Ford Full-Size Vans 1989-1996 Repair Guide

Powertrain Control Module



See Figure 1

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Fig. Fig. 1: Schematic of the Ford EEC-IV system

Most of the models covered by this information employ the fourth generation Electronic Engine Control system, commonly designated EEC-IV, to manage fuel, ignition and emissions on vehicle engines. Other models (depending on engine application), will be equipped with EEC-V.

The Powertrain Control Module (PCM) is responsible for the operation of the emission control devices, cooling fans, ignition and advance, and in some cases, automatic transmission functions. Because the EEC system oversees both the ignition timing and the fuel injector operation, a precise air/fuel ratio will be maintained under all operating conditions. The PCM is a microprocessor or small computer which receives electrical inputs from several sensors, switches and relays on and around the engine.

PCM's for EEC-IV systems use a 60-pin connector. For the EEC-V PCM, a 104-pin connector is used.

Based on combinations of these inputs, the PCM controls outputs to various devices concerned with engine operation and emissions. The engine control assembly relies on the signals to form a correct picture of current vehicle operation. If any of the input signals is incorrect, the PCM reacts to what ever picture is painted for it. For example, if the coolant temperature sensor is inaccurate and reads too low, the PCM may see a picture of the engine never warming up. Consequently, the engine settings will be maintained as if the engine were cold. Because so many inputs can affect one output, correct diagnostic procedures are essential on these systems.

The EEC system employs adaptive fuel logic. This process is used to compensate for normal wear and variability within the fuel system. Once the engine enters steady-state operation, the engine control assembly watches the oxygen sensor signal for a bias or tendency to run slightly rich or lean. If such a bias is detected, the adaptive logic corrects the fuel delivery to bring the air/fuel mixture towards a centered or 14.7:1 ratio. This compensating shift is stored in a non-volatile memory which is retained by battery power even with the ignition switched off. The correction factor is then available the next time the vehicle is operated.

The Powertrain Control Module (PCM) is usually located under the instrument panel or passenger's seat and is usually covered by a kick panel. A multi-pin connector links the PCM with all system components. The processor provides a continuous reference voltage to the B/MAP, EVP and TP sensors. EEC systems use a 5 volt reference signal. Different calibration information is used in different vehicle applications, such as California or Federal models. For this reason, careful identification of the engine, year, model and type of electronic control system is essential to ensure correct component replacement.

If the battery cable(s) is disconnected for longer than 5 minutes, the adaptive fuel factor will be lost. After repair it will be necessary to drive the truck at least 10 miles to allow the processor to relearn the correct factors. The driving period should include steady-throttle open road driving if possible. During the drive, the vehicle may exhibit driveability symptoms not noticed before. These symptoms should clear as the PCM computes the correction factor. The PCM will also store Code 19 indicating loss of power to the controller.

Electronic Engine Control

See Figure 2

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Fig. Fig. 2: Types of sensors and how they work with the PCM

The electronic engine control subsystem consists of the PCM and various sensors and actuators. The PCM reads inputs from engine sensors, then outputs a voltage signal to various components (actuators) to control engine functions. The period of time that the injectors are energized ("ON" time or "pulse width") determines the amount of fuel delivered to each cylinder. The longer the pulse width, the richer the fuel mixture.

The operating reference voltage (Vref) between the PCM and its sensors and actuators is 5 volts. This allows these components to work during the crank operation even though the battery voltage drops.

In order for the PCM to properly control engine operation, it must first receive current status reports on various operating conditions. The control unit constantly monitors crankshaft position, throttle plate position, engine coolant temperature, exhaust gas oxygen level, air intake volume and temperature, air conditioning (On/Off), spark knock and barometric pressure.


See Figure 3

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Fig. Fig. 3: Common PCM module mounting

The PCM is usually located in the left rear of the engine compartment, near the master cylinder.

  1. Disconnect the negative battery cable.
  3. Disconnect the module wiring leading to the unit.
  5. Unscrew, then remove the control unit from the vehicle.

To install:
  1. Attach the module to the bracket, then screw in place.
  3. Attach the wiring to the module.
  5. Connect the negative battery cable.