Ameer Randolph
Fuel injectors are a type of solenoid that deliver precise amounts of fuel to a car's cylinders. Over time, fuel injectors can wear out or clog up, which can prevent the engine from operating correctly. To ensure that fuel injectors are receiving the correct voltage, it is important to test their resistance and wiring. This can be done using a Digital Volt-Ohmmeter (DVOM) or a multimeter with an ohms setting. The engine should be turned off for this test, and the injectors should be accessed by following the instructions in the vehicle's manual. Once the injectors are located, the injector wiring harness can be tested by connecting the positive lead of the meter to the wiring harness terminal, which should read around 12 volts.
How to check fuel injector voltage
| Characteristics | Values |
|---|---|
| What you need | A current clamp and a test lead with a backpinning probe, or a Digital Volt-Ohmmeter (DVOM) or multimeter with ohms setting |
| Engine status | Off |
| Injector resistance | 12 to 17 ohms |
| Injector wiring | Ensure everything is connected properly |
| DVOM measurement | Measure DC voltage |
| DVOM negative lead | Touch to a ground source |
| DVOM positive lead | Connect to the wiring harness terminal |
| DVOM reading | One terminal should read 0 volts, the other 12 volts or around 12 |
| Injector voltage waveform | Very different to the current waveform |
| Current clamp connection | Connect to Channel A on your PicoScope and place the clamp around the fuel injector supply wire |
| Engine status | Running |
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What You'll Learn
Using a DVOM to measure DC voltage
To check the fuel injector voltage, you will need a Digital Volt-Ohmmeter (DVOM) or a multimeter with an ohms setting. The engine should be off for this test.
Now, for using a DVOM to measure DC voltage:
Firstly, ensure the system you are measuring is DC and not AC. This is important as AC systems can be lethal.
Next, turn the DVOM selector to DC volts. Touch the black lead (COM port) to the negative terminal and the red lead (DC volts port) to the positive terminal. The red lead should be connected to the positive test point, and the black lead to the negative test point. Modern multimeters will automatically detect polarity, but it is good practice to follow this order.
If your meter does not auto-range, set it to the lowest range. A typical voltage reading should be around 12 volts.
If the voltage reading is lower than expected, you may have a faulty connection, corroded terminals, or undersized wiring.
It is important to note that when finished, the probes should be removed in reverse order: red first, then black.
By following these steps, you can accurately measure DC voltage with a DVOM, which is essential for diagnosing electrical systems and ensuring proper functionality of components.
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Testing injector resistance
Testing the resistance of your fuel injectors can give you an idea of how dirty they are. You can use a Digital Volt-Ohmmeter (DVOM) or a multimeter with an ohms setting to do this. Make sure the engine is off, as no power is needed for this check. You can access the injectors in the engine or refer to the manual. There may be a plastic shield that needs to be removed to access the injectors.
Remove the injector wiring harness by first removing the slide lock, if there is one. Now, set the DVOM to measure ohms and check all the injectors one by one for their resistance. If the meter doesn't auto-range, set it to the lowest range. If you have high-impedance injectors, the range would typically be from 12 to 17 ohms. Most fuel injectors should read between 10 and 18 ohms, and all readings should be within the range suggested by the manufacturer. If the resistance readings are off, the faulty injector needs to be repaired or replaced.
It's important to note that resistance measurements can only show gross failures of the coil in the injector. A more conclusive test is to measure the inductance and coil "Q" with an impedance bridge, which measures the A.C. impedance of coils and capacitors (reactance and resistance) and will also measure pure resistance.
After your engine has reached operating temperature, take impedance readings. They should be around 16 ohms, and if they get down into the teens, you should consider getting new injectors or having yours reconditioned.
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Checking the wiring
To check the wiring of your fuel injectors, you will need a digital volt-ohmmeter (DVOM) or a multimeter with an ohms setting. You will also need to access the injectors, which may involve removing a plastic shield.
First, turn the key to the second (ON) position so that you receive battery power without turning on the engine. Set up your DVOM to measure DC voltage, and if the meter doesn't auto-range, set it to the lowest range possible. Take the negative lead of the DVOM and touch it to a ground source. You can look for an unpainted piece of the frame under the hood. Now, take the positive lead and connect it to the wiring harness terminal. One terminal should read 0 volts, and the other should read 12 volts or around 12.
Next, leave the ground lead in position and test the wiring harness of all the injectors. They should all read around 12 volts. A lower reading indicates excess resistance in the wire.
You can also manually fire the injector by putting 12 volts to one side and grounding the other when the car is turned off and your fuel pressure is at zero psi. You should show 12 volts on one side of the injector and it should ground out to fire.
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Monitoring the injector current
Testing Injector Current
Firstly, it's important to understand that the injector's current and voltage can be tested simultaneously to get a complete picture of its performance. To test the injector current specifically, you'll need a current clamp and a test lead with a backpinning probe. Connect the current clamp to Channel A on your PicoScope, and ensure it's the correct model, such as the PICO-TA018 20 A / 60 A Current Clamp or the PICO-TA473 PicoBNC+ Current Clamp. Place the clamp around the fuel injector supply wire, being careful to maintain the correct orientation. You may need to adjust the loom's outer shielding to fit the clamp properly.
Running the Engine
With the clamp connected, start the engine. You will observe the injector receiving a positive pulse to open, followed by a negative pulse to close. This process repeats as the engine runs, and the time between these pulses increases, allowing more fuel to be introduced over a longer period. This is visible in the slow motion of the oscilloscope.
Analyzing the Waveform
The waveform displayed on your PicoScope will show you the current drawn by the injector. You can observe this in conjunction with the primary ignition trace to identify any issues with the injector circuit. A loss of injector current, for example, signifies a fault within the injection circuit. Additionally, the waveform will show you the delay before the injector opens, the open time, and the delay before closing.
Comparing Injectors
You can also test each injector sequentially to ensure they are all functioning properly. By comparing the waveforms of multiple injectors, you can identify any discrepancies that may indicate a problem with a specific injector. This allows for precise diagnosis and repair.
It is important to note that the current in the injector may not increase instantly when voltage is applied. This is because the injector is inductive, and you may observe a bump in the waveform caused by the change in inductance as the pintel (plunger) moves.
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Testing the spray pattern
To test the spray pattern, you will need to set up a testing machine with the injector and adjust the settings for visibility. A working fuel injector spray should produce a clear-coloured, fine spray—a light misting.
One method to test the spray pattern is to use a testing machine with a pump and a measuring device, such as a flask with graduations of known value. Set up the injector over the flask with the pump on and observe any drips or flow issues, which could indicate a problem with the injector pintle sealing. If the injector is sealing properly, activate the pulser for a set time, such as 5 or 10 seconds, and calculate the flow rate. While the injector is pulsing, you can examine the spray pattern and atomisation to determine if the injector is functioning correctly. A more atomised spray pattern indicates better fuel and air mixture and even ignition.
Another method for testing the spray pattern is to perform a cylinder balance test. This involves unplugging each injector one at a time and observing the RPM drop. If the RPM does not drop significantly, it suggests that the injector is not contributing effectively to the power development of that cylinder.
It is important to note that testing the spray pattern will not provide information on the flow rate. To test the flow rate, a pump with a known pressure, such as 50 psi, is required, along with a pulse tool calibrated to pulse the injector at a known rate, such as 100 hz.
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Frequently asked questions
Fuel injectors are a particular type of solenoid that actuates pistons very quickly, allowing the injector to deliver precise amounts of fuel into the cylinder. Over time, fuel injectors can wear out or clog up, preventing the engine from operating correctly. Checking the voltage can help identify any issues.
You will need a Digital Volt-Ohmmeter (DVOM) or a multimeter with an ohms setting. Some other tools that may be useful include a screwdriver, an oscilloscope, and a current clamp.
First, turn the car on by turning the key to the "ON" position. This will ensure battery power is flowing without turning on the engine. Set the DVOM to measure DC voltage, and if the meter doesn't auto-range, set it to the lowest range. Touch the negative lead of the DVOM to a ground source, such as an unpainted piece of the car's frame under the hood. Then, place the positive lead on the wiring harness terminal.
Each terminal on the wiring harness will have a different reading. One terminal should read 0 volts, while the other will read around 12 volts. If you find a lower reading, it may indicate excess resistance in the wire.
