Ford TFI Ignition system Wave Form Analysis (tech article)

[SPArKy_Dave 8,872]

Here's a Picoscope waveform of what Ford TFI ignition signals look like. The blue signal is the PIP sensor inside the distributor, short for Profile Ignition Pickup. This is called the PIP signal, and it's how the distributor tells the ignition module and the PCM when a cylinder is at 10 degrees before Top Dead Center. One out of every 8 pulses is narrower than the other 7 are. This is called Signature PIP, and represents Cylinder #1 is at 10 degrees before TDC.

Once the PCM knows which cylinder is #1, it can fire injector #1 at the correct time. To fire the other 7 injectors, it doesn't need a special cylinder identification. The PCM just counts from 1 to 8 in the firing order - 1-3-7-2-6-5-4-8.
The green signal is called SPOUT, short for SPark OUTput. The computer receives the PIP signal, modifies the timing of this signal to reflect the ignition advance timing it wants the coil to fire at, and sends this modified signal back to the ignition module. The ignition module then fires the coil based on this SPOUT signal.

 

If the SPOUT signal is not present, the ignition module creates it's own SPOUT signal that is identical to the PIP signal, and fires the coil based on that instead. This is what happens when either the PCM loses communication with the ignition module, or when you unplug the SPOUT connector to set your timing. Unplugging SPOUT literally cuts the wire going from the PCM to the ignition module.
The notches you see in the top of the PIP and SPOUT signals are reflections of the one signal's edge in the other. Presence of these notches indicates the ignition module is receiving a SPOUT signal OK. The TFI ignition system can be standalone. It does not need the PCM in order to fire the coil. It only needs the PCM to alter the timing of the coil firing, to create the proper ignition advance. Without the PCM, the ignition will fire at 10 degrees BTDC every time, if that's where your base timing is set to.

These ignition signal waveforms were captured using a Picoscope 4423 4 channel digital storage oscilloscope. It's basically the world's fastest graphing voltmeter. Each one of these blue PIP signals lasts for about 11.32 milliseconds at an idle speed of 672 RPM. A typical voltmeter will only update about 4 times per second, or once every 250 milliseconds, which is nowhere near fast enough to capture these events.

At 672 RPM, one PIP signal lasts about 11.32 milliseconds, or 11/1,000 of a second. At 6,000 RPM, one PIP signal will last for approximately 1.25 milliseconds, or 1/1,000 of a second. A scope could capture that as if it were in slow motion. This scope can measure signals as fast as 1/2 of one nanosecond. Nothing gets past this scope. If you're at 6,000 RPM and one single PIP signal is missing, causing one cylinder to misfire one time, a scope can catch it in the act, which is why a scope is perfect for diagnostics.
At Drag Radial Performance, we specialize in these classic 5.0L Mustangs. Despite their age, we're moving the hobby forward in technology, picking up where these cars were left off 20 years ago, and applying today's technology in tuning and diagnostics to them. It's a great time to be rolling in a 5.0!