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Hi All, First of all, my apologies to any of you who may feel that these questions should 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 time 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 applications. This PIP signal is sent to the TFI module, and the ECU, (engine control unit). 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 built engine management computer, Megasquirt , in particular. I've taken some existing 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 is fairly basic theory. It is my understanding that the primary winding of the coil is connected to system 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 current 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 ballast 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 control. The question is, what is the value of this controlled current, and when does it occur? I say, the amount of energy available to fire the coil depends, not only on the 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 time 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? Thanks, Robert Hiebert
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