Chapter 7 Computer Control Fundamentals 123
Copyright by Goodheart-Willcox Co., Inc.
compressor, turn any electric cooling fans on, and increase
the engine idle speed to force more coolant through the
engine. Increasing idle speed also increases the rotational
speed of a belt-driven fan.
A few ECMs provide overheating protection by selec-
tively disabling cylinders in the engine to redistribute heat.
If the ECM detects an overheating condition, it will shut off
the fuel injectors and spark to selected cylinders in an alter-
nating pattern. This decreases the build-up of engine heat.
Sleep Mode
To reduce parasitic battery drain, some ECMs may
go into sleep mode. Most engine and powertrain control
modules power down shortly after the ignition switch is
turned to the off position. Some body control modules,
such as those for ABS, stability control, or power windows,
remain powered up for a brief period of time. Others, such
as security and keyless entry systems, do not have a sleep
mode and remain energized at all times.
Input Sensors
Input sensors are installed on the engine and vehicle
and measure the actual conditions in the engine, vehicle,
and surrounding atmosphere. The input sensors are the
ECM’s “nerves.” Input sensors monitor a wide variety of
engine and drive train functions. They monitor conditions
in the engine and drive train, outside atmospheric condi-
tions, and direct or indirect driver inputs.
Types of Sensors
There are many types of input sensors. Sensors are
grouped according to the vehicle systems they affect.
Typical sensors measure:
Engine temperature and speed.
Vehicle speed.
Crankshaft position.
Camshaft position.
Manifold vacuum.
Intake airfl ow.
Intake air temperature.
Barometric pressure.
Exhaust gas oxygen.
Engine knocking.
Figure 7-5 shows the location of some common engine
sensors. A brief description of input sensor operation is pro-
vided in this section. More detail about certain specialized
sensors is given in later chapters.
In many sensors, a reference voltage from the ECM is
passed through a resistor unit. This voltage is modifi ed as
the resistance of the sensor varies with conditions. Some of
these sensors contain a contact unit that slides along a resis-
tor, Figure 7-6. These units are sometimes called rheostats.
As the contact moves along the resistor, the resistance
is automatically set by the ECM when the vehicle reaches
certain speed and operating conditions. A few vehicles
require the idle speed control to be set by the technician,
usually with a scan tool.
Engine and Vehicle Protection
Most ECMs are designed to protect the engine and
vehicle from damage due to excessive engine or vehicle
speed. Manufacturers add programming to the ECM that
will limit the fuel pump output, fuel injector operation, or
ignition module operation if engine speed becomes exces-
sive, usually between 6000–7800 rpm. In some cases, the
ECM will not allow the vehicle to exceed a predetermined
road speed. The ECM will also shut down the engine dur-
ing extreme deceleration conditions, such as during a
collision.
Clear Flood Mode
Some ECMs will shut down the fuel injectors if the
throttle position sensor indicates the throttle valve is being
held past a certain angle during start-up. This is called
clear fl ood mode. It prevents engine damage if the driver
depresses the accelerator while cranking the engine.
Clear fl ood mode also allows technicians to clear the
cylinders should they become fl ooded with fuel. Some
technicians will use the clear fl ood mode while testing the
ignition or starting system when engine operation is not
desired. Clear fl ood mode is discussed in more detail in
Chapter 17.
Caution: Not all ECMs have clear fl ood
capability. Check the vehicle’s service
manual before attempting to use this feature.
Voltage Monitoring
The ECM monitors battery voltage in order to compen-
sate for electrical loads placed on the charging system. If the
ECM detects a drop in battery voltage, engine idle speed is
slightly increased to compensate. This increases alternator
output in response to electrical loads. If the battery voltage
drops below 10–10.5 volts or goes above 14.5–15 volts for
an extended period, the ECM stores a trouble code.
Some ECMs also have control over the voltage regula-
tor. These ECMs can increase alternator output to increase
battery voltage. If excessive charging is detected, they can
shut down the alternator for a time.
Overheating Protection
The ECM monitors engine coolant temperature in
order to adjust inputs to the output devices to compensate
for changes in engine temperature. Another feature pro-
grammed into most ECMs is overheating protection. On
most engines, the ECM will shut off the air conditioning