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More information
The Reference Glossary describes the
various sensors and actuators used in
the EEC-IV and MCU systems. You can
learn more by reading these definitions.
THE MCU SYSTEM
(Make sure you have read
everything in the beginning part of
this section before continuing!)
The MCU system is similar (but simpler)
than the EEC-IV version just described.
The MCU computer module is located in
the engine compartment. The MCU uses
sensors to monitor engine operation and
actuators to control things.
What MCU controls
The original MCU just controls fuel
delivery (Air/Fuel ratio) and the
Thermactor Air System. Features added
later included limited control of idle
speed, spark timing retard and fuel
evaporation canister. To do these tasks,
the MCU needs information about
engine temperature, throttle position,
tach signal and knock conditions.
How MCU measures Engine Tempera-
ture
• Some MCU systems use a single
electrical switch (“Low Temperature
Switch”). The switch is activated by
vacuum. The vacuum comes from a
“Ported Vacuum Switch” which is
temperature controlled. When engine
temperature reaches a certain value,
the Ported Vacuum Switch sends
vacuum to the Low Temperature
Switch, which toggles and sends a
signal to the MCU computer. The Low
Temperature and Ported Vacuum
switches may be separate units or
combined into one assembly.
• Other MCU systems use two
switches: Mid and Dual Temperature.
The Mid Temperature Switch is similar
to the Low Temperature Switch. The
Dual Temperature Switch sends a
signal when engine temperature is
either cold OR very hot.
How MCU measures Throttle Position
• Some MCU systems use an Idle
Tracking Switch. This is an electrical
switch mounted near the throttle
linkage on the carburetor. The switch
is open when the throttle is resting in
idle position. The switch closes as
soon as the throttle is moved off idle.
A Wide-Open-Throttle (WOT) Vacuum
Switch is also used. Weak manifold
vacuum due to WOT operation causes
the WOT Vacuum Switch to send a
signal to the MCU computer.
• Other versions of MCU monitor
engine vacuum to sense idle (high
vacuum), cruise (moderate vacuum)
or WOT (low vacuum) conditions.
Vacuum operated electrical switches
are used. The switches toggle at
various vacuum levels and send
signals to the MCU computer. These
parts are sometimes called “Low”,
“Mid” and “High” vacuum switches (a
“Zone Vacuum Switch” assembly).
Other names are “Wide-Open-
Throttle”, “Crowd” and “Closed
Throttle” vacuum switches.
Tach Signal information
The MCU system monitors this ignition
signal to measure engine RPM. A wire
connects the computer to the Tach
terminal on the ignition coil. The
computer watches RPM to insure
smooth operation when the air/fuel
mixture is changed.
Knock Sensor information
Some MCU systems have a Knock
Sensor which sends a pulse signal to the
computer when an engine knock
condition occurs.
How MCU controls Fuel Delivery
The MCU computer controls air/fuel
delivery using a “Feedback Carburetor”.
The choke and idle cam mechanisms
are similar to those on a conventional
carburetor.
• One version has the computer
controlling a fuel metering rod inside
the carburetor. The computer controls
