Chapter
E
: Computerized Engine Control Diagnosis And Repair (Including OBD II)
100
On/Off solenoid
An on/off solenoid is used to regulate the flow of air or
fluid, or to move a mechanical linkage. Solenoids use a
two wire circuit: power and ground. On a typical auto-
motive application, voltage is applied to one solenoid
terminal, and the PCM switches the other circuit to
ground. Most solenoids are energized for varying
lengths of time as determined by PCM programming,
switch inputs from the driver, or input sensor signals.
The operation of an on/off solenoid can be checked by
monitoring voltage on the power circuit with a DMM. On
a solenoid that switches to ground, full voltage, which
may be either system voltage or a lower regulated volt-
age, should be available when the solenoid is switched
off because the circuit is open. When the solenoid en-
ergizes, or switches on, the circuit is complete and the
measured voltage should drop to ground. Resistance
specifications are available for most solenoids, and this
check can also be made with a DMM.
A more accurate way to check a solenoid is to measure
the amperage draw through the driver circuit. For this
test, the circuit must be under power and loaded so that
current is applied to the device. To test with a DMM, set
the meter to the amperage scale and connect it in se-
ries between the solenoid negative terminal and ground,
or the actual driver circuit if it can be energized. Leave
the circuit energized for one to two minutes to check the
draw. Compare results to specifications.
Pulse Width Modulation
Many automotive solenoids are pulsed on and off rapid-
ly at a specific number of cycles per second, or Hertz
(Hz). The operating cycle of a pulsed solenoid is the se-
quence from off to on and back to off again. An
actuator solenoid can operate at any fixed number of
cycles per second, or fixed frequency: 10 Hz, 20 Hz, 60
Hz, and so on.
The pulse width, which is usually measured in
milliseconds (ms), is the amount of time a solenoid is en-
ergized, or the “on” part of the cycle. Although the op-
erating frequency of a solenoid may be fixed, the ratio
of “on-time” to “total cycle time” can be varied. This
PWM control allows a digital output signal to provide
varied or analog control of a mechanical device.
The duty cycle of a PWM solenoid is the percentage of
the complete off-on-off cycle during which the solenoid
is energized, or on. It is the ratio of pulse width to com-
plete cycle width, figure 5-19. Duty cycle is usually ex-
pressed as a percentage. A 25 percent duty cycle
means that the solenoid is energized for 25 percent of
the total off-on-off cycle time; a 75 percent duty cycle
means that the solenoid is energized for 75 percent of
0
VOLT
5
VOLT
DROPOUT
FIg. 5-15.
Scope trace showing signal dropout during a
potentiometer sweep test.
REFERENCE
SIGNAL
GROUND
~
~
PCM
PIEZORESISTIVE
DEVICE
MANIFOLD
PRESSURE
MAP SENSOR
Fig. 5-16.
Manifold vacuum acts on a piezoresistive crystal to
provide a variable analog signal to the PCM based on engine load.
6V
BACK
SELECT
SEARCH
4
2
0
5ms/DIV
-2
-4V
HOLD
102
RECALL
5.28
50.0
5.00
Hz FREQUENCY
V
%
ms
PEAK-PEAK
DUTY CYCLE
PULSE WIDTH
Fig. 5-17.
A digital MAP sensor transmits a variable frequency signal
to the PCM.
Summary of Contents for ASE-A8
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