How to test injection pulses

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Aquí describo como hacer las pruebas para saber si hay pulsos de inyeccion de la computadora. Asi descartar problemas de CKP, cableado o inmovilizador.

How to test injection pulses

Usually, when the car does not want to turn on and we have discarded everything (gasoline pump, spark plug, battery ...), all we have left is the CKP, but how to find out if it is not a problem of a main fuse that went for a short cable, or another problem?

The best procedure that can be done to make a good diagnosis is the following:

Check the electrical integrity of the injector:

We remove the injectors connectors and measure with a multimeter the resistance of the injector between its two terminals.

Table of resistances:

Cold injector: 22ºC and 50ºC

11 – 16 ohm

Hot injector: more than 50ºC

20 – 26 ohm


If the values are very outside that range the injector is shorted or the winding is opened, and it does not inject gasoline.

If the values are slightly outside those ranges, the injector is in the process of deterioration. It is still injecting gasoline, but it is inefficient.

Check if positive voltage reaches the branch of injectors:

We place the ignition switch in the ON position. (turn on the lights on the board)

We place the test lamp to ground and touch the two terminals of the connector of the injector that comes from the computer.

positive injector testprueba positivo de inyector

IMPORTANT: Use test lamp whose resistance is greater than 15ohms. Do not use the stop light bulbs since they consume a lot of current and can burn a fuse or damage the computer.

If you are not sure if the bulb does not have a resistance greater than 15ohms, place a resistor in series to the bulb or assemble a test bulb with a coil-type knob as shown below:

buscar positivo inyector

R1: 15 ohms per 2W

LAMP: Muelita type lamp

If you turn on one of the terminals, everything is in order.

This lamp test is necessary to rule out that there is a false contact in the positive line which is not possible to see with a voltmeter.


Check the injection pulse.

If all the previous steps were correct, proceed to check the injection pulse.

1First, assemble a tester with a high-brightness LED diode, as shown in the following image:

probador de pulsos de inyector


R1 = 1k ohm, necessary to limit the current of the LED and that does not burn.

Led = High brightness green.

The tips can be done with paper clips or needles. This is done so that they can enter the space between the cable and the connector.


It is important that it be of high brightness so that the flash is seen well.

You can lower resistor R1 to about 880 ohm if you use ordinary diodes. But I do not recommend it because the flash does not look good.

Connection of the LED in the injectorconexion del led en el inyector
2 Connect the small led tester as shown in the previous image to the injector.

Try inserting the tips between the cable space and the connector. Do it calmly and without damaging the rubber protector of the connector.

Although the Led has polarity, in this case it is not necessary to find the polarity of the cables as suggested in some videos and other texts. This is due to the nature of the coil. When the feeding current is interrupted, a reverse electromotive force is produced that tends to infinity, which opposes this change (inverse voltage is generated). This electromotive force is enough to be flashing the led no matter what polarity is placed because it is an oscillating phenomenon. This is why, for a pulsating working injector, a current always originates in delay to the voltage, which is dangerous for the electronic components and must be discharged correctly. More of this phenomenon? On Wikipedia

So, at the end of the day, you will always see a flash no matter in what polarity you place it.


3 Once the tester is connected, start the engine and you should see flashes on the led indicating the injection pulse.

destello led en inyector

If the led does not flash, then there are no injection pulses, so you will have to check the wiring to the computer and verify that the output is not damaged.


If you do this pulse test on modern computers (2011 for here) without the injector connected, you will see the LED lit poorly indicating the existence of the test current. If you turn on the car, you will see some initial flashes and then it will stop. The computer detects that the connected is not an injector and turns off the pulse signal.

Considerations to perform this pulse test.

Do not perform the pulse test with the lamp. It is not that the output of the injector is burned, but that the injection pulse is so short that it does not give the filament a chance to heat up to emit light.

Try to always perform this test with the injector connected to the line (for modern computers), since for example if you connect a led with a limiting resistor of about 1k ohm between the pair of terminals of the injector connector, the handler will try to operate its led as if it were one more injector. Then the compensation forces the led to be on in the held state and only in a few moments (opening step) will it see an increase in brightness.

Always bear in mind that we have a RS measuring resistor that modern computers acknowledge the status of the injector. If the values ​​are incorrect, it triggers the fault in the computer and the output is disabled. It is very important to know this in order not to make a bad diagnosis.

Be careful not to short-circuit to positive. In general, the system is protected for short between lines and line to ground.

Well, it's all for the moment, I hope you like this article.

A video of how I did it:

Do you have questions or doubts?

Questions or suggestions can use the form below and I will gladly answer them.

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Written by Dennis García.

Chica Mecánica



Me encanto.

Inmejorable. Muchas gracias por tus aportes.

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    How to test injection pulses

    How to test injection pulses

    Aquí describo como hacer las pruebas para saber si hay pulsos de inyeccion de la computadora. Asi descartar problemas de CKP, cableado o inmovilizador.

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    4 Question(s) answered

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    • Asked by Pablo
      on 04/12/2019
      Hice el probador con el led.pero se prende una vez x segundo.el auto es ford escort 98 Answer:
      Caramba Pablo, depende de que tipo de inyección tiene el escort, ¿ese es monopunto?
    • Asked by Mario
      on 05/27/2019
      Hola en una camioneta pruebo la señal negativa que envia compuatdora pulsos en todas envia y en uno luego envio y luego se deteniene que podria ser? Gracias. Answer:
      Hola Mario. Entendí que probaste y hay pulso negativo en todos, pero hay uno que al poco tiempo se detiene. En ese caso, es posible que la salida se esté recalentando y el transistor de la computadora se proteje. Mide cuantos ohmios tiene ese inyector para descartar que este algo dañado y revisa el conector y el cable no exista un corto.
    • Asked by Armando (Pelao)
      on 06/23/2019
      Si CAM BUS es lo mismo que MULTIPLEXADO, atte. Armando , saludos y bendiciones Answer:
      No es lo mismo Armando. Cambus es un protocolo de comunicación, Multiplexado es un sistema combinacional, que une varias señales en una sola o viceversa.
    • Asked by daniel roa
      on 07/31/2019
      hola xfa si me pueden ayudar que puedo aser tengo un auto chino su motor es 800 pasa lo sigiende que tiene 3 bujias y tres inchecion y solo ados le llega pulso y chispa y a uno nada ni pulso ni chispa y lo revise y no tiene pulso solo le chega el positivo y negativo xfa si no es tanda molestia y avisar en nel wuassap 69269017 Answer:
      Tienes un cable roto o la computadora tiene esas salida dañada Daniel.

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