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Why Does This Hall Effect Distributor Pickup Only Work When Probed With An LED Test Light???

Discussion in 'Automotive Electronics' started by tempest411, Sep 16, 2016.

  1. tempest411

    tempest411 New Member

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    Hello,

    I have a Ford TFI system that I converted from a style that had a distributor mounted ignition module to a style used by Ford in later years which relocated the ignition module to the fender on a heat sink. I made the harness from scratch and used Motorcraft components. Upon completion I was very happy to see that it starts right up and runs great in it normal configuration. However, to set the base timing a removable jumper must be pulled from the SPOUT (Spark-Output) line, which takes control away from the computer and lets the module fire the ignition system at a fixed dwell. Mine will not do that unless I either goose the throttle while cranking and maintain it above 1500 rpm, or, as I discovered in the process of testing, install (back-probed) an LED test light between the distributor pickup output, called the 'PIP' and +12V. With that LED test light installed, it starts up and runs like a champ at base timing. I did find one author that mentioned this scenario (with the test light), and he stated that nine times out of ten it indicates the hall effect sensor in the distributor is bad. With that in mind, but being suspicious in my particular case as my distributor pickup was new, I made use of another distributor I had. I just plugged it in and spun the shaft. I have a spark tester between the coil and the distributor cap. It mirrors exactly what is happening with the distributor installed in the engine. I have also changed the ignition module, and loop checked all the wiring. I have looked for shorts to ground where they shouldn't be, and between all pathways to insure they are not shorted together. The only time I can get the module to perform it's switching function to the coil properly (without the LED test light installed), is to change the load-I experimented with a 12V incandescent bulb (#194) in place of the coil. The coil does check out as far as primary and secondary resistance, and is also new...One peculiar thing I noticed is that-with the LED test light removed, coil connected-if I happen to stop the distributor shaft at a place where the shutter is in the Hall Effect sensor, the coil will start firing repeatedly on it's own, with no distributor shaft rotation, but will stop if you rotate the distributor shaft so the shutter is no longer in the sensor, or you connect the LED light. I'm pretty much at my wit's end at this point. It's not that complicated a circuit! I suppose if I could find out hat the LED is actually doing for this circuit I could find out where the problem is...Any suggestion are very appreciated!

    Rick
     

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  2. Diver300

    Diver300 Well-Known Member

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    Your circuit diagram only really shows the connections, and without knowing what the different modules do, it isn't a much uses.

    From the symptoms, it sounds like you need a pull-up resistor. A lot of automotive Hall sensors have an output that is simply a switch to ground, and to get the output to go up, a pull-up is needed. In some cases, the Hall sensor assembly contains the resistor, but in other cases it doesn't.

    I suggest that you put a pull-up resistor of 1 kΩ where you have the test light.
     
  3. tempest411

    tempest411 New Member

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    My apologies...The Hall Effect output (Profile Ignition Pickup-PIP) is sent to two modules; the ECU (engine/powertrain control computer) and the ignition module. In it's normal configuration the ECU sees the PIP signal and along with other sensor inputs sends out a signal to the ignition control module (on the SPOUT wire), which is just a solid state switching mechanism for the coil. As long as there is a signal coming from the ECU on the SPOUT wire, the ignition module responds to that input. The ECU via SPOUT signal will tailor the spark by varying timing and dwell. There is an instance however when we do not want a signal to the coil that varies at all, such as when the 'base timing' is set. For this, a jumper on the SPOUT line is removed and the ignition module then looks to the raw signal coming from the distributor on it's own PIP input. when in this mode the timing is fixed at what ever you have the distributor at, and the dwell is fixed as well. It is in this mode I am have trouble with. The engine will run perfectly fine when the SPOUT is connected, but once removed, it will die unless the LED test light is in the circuit, or the speed of the engine is maintained at or above 1500 rpms. I'm not sure what the significance of the engine speed is, but the test light I can see...sort of. The test light is something I made from odds and ends I had on hand. An LED light, a 600 Ohm resistor, and some wire with alligator clips. It's probably the resistor influencing the circuit here, yes?

    I did take some measurements this morning after work (graveyard shift) of two different PIP signals. With my DVOM set to DC, between the PIP output and ground (no test light connected), I read 5V on one PIP and 2V on another. With the test light connected in the circuit both PIPs went to about 8V at their outputs. I have read the threshold voltage for correct operation of the module from the PIP signal is 5.75V. What have I done to create the need for an additional resistor in this circuit, when the factory harness did not have it?
     

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  4. dave

    Dave New Member

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  5. Diver300

    Diver300 Well-Known Member

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    Without the internal circuit of each module, I can't say why the circuit doesn't have anything pulling up the signal. It is most likely that the combination of components that you have used doesn't quite match what Ford would have put on one car. It is quite usual for changes between model years to be quite small, and Ford could have used a sensor with a pull up inside it at one model year, and then decided to put the pull up in the ECU for other model years. Then if you use an ECU with no pull up with a sensor that also has no pull up, that gives the condition that you have now.

    On a similar theme, is there something else connected to that line that would have been fitted on a production car, that provided the pull-up?

    I haven't quite followed why the jumper is removed, but if that is only intended to be done as a diagnostic procedure, it could be that Ford only wanted the jumper removed when some bit of test equipment was wired to the car, so the test equipment would be the pull-up. There are lots of places where test procedures need specific bits of equipment to function in place of car components during diagnostics. This could be one of them.

    It is also possible that you have a faulty component. If the resistor cures it, that will be far cheaper than anything from Ford, and it won't do any harm to add the resistor if it turns out not be needed.
     
  6. tempest411

    tempest411 New Member

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    Thank you very much for your thoughts on this. The components I'm using, except for the ECU, were all used together from the factory. It is possible that my ECU has a bad resistor in it, though people actually use these same ignition components with completely different ECUs, such as Mega/MicroSquirt. My next move is to obtain a third example of a PIP and test that. If it doesn't do the trick I'll just patch in a resistor and be done with it!
     

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