Rajan Wilcox
Testing the fuel injectors on a Mercedes-Benz vehicle can be done through a variety of methods, depending on the make and model of the car. For older models from the 1960s to the 1980s with Bosch mechanical fuel injection systems, a pressure tester can be used. Newer models with electrical wires plugged into the injectors may require a different testing method, such as a diagnostic test by a professional mechanic. It is important to ensure that the vehicle's electrical system is active during testing without the engine running. This can be done by inserting the key and turning it to the on position without starting the engine.
Testing Mercedes Fuel Injectors
| Characteristics | Values |
|---|---|
| Vehicles | Mercedes Benz, Porsche, Rolls Royce |
| Models | 170D, 180D, 190D, 200D, 220D, 240D, 300D, 300CD, 300TD, 300SD, 300SDL, 350SD, 350SDL |
| Years | 1950s–1990s, 1960s–1980s |
| Fuel Injection System | Bosch mechanical, Bosch C.I.S. |
| Tester | Bench mount, will not work on electrically operated fuel injectors |
| Wires | Two, one is a 12-volt constant, can be grey and black or any other colour |
| Voltage | Constant voltage indicated by test light turning on |
| Power Issue | No light indicates a problem, wrap electric tape around holes in the wiring |
| Cleaning | Carb cleaner, breadboard, momentary switch, 6V or 12V battery |
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Test light connection
To test the fuel injectors in your Mercedes, you can use a test light connection. This is best to observe the pulse produced by the PCM (Powertrain Control Module).
First, locate the two wires going into each injector. Each fuel injector will have a metal clip plugged into it with two wires coming out. These wires are often grey and black, but can be other colours. One of these wires is a 12-volt constant that should be continuously receiving power.
You can then test each wire for voltage. Take the sharp end of the test light and press it into the rubber coating around each wire until it penetrates the metal wiring. One of the wires should make the test light turn on.
With the key in the 'on' position, connect the test light or voltmeter to the negative side of the battery. Start the engine and allow it to idle. Then, using a screwdriver, touch the end of the screwdriver to the injector and place your ear to the handle to listen for an audible clicking sound, which confirms the injector is working.
If the test light is failing to light up, the injector may be faulty or there could be an issue with the vehicle's electronic control unit.
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Locating wires
Locating the wires is the first step in testing your Mercedes' fuel injectors. Each fuel injector has a metal clip plugged into it, with two wires coming out of it. These wires are often grey and black but can be any colour. They will be the only wires coming from each injector.
You will need to test each wire for voltage. To do this, take the sharp end of the test light and press it into the rubber coating around each wire until it penetrates the metal wiring. One of the two wires should make the test light turn on when it comes into contact with the wire inside the protective coating. If the test light turns on with one wire, the injector is receiving the necessary constant voltage.
Repeat this process for each injector. If you locate one injector with a power issue, this doesn't mean the others are fine. Once you identify a faulty injector, make a note of it and continue to test the rest.
Follow the wires on the injectors that fail to engage the test light to ensure there are no breaks in the wire that may prevent electricity from reaching it. If there are no breaks, the problem may be with the vehicle's electronic control unit.
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Voltage testing
To test the voltage of your Mercedes fuel injectors, you'll need to access the wires connected to each injector. Each fuel injector will have a metal clip plugged into it, with two wires—one black, one grey, though the colours may vary—that are exposed as they emerge from the plastic clip.
Turn the key to the "on" position without starting the engine. The vehicle's electrical system must be active, but the engine should not be running. Insert the key and activate the electrical system, but don't engage the engine starter. This will power the interior lighting, radio, and other electronics. Ensure the headlights and stereo are off to conserve power.
Now, test each wire for voltage. Use a test light and press its sharp end into the rubber coating around each wire, penetrating the metal wiring inside. One of the wires should illuminate the test light when it makes contact. This indicates that the injector is receiving constant voltage. If neither wire causes the light to turn on, there's an issue with the power reaching the fuel injector, resulting in a failure to fire.
Repeat this process for each injector. If you find one injector with a power issue, make a note of it and continue testing the others. It's possible that other injectors may have similar problems.
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Battery power
To test your Mercedes fuel injectors with battery power, you will need to use a DVOM (Digital Volt-Ohm Meter) to measure the voltage and resistance. Here is a step-by-step guide:
Step 1: Turn the Car On
Turn the key to the second ("ON") position. You want the battery power flowing, but do not start the engine.
Step 2: Set Up the DVOM
Set the DVOM to measure DC voltage. If your meter does not auto-range, use the lowest range possible.
Step 3: Connect the Negative Lead
Connect the negative lead of the DVOM to a ground source. You can find a ground source by looking for an unpainted piece of the car's frame under the hood. Some DVOMs have alligator clips, which can be helpful for this step.
Step 4: Connect the Positive Lead
Connect the positive lead to the wiring harness terminal. The wiring harness will have two terminals that the prongs on the injector insert into. One will be connected to the ground and should read 0 volts.
Step 5: Test the Voltage
The voltage at the wiring harness terminal should be around 12 volts. If it is lower, it indicates excess resistance in the wire.
Step 6: Repeat for All Injectors
Leave the ground lead in position and test all injector wiring harnesses. This will help you identify any injectors that may be malfunctioning.
Additional Tips:
- You can also test the resistance of the injectors using a DVOM. Set the DVOM to measure ohms and test the injector wiring harnesses.
- If your injectors are receiving the correct voltage and have normal resistance, the next step is to remove the injector and test the spray pattern it produces on an injector tester.
- Always refer to your vehicle's repair guide or owner's manual for specific instructions and safety precautions before attempting any diagnostic or repair work.
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Cleaning injectors
Locate the Fuel Injectors: Start by identifying the fuel injectors in your Mercedes engine. They are typically located on the intake manifold and are responsible for spraying fuel into the cylinders.
Disconnect the Fuel Injectors: Before cleaning, ensure the engine is turned off and disconnect the fuel injectors from the fuel lines and electrical connectors. This step is crucial for safety and to prevent any fuel leaks.
Prepare the Cleaning Solution: Use a specialized fuel injector cleaning solution or solvent that is designed to remove carbon deposits and other contaminants from the injectors. These solutions are typically available at automotive stores.
Soak the Fuel Injectors: Place the disconnected fuel injectors in a container filled with the cleaning solution. Allow them to soak for the recommended time, usually several hours, to loosen and dissolve any built-up deposits.
Scrub and Rinse the Fuel Injectors: After soaking, use a soft-bristled brush to gently scrub away any remaining debris or deposits from the injectors. Pay close attention to the tiny holes and ensure they are clear of any blockages. Then, rinse the injectors with clean solvent or compressed air to remove any residual cleaning solution.
Dry and Reinstall the Fuel Injectors: Ensure the fuel injectors are completely dry before reinstalling them. You can use compressed air to speed up the drying process. Carefully reconnect the fuel injectors to their respective fuel lines and electrical connectors, ensuring a secure and proper connection.
It is important to note that cleaning the fuel injectors is just one aspect of proper fuel system maintenance. Regular fuel filter replacements, fuel line inspections, and engine tune-ups are also essential to ensure optimal performance and fuel efficiency. Additionally, if your Mercedes-Benz is equipped with a specific fuel injection system, such as Bosch mechanical fuel injection, refer to specialized resources or consult a Mercedes-Benz technician for detailed cleaning and maintenance instructions.
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Frequently asked questions
If your car uses a Bosch mechanical fuel injection system, you can use a pressure tester to test the fuel injectors. However, this will not work on electrically operated fuel injectors.
Fuel injectors spray fuel into the cylinders of your engine where it is combined with air.
First, locate the two wires going into each injector. One of these wires should be a 12-volt constant that is continuously receiving power. Then, test each wire for voltage by pressing the sharp end of a test light into the rubber coating around the wire. If the test light turns on with one wire, the injector is receiving the necessary constant voltage.
You can test if your fuel injector is clogged by using a floral water tube, a nail, a carb cleaner straw, a breadboard, a momentary switch, and a 6V or 12V battery. First, place the injector into the water tube with the open side facing down. Then, use the nail to punch a hole in the rounded side of the water tube and insert the carb cleaner straw. Next, hook up the battery to the electrical ports on the injector. Finally, add carb cleaner and tap the momentary switch to open the injector. If the injector is working properly, it should produce a puff of carb cleaner.
