Ford TFI modules: Gray or Black?
Most technicians who deal with Ford driveability and no-start problems have become very familiar with
the Thick Film Ignition (TFI) system. Ford started using the six-pin TFI module with the EEC-IV computer
system in 1983, and for years it remained basically unchanged.
The early TFI system, which Ford calls the "Push Start" TFI system, uses a gray TFI module. Originally,
the module was mounted on the distributor. In the late '80s Ford began to relocate it away from the
distributor on some vehicles to provide better protection from the effects of engine heat, but system
operation remained the same. It uses a Hall effect pickup (stator) in the distributor, which generates
a battery voltage, 50% duty cycle square wave, called the PIP signal, to the EEC-U PCM and the TFI module.
The PCM processes this signal and sends out another battery voltage, 50% duty cycle square wave, called
the SPOUT signal, to the TFI module. As long as the TFI module is receiving a SPOUT signal, it will fire
the coil at the rising edge of that signal (except during engine cranking, when SPOUT is ignored) and
the vehicle will run with the amount of timing advance commanded by the computer. If the TFI module
does not receive the SPOUT signal, it will fire the coil at the rising edge of the PIP signal, a nd
the vehicle will run at base timing. This is true on all TFI systems.
Ignition dwell with the Push Start (gray module) system is controlled by the TFI module alone, and increases
with engine rpm. The Ignition Diagnostic Monitor (IDM) signal on a Push Start TFI system comes from the coil
negative circuit and is filtered through a 22k ohm resistor to pin #4 on the EEC-IV computer. The computer
monitors this circuit to verify a coil firing for each PIP signal, and sets codes if it sees missing or erratic
signals. Another feature that is unique to the Push Start TFI system is the start input on pin #4 of the module
connector. This is wired into the starter relay trigger circuit, and signals the TFI module that the engine is
cranking. When the module sees battery voltage on this circuit, the SPOUT signal is ignored.
In the early '90s, Ford began using a different TFI system on certain vehicles--the Computer Controlled Dwell (CCD)
TFI system. The TFI module on CCD TFI is always black in color. There are a few major differences between the two systems.
As the name implies, with the CCD system, the computer controls primary dwell. The CCD TFI module still ungrounds (fires)
the coil at the rising edge of the SPOUT signal, but now the falling edge of the SPOUT signal (which had
no meaning to the Push Start TFI module) is used by the CCD TFI module to ground the coil. The PIP signal
remains the same 50% duty cycle square wave, but SPOUT signal duty cycle varies according to how much dwell
is desired by the computer.
Another major difference between the two systems is the IDM circuit. Pin #4 on the CCD TFI module, which was
the start circuit input on the Push Start TFI module, is now the IDM signal, sent directly from the TFL module
to pin #4 on the EEC-IV computer. This signal is still a filtered (low voltage) version of the ignition primary
waveform, but is filtered internally in the TFI module rather than through an external resistor. There isn't any
start circuit input to the CCD TFI module; the module infers engine cranking from a low rpm input from the PIP signal.
Since these two TFI systems are so significantly different, yet so similar in appearance, parts application problems
will inevitably occur. A gray Push Start TFI module will plug right into a CCD system, and vice versa. To make matters
worse, parts books are often incorrect on TFI module applications! With the incorrect TFI module installed,
the vehicle will run, but driveability and MIL (malfunction indicator lamp) problems will result. For instance,
if a gray Push Start TFI module is installed in a CCD system, the computer will not be able to control ignition
dwell, and the MIL will illuminate with memory codes for the IDM circuit set, as the gray TFL module is incapable
of generating an IDM signal to the computer. If a black CCD TFI module is installed in a Push Start system, dwell
will remain fixed, since the SPOUT signal duty cycle never changes. If in doubt about which TFI module belongs
on a particular vehicle, consult the ignition system wiring diagram for the vehicle. If the wire going to pin
#4 on the EEC-IV computer comes directly from pin #4 of the TFI module, it is a CCD system. If not, it is a Push Start system.
A final note: As with every other rule, there is an exception. Some 1.9L Central Fuel (throttle body) Injected
Escorts and Tracers were built with black TFI modules that are not CCD modules. If you encounter one of these,
check the engineering number on the module. A CCD TFI module engineering number always starts with E9 or higher.
Written by David Sill, IDENTIFIX Ford Team Leader. Dave is a certified Ford EEC, Advanced Engine Performance
Specialist, ASE Master and L 1. He is a graduate of Ford's ASSET Program and has 14 years of diagnostic repair experience.
COPYRIGHT 2001 Adams Business Media
COPYRIGHT 2001 Gale Group