjusting the resistance. This change in resistance sends an electrical signal to the gauge, providing the operator with an accurate fuel level reading.
The basic function of a sending unit involves:

• Measuring: A physical measurement (e.g., fuel level, engine temperature, oil pressure).
  
• Converting: The measurement is converted into a variable resistance or voltage.
  
• Transmitting: The electrical signal is sent to a gauge or display.
  

Common Types of Sending Units

1. Fuel Sending Units
   Fuel sending units are used to measure the amount of fuel in a tank. They are commonly found in vehicles, construction equipment, and farm machinery. These units typically use a float mechanism that moves with the fuel level, sending a corresponding signal to the fuel gauge.
   
2. Oil Pressure Sending Units
   These units measure the oil pressure within the engine and send that information to the oil pressure gauge. They are vital for monitoring engine health, as low oil pressure can indicate potential issues.
   
3. Temperature Sending Units
   Temperature sending units measure the engine coolant temperature or the temperature of various fluids and send that data to the vehicle’s temperature gauge. These are essential for avoiding engine overheating.
   
4. Pressure Sending Units
   Pressure sending units measure fluid or air pressure in hydraulic or pneumatic systems. These systems often use a sending unit to relay pressure readings to operators, ensuring that machinery operates safely within the required pressure range.
   

Testing a Sending Unit
Testing a sending unit is important when troubleshooting issues like inaccurate readings or a non-functional gauge. Below are the steps to test the sending unit, as well as some tips for diagnosing and resolving common problems.
1. Check the Gauge
Before testing the sending unit itself, ensure that the problem isn't with the gauge or the wiring. If the gauge is faulty, it can give the appearance of a problem with the sending unit. To test the gauge:

• Ensure that the wiring connections to the gauge are clean and secure.
  
• If the gauge is an analog type, check for any visible signs of wear, such as frayed or broken wires.
  

2. Test the Sending Unit with a Multimeter
Using a multimeter is the most common and straightforward way to test a sending unit. Here’s how to do it:

• Set the Multimeter: Set the multimeter to measure resistance (Ohms).
  
• Disconnect the Sending Unit: Disconnect the wire from the sending unit to isolate it from the rest of the system.
  
• Measure the Resistance: Place the multimeter’s probes on the terminals of the sending unit.
  • Fuel Sending Unit: With the tank full, you should measure low resistance (close to 0 ohms). With the tank empty, you should measure high resistance (usually several hundred ohms).
    
  • Temperature Sending Unit: At cold temperatures, you should measure high resistance, which decreases as the engine warms up.
    
  • Oil Pressure Sending Unit: At high oil pressure, resistance should be low, and at low oil pressure, resistance should be higher.
    

3. Check for Grounding Issues
Sending units rely on a ground connection to complete the circuit. If the ground is corroded or has a loose connection, it can lead to inaccurate readings or a non-functional sending unit. To check for grounding issues:

• Inspect the ground wire connected to the sending unit and ensure it is clean and free of corrosion.
  
• Use a multimeter to test continuity between the ground wire and the chassis of the vehicle or equipment. A lack of continuity indicates a grounding issue.
  

4. Verify Voltage Supply
In some cases, sending units require a voltage supply to function. If the sending unit is not receiving the correct voltage, it may not work properly. Use a multimeter to check for the correct voltage at the sending unit's input terminal.

Common Issues with Sending Units and How to Fix Them

1. Inaccurate Readings
   

Inaccurate readings are one of the most common issues with sending units. This can be caused by a variety of factors, including:

• Faulty sending unit: Over time, sending units can wear out, especially if exposed to high temperatures or harsh conditions. If the resistance values are outside the normal range, the sending unit likely needs replacement.
  
• Contaminated fuel or fluid: Contaminants in the fluid can cause the float mechanism to stick, resulting in incorrect readings.
  
• Corroded connections: Corroded or loose wiring can cause voltage fluctuations that lead to incorrect readings.
  

Solution: Clean and inspect the sending unit. If necessary, replace the sending unit or its components. Also, clean or replace the wiring connections to ensure they are secure and free of corrosion.

1. Sending Unit Failures
   

If the sending unit completely fails, the gauge will either read incorrectly or not display any information at all. A failed sending unit can result from:

• Worn-out components: Over time, the mechanical parts of the sending unit can break down.
  
• Electrical failure: Internal wiring or the resistor in the sending unit can fail.
  

Solution: Replace the sending unit entirely if testing confirms a failure. In some cases, you may need to replace the gauge as well if it has also been affected.

1. Intermittent Functioning
   

Intermittent issues, where the sending unit works sporadically, are often caused by poor connections or a faulty ground.

• Loose or corroded connections can cause the sending unit to work intermittently, especially when vibrations from the vehicle or equipment affect the connection.
  
• Worn-out wiring can also create poor contact and cause the sending unit to fail intermittently.
  

Solution: Inspect and clean all wiring connections. Ensure that there is a solid ground connection and replace any damaged or worn-out wiring.

Conclusion
Testing and troubleshooting sending units is a critical task for maintaining accurate readings in vehicles and heavy equipment. Regular maintenance, including inspecting wiring connections and testing resistance values with a multimeter, can help you identify and resolve issues before they lead to more serious problems. By understanding how sending units work and knowing how to test and diagnose them, you can ensure your equipment operates efficiently and accurately, reducing downtime and improving performance.