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Thread: Injector firing test

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Injector firing test

    If I want to see that an injector is indeed being energized would connecting the black lead of a VOM to the tan wire and the red lead to the dark green/black wire show a voltage reading when the engine is cranked?
    Should grounding the black lead of the VOM and probing either pin 2 or pin 3 of the red LM connector with the red lead show voltage when cranking the engine?
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    Moderator Turbo Mopar Staff Force Fed Mopar's Avatar
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    Re: Injector firing test

    Get a set of noid lights, they are cheap, and use the GM PFI one. It'll flash if you are getting pulse.

    Injectors should have power on the green/black wire, and pulse on the other side if you are using the VOM, it's a ground trigger.
    Rob M.
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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Injector firing test

    Rob,There is in fact power at the gn/bk wire. I looked up noid lights, very cool, can't wait to get them.
    When you say the vom is a ground trigger, does that mean it prevents the injector from firing or prevents seeing a reading.
    Last edited by jeff1234; 06-14-2023 at 07:02 PM.
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  4. #4

    Re: Injector firing test

    Ground side triggering (or low-side switching) means that 12v is constant and the computer controls the ground connection.

    I will note that you can test fuel injector firing with a test light, but you do need to choose one to match the amperage draw. I have one that has an LED in it for injector testing. Hook one end of the test light to 12v then gentle probe the ground side switch side of the connector (just lay it in there, don't force it or you will spread the terminals and cause issues.) Have a helper crank the engine and you should see pulses.

    If you are wondering how you tell which terminal is ground side switched. Hook the test light to battery ground and probe the terminals when the ignition is turned on. Many/all will only have power at the connector briefly if the engine is not running or cranking so you need to be quick...

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    Re: Injector firing test

    Thank you.
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    Re: Injector firing test

    Update, first I will tell you that I built a self contained engine start stand, this is about the injectors on that engine. I got a noid kit and tested each injector where I found #3 had a weak signal. I reformed the tabs in the connector which solved that problem. I have a diagnostic plug hooked up that claimed a pulse width of 1.0, which is unknown ifit is accurate. My Chargers pulse width is around 2.7. Today I was reading the post, (Fires Up And Dies After Rebuild by Shadowjake) where I got the idea to pull the fuel rail and watch for fuel spray. Yes the rail was grounded. No spray. I am using a used fuel pump that tests at 70psi. My fuel pressure regulator is the original from "85", seems like a suspect, maybe. I pulled a vacuum on it and it held pressure. New fuel lines and the pump is hard wired to a switch, not turned on by the ASD, which I turn on before starting the engine. Dumb question, how much pressure is on the return line and if I remove the return and energize the pump should I see significant flow?
    The engine will fire and run for a couple of revs from fuel I introduced into the intake but doesn't continue.Any thoughts?
    Jeff
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    Re: Injector firing test

    I still need some help.
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    Rhymes with tortoise. Turbo Mopar Staff cordes's Avatar
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    Re: Injector firing test

    The return should be a decent amount of flow at idle. You shouldn't see a ton of pressure though, since it's after the regulator.

    Do you have the HEP hooked up and working properly? The computer won't trigger the coil, or ground the injectors unless it is seeing a proper signal from the HEP.

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    Re: Injector firing test

    Yes, I took the hep off the charger because it's verified good from running. I've got spark there is 12+volts on both terminals to ground until I hit the start switch. Then the negative side drops. I assume thus is normal. What I discovered this morning is that no fuel is entering in the fuel rail. I have 120 psi fuel pump pressure. I think the stock Bosch FPR is blocked, broken or plugged. There is zero fuel in the rail, dry. I couldn't pass 120 psi air through it either.

    - - - Updated - - -

    I also applied 20 in hg to the vac port, no change. The voltage reading from above was at the coil, I forgot to mention that.
    Last edited by jeff1234; 06-22-2023 at 02:25 PM.
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    Re: Injector firing test

    If the noid lights flash when you plug them in, and you have fuel pressure, then the fuel injectors themselves must be no good....

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    Re: Injector firing test

    No fuel pressure is the problem at present. Aries turbo pointed out to me that I had the lines backward. So, make new fittings and try again.
    Last edited by jeff1234; 06-22-2023 at 05:14 PM.
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    Re: Injector firing test

    That will be your problem. The FPR is acting as a check valve and preventing the fuel from reaching the rail.

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    Re: Injector firing test

    Under the heading of: you don't know what you don't know and coming from the carburetor era I never questioned the line sizes I just hooked big to big small to small. It seems counter intuitive that the small line could feed enough fuel to run the engine at any boost, but it does. The big line apparently assists the check valve in its work by offering reduced resistance to flow.
    Well, live and learn...a valuable lesson. Hopefully others will benefit from this post.
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    Re: Injector firing test

    The fuel rail pressure inlet should be larger than the return port out of the pressure regulator. The inlet is for 5/16" line, and the outlet on the regulator is 1/4". I can see what you mean though in regard to the outlet to the regulator being so large.

    ETA: I think more than a few of us have proof tested their pump while hooking up the lines backwards on accident.

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    Authorized Vendor Turbo Mopar Vendor jeff1234's Avatar
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    Re: Injector firing test

    Lol, yeah but I did mine out of ignorance. Hahaha
    Never the less, THE ENGINE STAND IS ALIVE.!!!
    Hope your problem gets solved.
    The only substitute for cubic inches is cubic dollars, how fast can you afford to go?

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