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Car trouble. I suspect the fuel injector circuit. I am trying to construct a com port reader that will place a timestamp into a log on the closing the 12v fuel injector circuit.

The fuel injector is a simple normally open 12v circuit of 2 wires. The + is tied into the ignition switch. The ground is held open by the ECM (Electronic Control Module). When it is time to squirt fuel into the cylinder, the ECM pulses/flashes the ground to complete the circuit.

I am planning on using Eltima's Serial Port Monitor to record the event of the closed circuit with a timestamp on a single injector and store the data into a log so that I can evaluate and analyze the data.

Originally, I thought I would simply run pin 3 of a DB9 to a back probe on the injector and run the engine. But with the circuit always powered by 12 volts, I would get a reading on pin 3 coinciding with the ignition switch and not with the pulsing ground.

Do you have any ideas on how to read the pulsing ground on the fuel injector?

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You could try isolating the switching signal with an opto coupler. enter image description here

Every time the ECM pulls the injector to ground a small current (about 10mA) goes through the internal LED and turns on the output transistor.

The pulse width/timing could be measured using a small microcontroller and converted to serial data.

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  • \$\begingroup\$ Take precautions to prevent the inductive kick-back from the fuel injector from frying your opto-coupler LED. Since a waveform from a fuel-injector doesn't resemble a serial data stream in any way, it is not at all clear how you can convince your serial port monitor to detect and record the pulses? \$\endgroup\$
    – Richard Crowley
    Jun 6, 2016 at 14:00
  • \$\begingroup\$ @RichardCrowley. Thanks for that. As far as how to convert the pulse into a serial data stream I would add a small microcontroller to measure the pulse width and timing between pulses and send out an appropriate signal. I've added this to the circuit. \$\endgroup\$
    – JIm Dearden
    Jun 6, 2016 at 14:17
  • \$\begingroup\$ It is my understanding that a change in the voltage on pin 3 will be read by the port monitor -- the monitor should record the change as junk (i.e. no intelligible data, no stop bit, etc) but what I am interested in is the EVENT of the change and not the transfer of the data (or in this case, non-data). I am hoping that reading the change will allow me to determine the start of the pulse, the duration, and frequency per second or minute. \$\endgroup\$
    – Mark Cain
    Jun 6, 2016 at 14:21
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    \$\begingroup\$ Use one of the RS-232 flow-control pins - they are digital levels; there is no data protocol on them. "Back in the old days" when we needed quick & dirty digital I/O from a PC, we often used those. \$\endgroup\$
    – JRobert
    Jun 6, 2016 at 14:25

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