Testing And Replacing Your 1985 Silverado's Fuel Sending Unit

If the fuel gauge on your 1985 Silverado is acting up, you may need to test or replace the fuel sending unit. The fuel sending unit is located in the fuel tank and measures the fuel level, communicating this information to the fuel gauge on your dashboard. Before you jump to any conclusions, there are some simple inspections and tests you can perform to help you streamline the repair. For example, you can check the fuel gauge fuses, or use a multimeter to test the fuel gauge, the sending unit wiring connections, and the output and input voltages. If all else fails, you may need to replace the fuel sending unit, which can be a complex task depending on your vehicle.

Check the fuel gauge fuses

Checking the fuel gauge fuses is an important step in diagnosing issues with your fuel gauge. Here's a detailed guide on how to do this for your 1985 Silverado:

First, locate the fuse box in your vehicle. Your 1985 Silverado may have more than one fuse box, so consult your owner's manual to identify the specific location of the fuse box for the fuel gauge. The fuse box is typically located on the driver's side of the car, or in the box below the steering column. Some vehicles may also have a fuse box under the hood.

Once you've located the correct fuse box, refer to the fuse map in your owner's manual to identify which fuse controls the fuel gauge. Pull out the fuse and inspect its condition. Look for any signs of damage or corrosion. If the fuse is blown or damaged, replace it with a new one of the same specifications.

After replacing the fuse, re-check the fuel gauge to see if it is functioning properly. If the issue persists, there may be another problem with the fuel gauge or its wiring.

If the fuse is not the issue, the next step is to examine the wiring. Start by inspecting the electrical harness at the gas tank and the ground connections. Corrosion or damage to the wiring or connections can cause issues with the fuel gauge. Clean any corrosion and ensure all connections are secure.

If the wiring and connections appear intact, the next step is to use a multimeter to test the voltage of the fuel gauge wire. Set the multimeter to 20V DC and check if the fuel gauge is receiving 12 volts. If the voltage is insufficient, there may be an issue with the wiring or another component in the circuit.

If the voltage is within the normal range, the issue may lie with the fuel gauge itself or other components such as the sending unit. Refer to your owner's manual or seek professional assistance for further troubleshooting steps.

Remember to exercise caution when working with electrical components, and always refer to your vehicle's repair or service manual for detailed instructions specific to your 1985 Silverado.

Test a fuel gauge with a multimeter

Testing a fuel gauge with a multimeter is a simple process, but it requires some basic knowledge of electrical systems and safety precautions when working with gasoline. Here is a step-by-step guide on how to test a fuel gauge with a multimeter:

Prepare Your Tools and Safety Equipment:

Before beginning, ensure you have a multimeter and the necessary safety gear, including eye protection and gloves. Working with electricity and gasoline can be dangerous, so take the necessary precautions.

Locate the Fuel Gauge and Terminals:

The fuel gauge is usually located behind the dashboard. Find the fuel gauge and identify the two terminals at the back, marked with a "+" or "I" for positive and a "-" or ground symbol for negative.

Check Voltage with the Multimeter:

Turn on the ignition and set your multimeter to measure voltage, typically 20 volts DC. Probe between the ground and positive terminals on the fuel gauge. If there is no voltage, the issue is likely with the ignition circuit. If there are 12 volts, the problem is either with the gauge, the sender, or the wiring.

Disconnect the Sending Wire:

With the power still on, disconnect the sending wire at the back of the gauge. It is usually marked with an "S." If the gauge jumps to its highest reading, the gauge is functional, and you can proceed. If not, the gauge is faulty and needs replacement.

Test the Ground Connection:

Another test is to jump a wire or screwdriver from the sending pin to the ground pin on the gauge. If there is no ground pin, you can use a longer wire to connect to the engine block. The gauge should go to its lowest reading. If it does, the gauge is working correctly.

Check the Sending Wire and Sender:

If the gauge is functional, the issue lies elsewhere in the system. Disconnect the sending wire from both the sender and the "S" pin on the gauge. Set your multimeter to the Ohms scale and check the resistance within the wire. If there is no resistance, the circuit is intact, and the sender is likely faulty.

Replace the Sender and Gauge if Necessary:

In most cases, the sender and fuel gauge need to match in terms of resistance. Therefore, it is recommended to replace both the sender and the gauge to ensure accurate readings. Pre-packaged installation kits are available for this purpose.

Additional Testing:

If you suspect a problem with the fuel gauge sending unit, you can perform further tests. Check your vehicle manual to understand how to access the fuel tank and sending unit. Remove any necessary panels or parts for access. Inspect the ground wiring for physical damage or corrosion. Refer to your service manual to determine the voltage standards and schematics for the sending unit wiring or connector.

Test Voltage and Voltage Drop:

With the ignition on, use the multimeter to check the voltage of the wiring or connector providing power to the sending unit. Follow the circuit to locate any issues if incorrect readings are found. Next, check for voltage drop by plugging the connector back in and testing again. The fuel gauge sending unit should create resistance, resulting in a voltage drop.

Remove the Sending Unit for Further Testing:

If there is no resistance, you may need to remove the sending unit for further inspection and testing. Set your multimeter to Ohms and refer to your owner's manual to determine the expected ohms range for the sending unit. For example, a range of 0-70 ohms would indicate 0 ohms when the tank is empty and 70 ohms when full, or vice versa.

By following these steps, you can effectively test a fuel gauge with a multimeter, identify any issues, and make the necessary repairs or replacements. Remember to consult your vehicle's manual and take the appropriate safety precautions throughout the process.

Test sending unit wiring connections

To test the wiring connections of the fuel sending unit of a 1985 Silverado, you will need a multimeter.

Firstly, inspect the top of the fuel sending unit for any damage, corrosion, or incorrect seating in the tank. If the top of the unit is dirty, clean the connectors to help conductivity and ensure a good ground connection.

Next, set the multimeter to 20 volts. With the battery turned on, back-probe the input connector on the sending unit and check for voltage. Refer to the service manual for the correct voltage. If the input voltage is lower than expected, inspect the wiring from the sending unit to the battery as this could be faulty.

To check the output voltage, measure the voltage from the sending unit to the fuel gauge. The voltage should be the same on both ends of the wire. If the voltage is different, there is likely corrosion or poor wiring between the sending unit and the fuel gauge.

If you are still unable to identify the issue, you may need to drop the tank and check the resistance performance of the fuel sending unit using a multimeter set to the Ohms setting. Refer to the service manual for the specific Ohms value for your fuel tank sending unit.

Check fuel sending unit ohms

To check the fuel sending unit ohms, you will need a digital multimeter to measure the resistance or ohms across the sending unit terminals. This will help diagnose whether the fuel gauge is faulty or if the issue lies with the sending unit itself.

Before you begin, it is important to prioritise safety. Work in a well-ventilated area and wear safety glasses and gloves. Disconnect the battery to prevent electrical shorts.

To access the sending unit, you will need to remove the rear seat or the access panel on the floor of the trunk. Once you have located the unit, clean the connectors to avoid any contamination in the electrical system.

Now, you can connect the multimeter. Set the device to the ohm measurement setting and disconnect the sending unit wiring. Attach the multimeter leads to the appropriate terminals: the positive lead to the signal wire and the negative lead to a grounding point on the chassis or the negative terminal on the battery.

With the multimeter connected, check the resistance readings. A functioning sending unit will display ohm values that change smoothly as the float moves up and down. If the readings are erratic or the multimeter shows infinite resistance, there may be a break in the wire or the resistor in the sending unit could be faulty.

Compare these readings to the specifications listed by the vehicle's manufacturer. If your readings deviate from the manufacturer's specifications, the sending unit may need to be replaced.

• Check the ground connection: Ensure the unit's ground connection is secure. A grounded wire can cause malfunctioning readings.
• Visual Inspection: Examine the sending unit for visible damage, including corrosion or damage to the wiring that could lead to inaccurate signals.
• Empty Tank Reading: Connect your multimeter to the sending unit and record the resistance with an empty tank; it typically reads high ohms.
• Full Tank Reading: Fill up the tank and record the reading again; a full tank usually shows low ohms.

If you are still experiencing issues, refer to the vehicle's service manual for specific information on the fuel sending unit, including the correct ohm range and removal procedures.

Replace a fuel sending unit

Before replacing the fuel sending unit, it is important to test whether the unit is faulty. A faulty fuel sending unit can cause the fuel gauge to read full or empty all the time, or the needle may move erratically.

To test the fuel sending unit, you will need to check the fuel gauge fuses. If other gauges are also acting up, it may be a fuse-related problem. Check the owner's manual to locate the fuses for the fuel gauge and pull the fuse to check its condition. If the fuse is blown, replace it and verify if this fixes the fuel gauge.

If the fuel gauge is working, the next step is to check the other system components, as either the wire running to the sender or the sender itself may be faulty. To check the wire, disconnect it from both the sender and the back of the gauge. Set a multimeter to the Ohms scale and check the resistance within the wire. If there is no resistance, the circuit is good and the sender is faulty.

If you have determined that the fuel sending unit needs to be replaced, the first step is to release the fuel tank pressure. This can be done by following the instructions in the vehicle's service manual.

Next, disconnect the fuel lines and electrical connections from the fuel sending unit. Clean the area around the plug and ensure it is free of any contaminants that could fall into the tank.

If applicable, use a non-ferrous tool or punch to remove the retaining ring. Pull out the old fuel sending unit, along with the gasket or O-ring, from the tank. Compare the old unit with the replacement unit to ensure compatibility.

Install the new fuel sending unit with a new gasket or O-ring, making sure the gasket is properly aligned between the fuel sending unit and the tank. Reinstall the retaining ring if applicable. If the fuel sending unit uses screws, use the included washers to prevent leaks and tighten them in a star pattern.

Finally, reconnect all electrical connectors and snap the fuel lines back into place. It is important to take safety precautions when working with fuel, such as having a fire extinguisher nearby and avoiding working near anything that could cause a spark. Wearing safety glasses and gloves is also recommended.

Frequently asked questions