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WHy am I not getting a pulse to my injectors

I have ran several test and I am getting 12V to my injectors when I turn the key forward for a split second. But when I try to turn the engine over I am not getting a pulse...Could someone please tell me why?

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There is a (I'm going to call it a fuse) square block with 4 or 5 prongs and 1 may not be getting current thru on the cranking pin.

Posted on Nov 18, 2014

Testimonial: "Hummm Cranking pin? Funny you should say that......Are you talking about a relay? I put a ignition switch in the truck but when I did it cranked and ran for a second then died......I noticed that the left injector just dripped a little gas the right looked like none...Now I did have to do some work in the fuse box no Bad fuses just a main lead wire 12V was broke away from the back so one hole leg was dead in the fuse panel. Once I fixed that I got a hot read on everything then....When you say square box do you mean a relay?"

  • Kar Volk Nov 19, 2014

    Yes thank you I was blanking on words tonight ! Hope all goes well !

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Do you still for sure have spark? If so, I would test the injector circuit with a cheap "noid light". You can make a tester with a 12 volt bulb and jumper wires, but parts stores sell a noid light which would fit your Chrysler injector connectors, so you could test the circuit. You plug it in to an injector connector and crank the engine over. If the light is blinking, the circuit is working and the injectors should be spraying gas into the cylinders.

The injectors have constant voltage to them anytime the key is on. It is a fused circuit. the fuse should be in the distribution box under the hood. So constant voltage to them, it is only when the pcm grounds the circuit for a split second that the pulse occurs. All injector grounds go into the pcm.

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Check the injector circuit. Here's how injectors usually work: when key is on, they have live power available to them. Key on, the circuit is turned on, so power passes through a fuse to the injector on the power wire. (Check the fuse- it's under the hood, most likely.)
To make the injector "pulse" or spray fuel out, the engine computer will internally ground the circuit for a split second. The injector has a power wire to it from that fuse. Then the ground wire at the injector goes into the computer.

If you have a test light, you can test the circuit. First, clip a test light ground lead to negative battery post. With key on, and the injector connector unplugged, check for battery voltage on the power wire. One of the wires should have voltage and will light up the test light. Now, move your test light lead to the battery POSITIVE post. Have a helper be cranking the engine over while you probe the ground wire-the one that did not light up the test light. If the injector circuit ground is good, the test light will now be blinking. (You are putting voltage into the ground wire to the computer, to check if it is properly grounding the circuit. Every time the light blinks, the computer has grounded the circuit.)
If the circuit passes both tests (key on, steady test light on power wire; engine cranking, blinking light on ground wire), then the injector circuit is good. The injector should be pulsing when it is connected and the engine is cranking. If no gas is coming out of the injector, could be that fuel is blocked somewhere (filter?) from reaching the injector, or it could be a shorted injector. Someone with a volt-ohm meter should check the resistance across the injector terminals and compare with manufacturers' specifications. This will determine if the injector is bad.

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Check for an injector fuse in the fuse box under hood. Pull off one injector connector and use a testlight to check for a steady power signal on one of the two wires, with key on. Should have power on one wire. If not, pull all injector connectors off and check again-a shorted injector can shut down the injector drive circuit-if power now, reconnect injectors one at a time. If one connection causes power to be lost, that is a shorted injector. Replace it. If still no power and fuse is good, you will need a wiring diagram to check injector drive circuit.
If you have voltage at the injectors with key on, move the test light ground lead to the battery positive post, and put the test light probe on the injector wire that did not light the test light. Now have a helper crank the engine over-you should have a blinking light on this ground circuit. If no light, or light is not blinking, the ground circuit between injector and computer is bad. See? Steady power is available to injector anytime key is on. When the computer internally grounds the injector circuit for a split-second, the injector is energized, causing the pulse.

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It is a fused circuit, so check the fuse first. The injector should have a hot wire to it when the key is on. Ground a test light and check for the hot wire with key on. If you have that, then move the test light lead to the positive post of the battery, and probe the ground wire terminal of the injector harness-now have someone crank the car over and check for a blinking light. If the light is blinking when cranking, the ground circuit is good. See? The injector always has power to it with key on. When the computer grounds the injector circuit for a split second, power is passed across the injector solenoid terminals causing it to pulse.

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A test light connected across the coil primary leads should flash on + off when engine cranks(coil sparks when 12v turned OFF due to collapse of magnetic field).If you get pulsed 12 volts,and no spark from the coil,the coil is bad.If you don't get the pulsed 12 volts,trace back to find out why(the PCM turns the coil ground on and off,12 volts comes from switched IGN source)

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Actually the injectors don't spray "all the time". They pulse open for a few milliseconds then close every time they are grounded by the ECM. The distributor has a pick-up coil (Variable Reluctance sensor) that provides an A/C (Alternating Current) pulse every 120 degrees of crankshaft revolution to the Ignition Control Module also contained inside the distributor. The I.C.M. amplifies, rectifies and passes along the pules as a +12V square wave to the ECM. The ECM uses these pulses to calculate RPM and determine when the engine is rotating vs. stopped. Together with this information and more from Coolant Temp., Throttle Position, Manifold Pressure and Air Temp. the computer constantly 'looks up' what the appropriate amount of time it should pulse the injectors for to deliver the required amount of fuel.
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