Why is this sensor dropping out?

Discussion in 'General Electronics Chat' started by Jon K, Jun 28, 2011.

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  1. Jon K

    Thread Starter New Member

    Nov 27, 2009
    Hi all - I read a lot here, but rarely post.

    I am pulling my hair out with a project. Its an engine management system I use to run my car. Right now the most obvious issue is that sometimes this sensor drops out (ie., no output).

    The sensor in question is a hall effect sensor that pulses square wave output with the presence of ferrous metal.

    The sensor has 3 wires - power in, signal out, and ground. The sensor is grounded back to the ECU to a pin called "sensor grounds" where common sensors share a ground for ground reference. The output goes to a pin that has an internal pull up resistor (1k) to 5V, and the ECU provides 5v power to the sensor when on.

    Problem is - I turn the key on, all systems are happy, and then when I go to start the car (with the starter turning) I see the square wave pulse up and down and then go high for a second even though the starter is still turning, and then start pulsing again. So, since the output of the sensor is a "sinking" output, I assume that means its a pulsed ground output - so it probably collects ground from the ground wire and spits it out over the signal wire, yes?

    When I use an oscilloscope on the signal wire and ground I can read that the signal wire is high (+5V DC) when the key is on and the starter is not turning. So that shows me that the pull-up to 5V is working inside the ECU at all times. So, in order for the signal wire to pulse and then drop out and stay high, that would mean the ground has to be dropping out, right? Or, maybe the ECU is dropping the +5V power supply to the sensor? Would that cause the same issue?

    So to test, what should I do? Hook up the scope between the +5V supply and ground, and if that voltage drops during cranking the car then one one of the two is dropping out? Or I could hook up to +12V constant on the car and if that drops out I know its the ground from the ECU to the sensor, or ....

    Thanks all for any insight. Trying to figure this out and its driving me mad! This is related to automotive, but of course is fundamental down to the hall sensor... so its not truly a modification or anything.

  2. PackratKing

    Well-Known Member

    Jul 13, 2008
    Excellent info on what it is doing though no mention yet as to what the sensor is supposed to do. There's a lot of ferrous metal in an engine to detect, ergo it must be mounted in aluminum ?? to detect ferrous ??

    Better yet I'll wait for your return........:D

    Good you specified it was part of an existing system vs. a mod, as ECU mods are not discussed on the forums for safety, emission control, and legal / liability reasons.
  3. #12


    Nov 30, 2010
    Existing systems aren't discussed either as they are part of a CAR.

    The bosses will be swooping down on this soon.
  4. bertus


    Apr 5, 2008

    I am closing this thread as it violates AAC policy and/or safety issues.

    6. Restricted topics. The following topics are regularly raised however are considered “off-topic” at all times and will results in Your thread being closed without question:

    • Any kind of over-unity devices and systems
    • Automotive modifications
    • Devices designed to electrocute or shock another person
    • LEDs to mains
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    This comes from our Tos:
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    There will be enough sites where automotive questions can be discussed :
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