Ford Aerostar 1986-1997 Repair Guide

Control Systems



Micro-processor Control Unit (MCU) System

The Micro-processor Control Unit (MCU) system is used on the 2.8L carbureted 6-cylinder engine. The heart of the system is the fuel control unit. This is necessary to keep the air-fuel ratio at the proper balance (14.7 to 1) to obtain maximum catalyst efficiency. The fuel control loop consists of an Exhaust Gas Oxygen (EGO) sensor, Micro-processor Control Unit (MCU), and Fuel Control Solenoid (FCU).

The EGO sensor senses whether the exhaust gas is rich or lean of proper chemical balance. This signal is sent to the MCU module, which sends a varying signal to the fuel control solenoid to move the air-fuel ratio back to the proper chemical balance. The operation is called a closed loop operation.

The other mode of operation is called an open loop. In this mode, the MCU module sends out a fixed signal to the fuel control solenoid. During this time the input from the EGO sensor is ignored, thus opening the loop.

The factor which determines when the system goes into an open or closed loop is based upon information from the switch inputs, such as the coolant temperature, manifold vacuum, and throttle position switches. Generally, the vehicle will be in closed loop when the vehicle is at operating normal temperature and at a steady part throttle cruise.

Other functions controlled by the MCU module include charcoal canister purge and the spark retard.

Because of the complexity of this system, no attempt should be made to repair the module. It should only be serviced by a qualified mechanic.

Electronic Engine Control IV (EEC-IV) System

The Electronic Engine Control IV system is used on the 2.3L EFI, 3.0L and 4.0L fuel injected engines. Minimal variations may occur between years and models. This system is designed to improve emission control, fuel economy, driveability, and engine performance. This is achieved by the means of an on-board control assembly which reads the inputs from various sensors and makes computations based on these inputs and then sends controlling outputs to various engine components in order to provide the optimum air/fuel ratio.

The electronic control assembly is calibrated to optimize emissions, fuel economy and driveability. The system controls the fuel injectors for air fuel mixture, spark timing, deceleration fuel cut-off, EGR function, curb and fast idle speed, evaporative emission purge, air condition cut-off during wide open throttle, cold engine start and enrichment, electric fuel pump and self test engine diagnostics.


See Figure 1

The removal and installation procedure is basically the same for EEC-IV and MCU modules, except that different modules may have multiple wire harnesses attached.

  1. Disconnect the negative battery cable.
  3. Locate the EEC-IV/MCU model in the vehicle. Depending on the production year and model, the module is located to the right of the steering column on the driver's side, or next to the glove box on the passenger side. The module is roughly the size of a cigar box and is silver in color.
  5. To access the module, remove the retaining screws securing the interior panel over the assembly.
  7. Loosen and remove the retaining bolts securing the EEC-IV/MCU to the bracket or body. Once the hardware is removed, lower the module to access the wire harness.

Click image to see an enlarged view

Fig. Fig. 1: EEC-IV module under steering column-1990 model shown

  1. Determine whether the module has multiple wire harnesses attached. If it does, mark or tag each harness to identify its relative position.
  3. Disconnect the wire harness(s) from the module.
  5. Inspect the connectors for signs of corrosion and clean if needed.

To install:
  1. Plug the wire harness (s) into the module. If the plug does not fit in easily, remove and start again. NEVER force a harness into the module assembly.
  3. Position the EEC-IV/MCU module to the mounting bracket or body, and secure in place using the mounting bolts. Tighten the bolts to 15-18 ft.lbs. (19-23 Nm).
  5. Install the interior panel over the module and secure in place using the retaining screws.
  7. Connect the negative battery cable.

When the battery cable has been disconnected and reconnected, some abnormal drive symptoms may occur while the powertrain control module relearns its adaptive strategy. The vehicle may need to be driven 10 miles (16 km) or more to relearn the adaptive strategy.