Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
99
Most thermistor failures are due to an open or short cir-
cuit, which can be detected with a digital multimeter
(DMM). However, voltage dropout and drift can also
cause problems with this type of sensor. Expect signal
voltage to change slowly because it responds to chang-
ing temperature. The best way to isolate signal dropout
and drift is to make a timed data recording. Use a
graphing multimeter
(GMM) to record signal voltage
from a cold start until the engine reaches normal oper-
ating temperature.
OUTPUT ACTUATORS
The actuators are the output devices activated by the
PCM to control vehicle operation. These devices con-
vert electrical energy into mechanical work. Actuators
usually contain some kind of induction coil and operate
at current levels that are much higher than input sensor
circuits, which only carry information. A feature of most
automotive actuators is that the PCM regulates the de-
vice by controlling on the ground side of the circuit. One
side of the actuator coil receives system voltage, and
the ground side circuit connects to the PCM.
Most of the system voltage is dropped across an actu-
ator, which reduces the current applied to the output dri-
ver transistors in the PCM. To regulate this voltage drop,
the coil windings of an actuator must meet a minimum
resistance specification. If coil resistance is too low,
higher voltage and current on the ground circuit can
overheat and destroy the transistors in the PCM driver
circuit.
Common actuators used on an electronic engine man-
agement system include solenoids, relays, and stepper
motors.
Solenoid
A solenoid is an electromechanical device that uses
magnetism to move an iron core. The core provides me-
chanical motion to some other system part. A solenoid
thus changes electrical voltage and current into me-
chanical movement. Solenoids operate fuel injectors,
carburetor metering rods and air bleeds, vacuum con-
trol valves, transmission hydraulic circuits, and other de-
vices in an engine control system. There are two types
of solenoids used in automotive systems:
• On/off
• Pulse width modulated
All solenoids are digital devices, which can only be
switched on or off. However, by regulating the amount
of time the induction coil is switched on in relation to the
amount of time that it is switched off, the PCM can vary
the movement of the core. This type of solenoid control
is known as pulse width modulation.
VHz~
~
10A
RPM
+
COM
A
C F
RPM
V
%
OFF
VHz
A
C
B
THROTTLE POSITION
SENSOR
REFERENCE
SIGNAL
GROUND
FIg. 5-14.
Checking the TP sensor with a DMM.
Fig. 5-13.
The position of the variable wiper on a potentiometer
determines whether the signal voltage will be high or low.
Summary of Contents for ASE-A8
Page 2: ......