Connect with us

Ignition coil theory and Ford TFI module questions

Discussion in 'Electronic Design' started by Guest, Nov 9, 2003.

Scroll to continue with content
  1. Guest

    Guest Guest

    Hi All,
    First of all, my apologies to any of you who may feel that these questions
    have been posted to some other group, but I felt that those who frequent
    this one would
    likely have the most knowledge concerning the subject matter.
    It concerns the workings of the Ford , "push start", TFI module.
    This module was used from the mid 80's to the mid 90's to control the dwell
    of the ignition coil, and provide a "limp home" mode, at base timing, in
    case the timed
    spark signal to the module is lost. Ford refers to the trigger signal as
    PIP, (profile ignition
    pickup). It is produced by a hall effect sensor in the distributor at system
    voltage, and has
    it's rising edge at 10 degrees before top dead center on the crankshaft. In
    the case of the
    distributor I'm working on, the duty cycle is 50% for all cylinders except #
    1, which is
    35%. This is used to identify #1 for fuel injector sequencing in some
    This PIP signal is sent to the TFI module, and the ECU, (engine control
    Which uses it to calculate RPM, and as a camshaft position indicator to
    calculate timed
    spark. The SPOUT signal, (timed spark out), is an output at system voltage
    and 50% duty
    cycle, and is sent back to the TFI module. It is delayed from the point of
    PIP, and
    calculated to occur so that it's rising edge coincides with the desired
    timing angle to fire
    the coil for the following cylinder in the firing order. The rotor on the
    distributor routes
    the coil secondary current to the appropriate spark plug. The TFI module
    fires the coil
    when it receives a rising edge of SPOUT. If it does not receive a SPOUT
    before the next
    PIP, it will fire the coil on that PIP, (base timing, limp home mode). It
    appears to charge
    the coil only when PIP is low.
    There is also an input the TFI module from the starter circuit which is
    supposed to
    provide increased dwell time during cranking to offset decreased system
    voltage. Also,
    some printed information available on the module has stated that if the
    "start" input is not
    present during crank, it will latch into "limp home" mode and stay there
    until the key has
    been cycled and start signal restored.
    I am, along with some others, are trying to use this module with a home
    engine management computer, "Megasquirt", in particular. I've taken some
    Megasquirt codes and have modified them so they should work with the TFI
    module but
    have run into some difficulties, probably because of my lack of
    understanding on exactly
    how this module works.
    The discussion has come up as to how the TFI handles the dwell time, which
    fairly basic theory.
    It is my understanding that the primary winding of the coil is connected to
    voltage for a short period of time, say 4 or 5 ms. During this time, the
    current ramps up,
    opposed by the induced voltage in the windings, up to a maximum steady state
    determined by the resistance of the circuit. When it's time to "fire', the
    circuit to the
    primary is opened. The field collapses, inducing a high voltage in the
    primary, and a
    proportional higher voltage in the secondary.
    In a conventional Kettering, (points) system. the current is limited by a
    resistor in series with the coil primary, and the time available to charge
    the coil due to the
    points being open and closed by a lobe on the distributor shaft.
    Information on the TFI module states that it controls dwell with current
    The question is, what is the value of this controlled current, and when
    does it
    I say, the amount of energy available to fire the coil depends, not only on
    inductance of the coil, but the amount of current flowing in the primary, at
    the time of
    current break. This is usually limited to 4 to 6 amps.
    So, my theory is, the coil is not actually charged until SPOUT is received
    .. At this
    point, it is charged and fired in quick succession.
    Others have said, the coil can be charged to it's fully charged state, any
    before SPOUT goes high, with the current still limited to about the same
    value, but then
    decreased to some milliamp value until time to fire the coil, then break the
    current, and
    the coil will fire.
    Who's right?
    Next question.
    The heart of the module is a Motorola IC.
    The numbers on it are:
    XC33191DW and XAA0001
    I tried a google search to see if I could get a data sheet, or any
    information at all
    on this chip, but only came up with a couple of sites which listed it as an
    obsolete, or hard
    to find part.
    Can anyone shed any more light on exactly how this little critter works?
    Robert Hiebert
  2. Jim Thompson

    Jim Thompson Guest

    I'm the one who coined the phrase "limp home". IIRC it was GM that
    wanted lost timing to result in drop dead operation of the ignition so
    they wouldn't violate any smog laws. I told them I wouldn't be a
    party to any such design that did that and we settled on a crude
    default timing mechanism that would allow you to "limp home" if you
    lost contact with the ECU.

    "Current Control" implies that the ignition coil driver device is
    turned on and the current ramps up. When the current reaches the
    proper value (~5A) the driver device goes into current-limit mode,
    comes out of saturation as necessary to hold the current constant.
    The driver device turns off at the firing point.

    This results in a hot-operating driver device.

    There are several cures for this situation:

    Ford (in the late '60's, early '70's) had a shaped rotor structure
    that produced a ramp, then a pulse from a pick-up coil (inductor).
    When the ramp voltage-value crossed a threshold the driver device was
    turned on.

    Operation is the same as with Current Control except that the
    driver-device condition is reported to the electronics at the time of

    If the device is still saturated the slicing point on the ramp is
    moved down to allow more inductor "charging" time before firing. If
    the driver device was out of saturation (Current Control mode) the
    ramp slicing point is moved up to reduce the amount of time the coil
    is charging.

    The result is low dissipation.

    Using a 1968 (big mama engine, but I can't remember the size) T-bird
    as a test vehicle I was never able to "out-run" this scheme (though I
    did manage to scare the hell out of my boss driving up and down 101 in
    San Jose at over 100MPH :)

    The same scheme can be done with an electronically generated ramp
    instead of the shaped rotor.

    In the past I have posted an ignition design of mine that
    "self-inverts" to produce a very efficient capacitive discharge
    ignition. I'll see if I can find it and post it on my website.

    ...Jim Thompson
  3. Robert Hiebert wrote...
    Hmm, there was a Motorola MC33191, a 16-pin DIP
    "automotive 12V ignition driver" IC, is that it?

    - Win

  4. Jim Thompson

    Jim Thompson Guest


    See "Self-Inverting-CD-Ignition.pdf" on the S.E.D/Schematics page of
    my website.

    ...Jim Thompson
  5. Jeff

    Jeff Guest

    Sometimes I think "Upper Management" has a requirement to pass a stupidity
    test. Add that to the marketing department's ever increasing ability to sell
    low cost junk for ever increasing prices.

    460 CID, maybe a 427?

    100 MPH is not scary, unless thats one bad highway (assuming the 101 is a
  6. It was with a '68 gas-guzzler with eiderdown suspension and bullock-cart
    steering design. (;-)

    In Britain, we had big Rover saloons that had somewhat similar
    characteristics. They had a habit of going straight on at roundabouts.
  7. Jim Thompson

    Jim Thompson Guest

    The T-bird was a nice car and, IIRC, handled quite well.

    101 is the main north-south highway thru San Jose and is normally
    packed with cars... that's the scary part ;-)

    ...Jim Thompson
  8. Jim Thompson wrote...
    I thought the CHP was the scary part!

    - Win

  9. Jeff

    Jeff Guest

    The HP nailed me once for 92 after entering the highway for just 0.35 miles,
    while going up a big hill. It was late at night in a double lane twined
    highway with almost no one around. The cop was just over the crest of the
    hill digging in his trunk. He ran to grab the radar, giving me a good amount
    of time to slam my brakes on for a while before he got a reading....
  10. Jim Thompson

    Jim Thompson Guest

    By legislative regulation CHP does NOT have radar... only local police
    units do.

    But I did get picked off once by a CHP airplane between Blythe and

    ...Jim Thompson
  11. I read in that Jim Thompson>) about 'Ignition coil theory and Ford TFI module questions', on
    Mon, 10 Nov 2003:
    They are armed aircraft??
  12. Jim Thompson

    Jim Thompson Guest

    Naaah! They time you from the air using a stopwatch and marks on the
    roadway. Then they have a car pull you over. Because I was
    see-sawing back-and-forth between 100+ and 55 depending upon whether a
    black & white was visible I got ticketed for 83 :-(

    This was back in the ridiculous Carter double-nickel days ;-)

    Car was 280ZX.

    ...Jim Thompson
  13. I read in that Jim Thompson>) about 'Ignition coil theory and Ford TFI module questions', on
    Mon, 10 Nov 2003:
    You could have insisted on the geometric mean - 74.2 mph. (;-)
  14. Oh really? That's odd. Here in PA it's the other way around, only the state
    cops are allowed to have radar, not the local ones...
  15. Jim Thompson

    Jim Thompson Guest

    AIUI many years ago the California legislature and CHP had a budget
    tiff and, to this day, the legislature won't fund radar.

    In Arizona *every* police officer has radar... even motorcycles. So
    most drivers have radar detectors.

    ...Jim Thompson
  16. Fred Abse

    Fred Abse Guest

    This was CHP. "Geometric" has four syllables. :)
  17. I read in that Fred Abse <[email protected]> wrote (in <>)
    about 'Ignition coil theory and Ford TFI module questions', on Tue, 11
    Nov 2003:
    So does 'arithmetic' - the arithmetic mean is 77.5 mph. 88 is more than
    the harmonic (3 syllables!) mean, 70.97 mph. There must have been a lot
    of '+' associated with that '100'.
  18. Jim Thompson

    Jim Thompson Guest

    Everyone is missing the point that stopwatches only get end-to-end
    timing... not an integral ;-)

    Now would *I* do a lot of "+" ? You bet your sweet bippy!

    ...Jim Thompson
  19. Jeff

    Jeff Guest

    How much "+"?

    A km in under 15 seconds is moving. Not recommend in traffic.
  20. Jim Thompson

    Jim Thompson Guest

    I've driven steadily at 135-140MPH (almost 1km/15-secs) down A3 (maybe
    A5, can't remember for sure now... parallels the Rhein south from
    Frankfurt, Germany).

    In speed-regulated Arizona I rarely drive over 110MPH... the roads
    here lack the quality and banking of the German equivalents.

    ...Jim Thompson
Ask a Question
Want to reply to this thread or ask your own question?
You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.
Electronics Point Logo
Continue to site
Quote of the day